LTC1562ACG [Linear]
Very Low Noise, Low Distortion Active RC Quad Universal Filter; 非常低噪声，低失真有源RC四核通用滤波器
| 型号： | LTC1562ACG |
| 厂家： | 
Linear |
| 描述： | Very Low Noise, Low Distortion Active RC Quad Universal Filter |
| 文件： | 总28页 (文件大小：493K) |
| 中文： | 中文翻译 | 下载： | 下载PDF数据表文档文件 |
LTC1562
Very Low Noise, Low Distortion
Active RC Quad Universal Filter
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FEATURES
DESCRIPTION
TheLTC®1562isalownoise, lowdistortioncontinuous-time
filter with rail-to-rail inputs and outputs, optimized for a
center frequency (fO) of 10kHz to 150kHz. Unlike most
monolithic filters, no clock is needed. Four independent 2nd
order filter blocks can be cascaded in any combination, such
as one 8th order or two 4th order filters. Each block’s
response is programmed with three external resistors for
center frequency, Q and gain, using simple design formulas.
Each 2nd order block provides lowpass and bandpass out-
puts. Highpass response is available if an external capacitor
replaces one of the resistors. Allpass, notch and elliptic
responses can also be realized.
■
Continuous Time—No Clock
■
Four 2nd Order Filter Sections, 10kHz to 150kHz
Center Frequency
■
±0.5% Typical Center Frequency Accuracy
■
±0.3% Typical Center Frequency Accuracy (A Grade)
■
Wide Variety of Response Shapes
■
Lowpass, Bandpass and Highpass Responses
■
103dB Typical S/N, ±5V Supply (Q = 1)
■
97dB Typical S/N, Single 5V Supply (Q = 1)
■
96dB Typical S/(N +THD) at ±5V Supply, 20kHz Input
Rail-to-Rail Input and Output Voltages
DC Accurate to 3mV (Typ)
■
■
■
“Zero-Power” Shutdown Mode
The LTC1562 is designed for applications where dynamic
range is important. For example, by cascading 2nd order
sections in pairs, the user can configure the IC as a dual 4th
order Butterworth lowpass filter with 94dB signal-to-noise
ratio from a single 5V power supply. Low level signals can
exploitthebuilt-ingaincapabilityoftheLTC1562.Varyingthe
gain of a section can achieve a dynamic range as high as
118dB with a ±5V supply.
■
Single or Dual Supply, 5V to 10V Total
■
Resistor-Programmable fO, Q, Gain
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APPLICATIONS
■
High Resolution Systems (14 Bits to 18 Bits)
■
Antialiasing/Reconstruction Filters
Data Communications, Equalizers
Dual or I-and-Q Channels (Two Matched 4th Order
■
■
Othercutofffrequencyrangescanbeprovideduponrequest.
Please contact LTC Marketing.
Filters in One Package)
■
Linear Phase Filtering
■
, LTC and LT are registered trademarks of Linear Technology Corporation.
Replacing LC Filter Modules
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TYPICAL APPLICATION
Amplitude Response
Dual 4th Order 100kHz Butterworth Lowpass Filter
10
0
–10
–20
R
, 10k
IN2
R
IN1
10k
20
19
18
16
15
13
12
11
1
2
INV C
V
INV B
V1 B
IN2
5V
R
, 13k
R
, 5.62k
Q2
Q1
V1 C
V2 C
–30
R22, 10k
SCHEMATIC INCLUDES PIN
R21, 10k
3
V2 B
+
NUMBERS FOR 20-PIN PACKAGE.
V
–40
OUT2
5
PINS 4, 7, 14, 17 (NOT SHOWN)
–
LTC1562
–5V
V
V
–
ALSO CONNECT TO V
–50
0.1µF
0.1µF
6
AGND
V2 D
SHDN
V2 A
V1 A
INV A
SEE TYPICAL APPLICATIONS
FOR OTHER CUTOFF FREQUENCIES
V
R23, 10k
–60
OUT1
8
R24, 10k
DC ACCURATE, NONINVERTING,
UNITY-GAIN, RAIL-TO-RAIL
INPUT AND OUTPUTS. PEAK
SNR ≈ 100dB WITH ±5V SUPPLIES
–70
9
R
V1 D
IN3
10k
R
, 13k
Q4
R
, 5.62k
–80
10
Q3
INV D
V
10k
100k
1M
IN1
1562 TA01
FREQUENCY (Hz)
R
IN4
, 10k
1562 TA03b
1
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PACKAGE/ORDER INFORMATION
LTC1562
W W U W
ABSOLUTE MAXIMUM RATINGS
(Note 1)
Total Supply Voltage (V+ to V–) .............................. 11V
Maximum Input Voltage
TOP VIEW
ORDER PART
NUMBER
INV B
V1 B
V2 B
1
2
3
4
5
6
7
8
9
20
19
18
17
16
INV C
V1 C
V2 C
at Any Pin ....................(V– – 0.3V) ≤ V ≤ (V+ + 0.3V)
Operating Temperature Range
–*
V
–*
V
LTC1562CG
LTC1562ACG
LTC1562IG
LTC1562AIG
+
–
V
V
LTC1562C................................................ 0°C to 70°C
LTC1562I............................................ –40°C to 85°C
Storage Temperature Range ................. –65°C to 150°C
Lead Temperature (Soldering, 10 sec).................. 300°C
SHDN
15 AGND
14
13 V2 D
12 V1 D
11 INV D
–*
–*
V
V
V2 A
V1 A
INV A 10
G PACKAGE
20-LEAD PLASTIC SSOP
*G PACKAGE PINS 4, 7, 14, 17 ARE
SUBSTRATE/SHIELD CONNECTIONS
–
AND MUST BE TIED TO V
TJMAX = 150°C, θJA = 136°C/W
Consult factory for Military grade parts.
VS = ±5V, outputs unloaded, TA = 25°C, SHDN pin to logic “low”,
unless otherwise noted. AC specs are for a single 2nd order section, RIN = R2 = RQ =10k ±0.1%, fO = 100kHz, unless noted.
ELECTRICAL CHARACTERISTICS
SYMBOL PARAMETER
CONDITIONS
MIN
TYP
MAX UNITS
V
Total Supply Voltage
Supply Current
4.75
10.5
V
S
I
V = ±2.375V, R = 5k, C = 30pF, Outputs at 0V
17.3
19
19.5
21.5
mA
mA
S
S
L
L
V = ±5V, R = 5k, C = 30pF, Outputs at 0V
S
L
L
V = ±2.375V, R = 5k, C = 30pF, Outputs at 0V
●
●
23.5
25.5
mA
mA
S
L
L
V = ±5V, R = 5k, C = 30pF, Outputs at 0V
S
L
L
Output Voltage Swing
V = ±2.375V, R = 5k, C = 30pF
●
●
4.0
9.3
4.6
9.8
V
V
S
L
L
P-P
P-P
V = ±5V, R = 5k, C = 30pF
S
L
L
V
OS
DC Offset Magnitude, V2 Outputs
(Lowpass Response)
V = ±2.375V, Input at AGND Voltage
V = ±5V, Input at AGND Voltage
S
●
●
3
3
15
15
mV
mV
S
DC AGND Reference Point
V = Single 5V Supply
S
2.5
V
Center Frequency (f ) Error (Note 2)
LTC1562
LTC1562A
O
V = ±5V, V2 Output Has R = 5k, C = 30pF
V = ±5V, V2 Output Has R = 5k, C = 30pF
S L L
0.5
0.3
1.0
0.6
%
%
S
L
L
H
LP Passband Gain (V2 Output)
BP Passband Gain (V1 Output)
V = ±2.375V, f = 10kHz,
●
●
0
+0.05 +0.1
dB
L
S
IN
V2 Output Has R = 5k, C = 30pF
L
L
H
B
V = ±2.375V, f = f ,
S IN O
+0.2 +0.5
dB
V2 Output Has R = 5k, C = 30pF
L
L
2
LTC1562
VS = ±5V, outputs unloaded, TA = 25°C, SHDN pin to logic “low”,
unless otherwise noted. AC specs are for a single 2nd order section, RIN = R2 = RQ =10k ±0.1%, fO = 100kHz, unless noted.
ELECTRICAL CHARACTERISTICS
SYMBOL
PARAMETER
CONDITIONS
V = ±2.375V, LP Output Has R = 5k, C = 30pF
MIN
TYP
MAX UNITS
Q Error
+3
%
S
L
L
Wideband Output Noise,
Lowpass Response (V2 Output)
V = ±2.375V, BW = 200kHz, Input AC GND
V = ±5V, BW = 200kHz, Input AC GND
S
24
24
µV
µV
S
RMS
RMS
Input-Referred Noise, Gain = 100
BW = 200kHz, f = 100kHz, Q = 1, Input AC GND
4.5
µV
O
RMS
THD
Total Harmonic Distortion,
Lowpass Response (V2 Output)
f
= 20kHz, 2.8V , V1 and V2 Outputs Have
–96
dB
IN
P-P
R = 5k, C = 30pF
L
L
f
= 100kHz, 2.8V , V1 and V2 Outputs Have
–78
dB
IN
P-P
R = 5k, C = 30pF
L
L
+
Shutdown Supply Current
SHDN Pin to V
1.5
1.0
15
µA
µA
+
SHDN Pin to V , V = ±2.375V
S
Shutdown-Input Logic Threshold
Shutdown-Input Bias Current
Shutdown Delay
2.5
–10
20
V
µA
µs
µs
pA
SHDN Pin to 0V
–20
+
SHDN Pin Steps from 0V to V
+
Shutdown Recovery Delay
SHDN Pin Steps from V to 0V
100
5
Inverting Input Bias Current, Each Biquad
The
●
denotes specifications that apply over the full operating
Note 2: f change from ±5V to ±2.375 supplies is –0.15% typical,
O
f temperature coefficient, –40°C to 85°C, is 25ppm/°C typical.
O
temperature range.
Note 1: Absolute Maximum Ratings are those values beyond which the life
of a device may be impaired.
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TYPICAL PERFOR A CE CHARACTERISTICS
fO Error vs Nominal fO (VS = ±5V)
Q Error vs Nominal fO (VS = ±5V)
fO Error vs Nominal fO (VS = ±2.5V)
35
30
25
20
15
10
5
1.50
1.25
1.50
1.25
T
T
= 70°C
= 25°C
A
A
1.00
1.00
R
= R
IN
Q
Q = 5
Q = 5
0.75
0.75
Q = 2.5
Q = 10
0.50
0.50
Q = 2.5
0.25
0.25
Q = 5
0
0
–0.25
–0.50
–0.75
–1.00
–1.25
–1.50
–0.25
–0.50
–0.75
–1.00
–1.25
–1.50
Q = 2.5
Q = 1
Q = 1
0
Q = 1
–5
50 60 70 80 90 100 110 120 130 140 150
50 60 70 80 90 100 110 120 130 140 150
50 60 70 80 90 100 110 120 130 140 150
NOMINAL f (kHz)
O
NOMINAL f (kHz)
O
NOMINAL f (kHz)
O
1562 G03
1562 G01
1562 G02
3
LTC1562
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TYPICAL PERFOR A CE CHARACTERISTICS
Peak BP Gain vs Nominal fO
(VS = ±5V) (Figure 3, V1 Output)
Peak BP Gain vs Nominal fO
(VS = ±2.5V) (Figure 3, V1 Output)
Q Error vs Nominal fO (VS = ±2.5V)
35
30
25
20
15
10
5
3.0
2.5
2.0
1.5
1.0
0.5
0
3.0
2.5
2.0
1.5
1.0
0.5
0
T
T
Q
= 70°C
= 25°C
T
T
Q
= 70°C
= 25°C
T
T
Q
= 70°C
= 25°C
A
A
A
A
A
A
Q = 10
Q = 10
R
= R
R
= R
IN
R
= R
IN
IN
Q = 10
Q = 5
Q = 5
Q = 5
Q = 2.5
Q = 1
Q = 2.5
Q = 2.5
Q = 1
Q = 1
0
–5
–0.5
–0.5
50 60 70 80 90 100 110 120 130 140 150
50 60 70 80 90 100 110
120 130
140 150
120
130 140 150
50 60 70 80 90 100 110
NOMINAL f (kHz)
O
NOMINAL f (kHz)
NOMINAL f (kHz)
O
O
1562 G04
1562 G5
1562 G6
LP Noise vs Nominal fO
(VS = ±5V, 25°C) (Figure 3,
V2 Output) (RIN = R2)
BP Noise vs Nominal fO
(VS = ±5V, 25°C) (Figure 3,
V1 Output) (RIN = RQ)
Distortion vs External Load
Resistance (VS = ±5V, 25°C)
(Figure 8)
60
55
50
45
40
35
30
25
20
15
10
60
55
50
45
40
35
30
25
20
15
10
0
2nd ORDER LOWPASS
–10
f
= 100kHz
O
Q = 0.7
–20
–30
–40
OUTPUT LEVEL 1V
±5V SUPPLIES
(2.83V
)
RMS
P-P
Q = 5
Q = 5
–50
–60
Q = 2.5
Q = 1
Q = 2.5
Q = 1
–70
–80
f
= 50kHz
= 20kHz
IN
–90
f
IN
–100
60
100
120 130
60
70 80 90
110
140
70 80 90
100
110 140
120 130
10k
2k
EXTERNAL LOAD RESISTANCE (Ω)
1k
5k
NOMINAL f (kHz)
O
NOMINAL f (kHz)
O
1562 G07
1562 G08
1562 G09
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PIN FUNCTIONS
Power Supply Pins: The V+ and V– pins should be
bypassed with 0.1µF capacitors to an adequate analog
ground or ground plane. These capacitors should be
connected as closely as possible to the supply pins. In the
20-lead SSOP package, the additional pins 4, 7, 14 and 17
are internally connected to V– (Pin 16) and should also be
tied to the same point as Pin 16 for best shielding. Low
noise linear supplies are recommended. Switching sup-
plies are not recommended as they will lower the filter
dynamic range.
Analog Ground (AGND): The AGND pin is the midpoint of
an internal resistive voltage divider, developing a potential
halfway between the V+ and V– pins, with an equivalent
series resistance nominally 7kΩ. This serves as an inter-
nal ground reference. Filter performance will reflect the
quality of the analog signal ground and an analog ground
plane surrounding the package is recommended. The
analog ground plane should be connected to any digital
ground at a single point. For dual supply operation, the
AGND pin should be connected to the ground plane
4
LTC1562
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PIN FUNCTIONS
(Figure 1). For single supply operation, the AGND pin
should be bypassed to the ground plane with at least a
0.1µF capacitor (at least 1µF for best AC performance)
(Figure 2).
Shutdown (SHDN): When the SHDN input goes high or is
open-circuited, the LTC1562 enters a “zero-power” shut-
down state and only junction leakage currents flow. The
AGND pin and the amplifier outputs (see Figure 3) assume
a high impedance state and the amplifiers effectively
disappear from the circuit. (If an input signal is applied to
a complete filter circuit while the LTC1562 is in shutdown,
some signal will normally flow to the output through
passive components around the inactive op amps.)
ANALOG
GROUND
PLANE
1
2
20
19
18
17
16
15
14
13
12
11
3
–
V
4
0.1µF
A small pull-up current source at the SHDN input defaults
the LTC1562 to the shutdown state if the SHDN pin is left
floating. Therefore, the user must connect the SHDN pin
to a logic “low” (0V for ±5V supplies, V– for 5V total
supply) for normal operation of the LTC1562. (This con-
vention permits true “zero-power” shutdown since not
even the driving logic must deliver current while the part
is in shutdown.) With a single supply voltage, use V– for
logic “low”—do not connect SHDN to the AGND pin.
5
+
V
LTC1562
6
0.1µF
7
8
9
10
SINGLE-POINT
SYSTEM GROUND
DIGITAL
GROUND PLANE
(IF ANY)
1/4 LTC1562
*R1 AND C ARE PRECISION
INTERNAL COMPONENTS
1562 F01
1
sR1C*
C
Figure 1. Dual Supply Ground Plane Connection
(Including Substrate Pins 4, 7, 14, 17)
–
+
ANALOG
GROUND
PLANE
1
2
20
19
18
17
16
15
14
13
12
11
3
4
V2
INV
V1
R
Q
R2
5
+
1562 F01
V
LTC1562
6
0.1µF
Z
IN
7
1µF
+
8
V
IN
–
9
+
V /2
REFERENCE
10
IN EACH CASE,
RESPONSE RESPONSE
AT V1 AT V2
Z
TYPE
10kΩ
R2
IN
f
= (100kHz)
O
(
)
R
C
BANDPASS LOWPASS
HIGHPASS BANDPASS
RQ 100kHz
SINGLE-POINT
SYSTEM GROUND
Q =
R2
f
DIGITAL
O
GROUND PLANE
(IF ANY)
Figure 3. Equivalent Circuit of a Single 2nd Order Section
(Inside Dashed Line) Shown in Typical Connection. Form of ZIN
Determines Response Types at the Two Outputs (See Table)
1562 F01
Figure 2. Single Supply Ground Plane Connection
(Including Substrate Pins 4, 7, 14, 17)
5
LTC1562
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PIN FUNCTIONS
INVA,INVB,INVC,INVD:EachoftheINVpinsisavirtual-
ground summing point for the corresponding 2nd order
section. For each section, external components ZIN, R2,
RQ connect to the INV pin as shown in Figure 3 and
described further in the Applications Information. Note
that the INV pins are sensitive internal nodes of the filter
and will readily receive any unintended signals that are
capacitively coupled into them. Capacitance to the INV
nodes will also affect the frequency response of the filter
sections. For these reasons, printed circuit connections to
the INV pins must be kept as short as possible, less than
one inch (2.5cm) total and surrounded by a ground plane.
orderfiltersection(seeFigure3andApplicationsInforma-
tion). Each output is designed to drive a nominal net load
of 5kΩ and 30pF, which includes the loading due to the
external RQ. Distortion performance improves when the
outputs are loaded as lightly as possible. Some earlier
literature refers to these outputs as “BP” rather than V1.
V2 A, V2 B, V2 C, V2 D: Output Pins. Provide a lowpass,
bandpass or other response depending on external cir-
cuitry (see Applications Information section). Each V2 pin
also connects to the R2 resistor of the corresponding 2nd
orderfiltersection(seeFigure3andApplicationsInforma-
tion). Each output is designed to drive a nominal net load
of 5kΩ and 30pF, which includes the loading due to the
external R2. Distortion performance improves when the
outputs are loaded as lightly as possible. Some earlier
literature refers to these outputs as “LP” rather than V2.
V1 A, V1 B, V1 C, V1 D: Output Pins. Provide a bandpass,
highpass or other response depending on external cir-
cuitry (see Applications Information section). Each V1 pin
also connects to the RQ resistor of the corresponding 2nd
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BLOCK DIAGRA
Overall Block Diagram Showing Four 3-Terminal 2nd Order Sections
INV
V1
V2
INV
V1
V2
A
B
C
C
–
+
–
+
+
V
∫
∫
+
–
V
V
SHUTDOWN
SWITCH
R
R
2ND ORDER SECTIONS
C
D
SHUTDOWN
SWITCH
AGND
SHDN
+
–
+
–
V
∫
∫
–
C
C
1562 BD
INV
V1
V2
INV
V1
V2
6
LTC1562
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APPLICATIONS INFORMATION
Functional Description
Setting fO and Q
Each of the four 2nd order sections in the LTC1562 can be
programmed for a standard filter function (lowpass,
bandpass or highpass) when configured as in Figure 3
with a resistor or capacitor for ZIN. These transfer func-
tions all have the same denominator, a complex pole pair
with center frequency ωO = 2πfO and quality parameter Q.
(The numerators depend on the response type as de-
scribed below.) External resistors R2 and RQ set fO and Q
as follows:
The LTC1562 contains four matched, 2nd order, 3-termi-
nal universal continuous-time filter blocks, each with a
virtual-ground input node (INV) and two rail-to-rail out-
puts (V1, V2). In the most basic applications, one such
block and three external resistors provide 2nd order
lowpass and bandpass responses simultaneously (Figure
3, with a resistor for ZIN). The three external resistors set
standard 2nd order filter parameters fO, Q and gain. A
combination of internal precision components and exter-
nal resistor R2 sets the center frequency fO of each 2nd
order block. The LTC1562 is trimmed at manufacture so
that fO will be 100kHz ±0.5% if the external resistor R2 is
exactly 10k.
1
10kΩ
R2
fO =
Q =
=
100kHz
(
)
2πC (R1)R2
However, lowpass/bandpass filtering is only one specific
applicationforthe2ndorderbuildingblocksintheLTC1562.
HighpassresponseresultsiftheexternalimpedanceZIN in
Figure 3 becomes a capacitor CIN (whose value sets only
gain, not critical frequencies) as described below.
Responses with zeroes are available through other con-
nections(see Notches and Elliptic Responses). Moreover,
the virtual-ground input gives each 2nd order section the
built-in capability for analog operations such as gain
(preamplification), summing and weighting of multiple
inputs,handlinginputvoltagesbeyondthepowersupplies
or accepting current or charge signals directly. These
OperationalFilterTM frequency-selectivebuildingblocks
are nearly as versatile as operational amplifiers.
RQ
RQ
(10kΩ)R2
RQ 100kHz
=
=
R2
fO
(R1)R2
R1 = 10k and C = 159pF are internal to the LTC1562 while
R2 and RQ are external.
A typical design procedure proceeds from the desired fO
and Q as follows, using finite-tolerance fixed resistors.
First find the ideal R2 value for the desired fO:
2
100kHz
R2 Ideal =
10kΩ
(
(
)
)
fO
Then select a practical R2 value from the available finite-
tolerance resistors. Use the actual R2 value to find the
desired RQ, which also will be approximated with finite
tolerance:
The user who is not copying exactly one of the Typical
Applications schematics shown later in this data sheet is
urged to read carefully the next few sections through at
least Signal Swings, for orientation about the LTC1562,
before attempting to design custom application circuits.
Also available free from LTC, and recommended for de-
signing custom filters, is the general-purpose analog filter
design software FilterCADTM for Windows®. This software
includes tools for finding the necessary f0, Q and gain
parameters to meet target filter specifications such as
frequency response.
RQ = Q (10kΩ)R2
The fO range is approximately 10kHz to 150kHz, limited
mainly by the magnitudes of the external resistors
required. As shown above, R2 varies with the inverse
square of fO. This relationship desensitizes fO to R2’s
Operational Filter and FilterCAD are trademarks of Linear Technology Corporation.
Windows is a registered trademark of Microsoft Corporation.
7
LTC1562
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APPLICATIONS INFORMATION
Basic Lowpass
tolerance (by a factor of 2 incrementally), but it also
implies that R2 has a wider range than fO. (RQ and RIN also
tend to scale with R2.) At high fO these resistors fall below
5k,heavilyloadingtheoutputsoftheLTC1562andleading
to increased THD and other effects. At the other extreme,
a lower fO limit of 10kHz reflects an arbitrary upper
resistor limit of 1MΩ. The LTC1562’s MOS input circuitry
can accommodate higher resistor values than this, but
junction leakage current from the input protection cir-
cuitry may cause DC errors.
When ZIN of Figure 3 is a resistor of value RIN, a standard
2nd orderlowpass transferfunctionresultsfrom VIN toV2
(Figure 5):
V2(s)
–HLωO2
= HLP(s) =
s2 + ω /Q s + ω2
V (s)
IN
(
)
O
O
The DC gain magnitude is HL = R2/RIN. (Note that the
transfer function includes a sign inversion.) Parameters
ωO (=2πfO)andQaresetbyR2andRQ asabove. Fora2nd
orderlowpassresponsethegainmagnitudebecomesQHL
The 2nd order transfer functions HLP(s), HBP(s) and
HHP(s) (below) are all inverting so that, for example, at DC
the lowpass gain is –HL. If two such sections are cas-
caded,thesephaseinversionscancel.Thus,thefilterinthe
application schematic on the first page of this data sheet
is a dual DC preserving, noninverting, rail-to-rail lowpass
filter, approximating two “straight wires with frequency
selectivity.”
R
IN
V
IN
R
R2
Q
V
OUT
INV
V1
2nd ORDER
1/4 LTC1562
V2
Figure 4 shows further details of 2nd order lowpass,
bandpass and highpass responses. Configurations to
obtain these responses appear in the next three sections.
1562 F05
Figure 5. Basic Lowpass Configuration
BANDPASS RESPONSE
LOWPASS RESPONSE
HIGHPASS RESPONSE
H
H
H
H
H
B
P
L
L
P
H
0.707 H
0.707 H
0.707 H
H
B
f
f
f
f
P
f
C
f
f
P
L
O
H
C
f (LOG SCALE)
f (LOG SCALE)
f (LOG SCALE)
–1
fO
H – fL
2
Q =
;fO
= fLfH
2
1
2Q2
1
2Q2
f
fC = fO 1–
+
1–
+ 1
1
2Q2
1
2Q2
f
C = fO
1–
+
1–
+ 1
2
–1
2Q
1
1
2Q2
fL = fO
+
+1
+1
fP = fO 1–
2Q
–1
1
2Q2
fP = fO 1–
2
1
1
f
H = fO
+
1
HP = HL
2Q
2Q
1
1
4Q2
1
1–
H
P = HH
Q
1
Q
1
1–
4Q2
Figure 4. Characteristics of Standard 2nd Order Filter Responses
8
LTC1562
U
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APPLICATIONS INFORMATION
at frequency fO, and for Q > 0.707, a gain peak occurs at
Parameters ωO = 2πfO and Q are set by R2 and RQ as
above. The highpass gain parameter is HH = CIN/159pF.
For a 2nd order highpass response the gain magnitude at
frequency fO is QHH, and approaches HH at high frequen-
cies (f >> fO). For Q > 0.707, a gain peak occurs at a
frequency above fO as shown in Figure 4. The transfer
function includes a sign inversion.
a frequency below fO, as shown in Figure 4.
Basic Bandpass
Therearetwodifferentwaystoobtainabandpassfunction
in Figure 3, both of which give the following transfer
function form:
C
–H ω /Q s
IN
(
)
B
O
HBP(s) =
V
IN
s2 + ω /Q s + ω2
R
Q
R2
(
)
O
O
V
OUT
INV
V1
2nd ORDER
1/4 LTC1562
V2
ωO = 2πfO and Q are set by R2 and RQ as described previ-
ously in Setting fO and Q. When ZIN is a resistor of value
RIN, a bandpass response results at the V1 output (Figure
6a) with a gain parameter HB = RQ/RIN. Alternatively, a
capacitor of value CIN gives a bandpass response at the V2
output (Figure 6b), with the same HBP(s) expression, and
the gain parameter now HB = (RQ/10kΩ)(CIN/159pF). This
transferfunctionhasagainmagnitudeofHB (itspeakvalue)
whenthefrequencyequalsfO andhasaphaseshiftof180°
at that frequency. Q measures the sharpness of the peak
(theratiooffO to–3dBbandwidth)ina2ndorderbandpass
function, as illustrated in Figure 4.
1562 F07
Figure 7. Basic Highpass Configuration
Signal Swings
The V1 and V2 outputs are capable of swinging to within
roughly 100mV of each power supply rail. As with any
analog filter, the signal swings in each 2nd order section
must be scaled so that no output overloads (saturates),
even if it is not used as a signal output. (Filter literature
often calls this the “dynamics” issue.) When an unused
output has a larger swing than the output of interest, the
section’s gain or input amplitude must be scaled down to
avoid overdriving the unused output. The LTC1562 can
still be used with high performance in such situations as
long as this constraint is followed.
C
IN
R
IN
V
IN
V
IN
R
Q
R2
R
Q
R2
V
OUT
V
OUT
INV
V1
2nd ORDER
1/4 LTC1562
V2
INV
V1
2nd ORDER
1/4 LTC1562
V2
For an LTC1562 section as in Figure 3, the magnitudes of
the two outputs V2 and V1, at a frequency ω = 2πf, have
the ratio,
1562 F06
(a) Resistive Input
(b) Capacitive Input
Figure 6. Basic Bandpass Configurations
| V2(jω)| (100kHz)
=
| V1(jω)|
f
Basic Highpass
regardless of the details of ZIN. Therefore, an input fre-
quency above or below 100kHz produces larger output
amplitude at V1 or V2, respectively. This relationship can
guide the choice of filter design for maximum dynamic
range in situations (such as bandpass responses) where
there is more than one way to achieve the desired fre-
quency response with an LTC1562 section.
When ZIN of Figure 3 is a capacitor of value CIN, a highpass
response appears at the V1 output (Figure 7).
V1(s)
–HHs2
s2 + ω /Q s + ω2
= HHP(s) =
V (s)
IN
(
)
O
O
9
LTC1562
U
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APPLICATIONS INFORMATION
Because 2nd order sections with Q ≥ 1 have response
peaks near fO, the gain ratio above implies some rules of
thumb:
level inputs require further dynamic range, reducing the
valueofZIN booststhesignalgainwhilereducingtheinput
referred noise. This feature can increase the SNR for low
level signals. Varying or switching ZIN is also an efficient
waytoeffectautomaticgaincontrol(AGC). Fromasystem
viewpoint, this technique boosts the ratio of maximum
signal to minimum noise, for a typical 2nd order lowpass
response (Q = 1, fO = 100kHz), to 118dB.
fO < 100kHz V2 tends to have the larger swing
fO > 100kHz V1 tends to have the larger swing.
The following situations are convenient because the
relative swing issue does not arise. The unused output’s
swing is naturally the smaller of the two in these cases:
Input Voltages Beyond the Power Supplies
Lowpass response (resistor input, V2 output, Figure 5)
with fO < 100kHz
Bandpass response (capacitor input, V2 output, Figure
6b) with fO < 100kHz
Bandpass response (resistor input, V1 output, Figure
6a) with fO > 100kHz
Highpass response (capacitor input, V1 output, Figure
7) with fO > 100kHz
Properly used, the LTC1562 can accommodate input
voltage excursions well beyond its supply voltage. This
requires care in design but can be useful, for example,
whenlargeout-of-bandinterferenceistoberemovedfrom
a smaller desired signal. The flexibility for different input
voltages arises because the INV inputs are at virtual
ground potential, like the inverting input of an op amp with
negative feedback. The LTC1562 fundamentally responds
to input current and the external voltage VIN appears only
across the external impedance ZIN in Figure 3.
The LTC1562-2, a higher frequency derivative of the
LTC1562, has a design center fO of 200kHz compared to
100kHz in the LTC1562. The rules summarized above
apply to the LTC1562-2 but with 200kHz replacing the
100kHz limits. Thus, an LTC1562-2 lowpass filter section
with fO below 200kHz automatically satisfies the desirable
condition of the unused output carrying the smaller signal
swing.
To accept beyond-the-supply input voltages, it is impor-
tant to keep the LTC1562 powered on, not in shutdown
mode, and to avoid saturating the V1 or V2 output of the
2nd order section that receives the input. If any of these
conditions is violated, the INV input will depart from a
virtual ground, leading to an overload condition whose
recovery timing depends on circuit details. In the event
that this overload drives the INV input beyond the supply
voltages, the LTC1562 could be damaged.
R
IN
10k
V
IN
R
R2
10k
Q
6.98k
The most subtle part of preventing overload is to consider
the possible input signals or spectra and take care that
none of them can drive either V1 or V2 to the supply limits.
Note that neither output can be allowed to saturate, even
if it is not used as the signal output. If necessary the
passband gain can be reduced (by increasing the imped-
ance of ZIN in Figure 3) to reduce output swings.
V
OUT
R
L
INV
V1
2nd ORDER
1/4 LTC1562
V2
C
L
(EXTERNAL
30pF
LOAD RESISTANCE)
1562 F08
Figure 8. 100kHz, Q = 0.7 Lowpass Circuit for
Distortion vs Loading Test
The final issue to be addressed with beyond-the-supply
inputs is current and voltage limits. Current entering the
virtual ground INV input flows eventually through the
output circuitry that drives V1 and V2. The input current
magnitude ( VIN / ZIN in Figure 3) should be limited by
design to less than 1mA for good distortion performance.
Ontheotherhand,theinputvoltageVIN appearsacrossthe
Low Level or Wide Range Input Signals
The LTC1562 contains a built-in capability for low noise
amplification of low level signals. The ZIN impedance in
each2ndordersectioncontrolstheblock’sgain. Whenset
for unity passband gain, a 2nd order section can deliver an
outputsignalmorethan100dBabovethenoiselevel.Iflow
10
LTC1562
U
W U U
APPLICATIONS INFORMATION
external component ZIN, usually a resistor or capacitor.
This component must of course be rated to sustain the
magnitude of voltage imposed on it.
ApracticallimitationofthistechniqueisthattheCT capaci-
torvaluesthattendtoberequired(hundredsorthousands
of pF) can destabilize the op amp in Figure 3 if RINB is too
small,leadingtoACerrorssuchasQenhancement.Forthis
reason, when RINA and RINB are unequal, preferably the
larger of the two should be placed in the RINB position.
Lowpass “T” Input Circuit
The virtual ground INV input in the Operational Filter block
provides a means for adding an “extra” lowpass pole to
any resistor-input application (such as the basic lowpass,
Figure 5, or bandpass, Figure 6a). The resistor that would
otherwise form ZIN is split into two parts and a capacitor
to ground added, forming an R-C-R “T” network (Figure
9). This adds an extra, independent real pole at a fre-
quency:
Highpass “T” Input Circuit
A method similar to the preceding technique adds an
“extra” highpass pole to any capacitor-input application
(such as the bandpass of Figure 6b or the highpass of
Figure7).ThismethodsplitstheinputcapacitanceCIN into
twoseriespartsCINA andCINB,witharesistorRT toground
between them (Figure 10). This adds an extra 1st order
highpass corner with a zero at DC and a pole at the
frequency:
1
fP =
2πRPCT
where CT is the new external capacitor and RP is the
parallel combination of the two input resistors RINA and
RINB. This pair of resistors must normally have a pre-
scribed series total value RIN to set the filter’s gain as
described above. The parallel value RP can however be set
arbitrarily (to RIN/4 or less) which allows choosing a
convenient standard capacitor value for CT and fine tuning
the new pole with RP.
1
fP =
2πRTCP
where CP = CINA + CINB is the parallel combination of the
two capacitors. At the same time, the total series capaci-
tance CIN will control the filter’s gain parameter (HH in
Basic Highpass). For a given series value CIN, the parallel
value CP can still be set arbitrarily (to 4CIN or greater).
C
INA
C
INB
R
R
INB
INA
V
V
IN
IN
C
R
R2
R
T
R
R2
T
Q
Q
INV
V1
2nd ORDER
1/4 LTC1562
V2
INV
V1
2nd ORDER
1/4 LTC1562
V2
1562 F09
1562 F10
Figure 9. Lowpass “T” Input Circuit
Figure 10. Highpass “T” Input Circuit
The procedure therefore is to begin with the target extra
pole frequency fP. Determine the series value RIN from the
gain requirement. Select a capacitor value CT such that RP
= 1/(2πfPCT) is no greater than RIN/4, and then choose
RINA and RINB that will simultaneously have the parallel
value RP and the series value RIN. Such RINA and RINB can
be found directly from the expression:
Theprocedurethenistobeginwiththetargetcorner(pole)
frequency fP. Determine the series value CIN from the gain
requirement(forexample,CIN =HH(159pF)forahighpass).
Select a resistor value RT such that CP = 1/(2πRTfP) is at
least4CIN,andselectCINAandCINBthatwillsimultaneously
have the parallel value CP and the series value CIN. Such
CINA and CINB can be found directly from the expression:
RIN2 – 4R R
1
2
1
2
2
1
2
1
2
RIN ±
(
)
IN
P
CP ±
C – 4C C
P
IN P
11
LTC1562
U
W U U
APPLICATIONS INFORMATION
–3dB frequencies fL and fH are widely separated from this
peak.
This procedure can be iterated, adjusting the value of RT,
to find convenient values for CINA and CINB since resistor
values are generally available in finer increments than
capacitor values.
The LTC1562’s fO is trimmed in production to give an
accurate 180° phase shift in the configuration of Figure
6a with resistor values setting f0 = 100kHz and Q = 1.
Table 1 below shows typical differences between fO
values measured via the bandpass 180° criterion and fO
values measured using the two other methods listed
above (Figure 6a, RIN = RQ).
Different “fO” Measures
Standard 2nd order filter algebra, as in Figure 4 and the
various transfer-function expressions in this data sheet,
uses a center frequency parameter fO (or ωO, which is
2πfO). fO can also be measured in practical ways, includ-
ing:
Table 1
f
Q = 1
BP-PEAK f
Q = 1
√ƒ ƒ f
Q = 5
BP-PEAK f
Q = 5
√ƒ ƒ f
L H O
O
• The frequency where a bandpass response has 180°
phase shift
(BP 180
°)
O
L H
O
O
60kHz
+0.3%
+0.6%
+0.8%
+0.3%
+0.6%
+0.8%
+0.05%
+0.1%
+0.05%
+0.1%
100kHz
140kHz
• The frequency where a bandpass response has peak
gain
+0.15%
+0.15%
• The geometric mean of the –3.01dB gain frequencies in
a bandpass (√ƒLƒH in Figure 4)
LTC1562 Demo Board
The LTC1562 demo board is assembled with an LTC1562
or LTC1562A in a 20-pin SSOP package and power supply
decoupling capacitors. Jumpers on the board configure
theLTC1562fordualorsinglesupplyoperationandpower
shutdown. Pads for surface mount resistors and capaci-
tors are provided to build application-specific filters. Also
provided are terminals for inputs, outputs and power
supplies.
An ideal mathematical 2nd order response yields exactly
the same frequency by these three measures. However,
real 2nd order filters with finite-bandwidth circuitry show
small differences between the practical fO measures,
which may be important in critical applications. The issue
is chiefly of concern in high-Q bandpass applications
where, as the data below illustrate, the different f0 mea-
surements tend to converge anyway for the LTC1562. At
low Q the bandpass peak is not sharply defined and the
12
LTC1562
U
TYPICAL APPLICATIONS (Basic)
Quad 3rd Order Butterworth Lowpass Filter, Gain = –1
Amplitude Response
V
OUT1
V
OUT2
10
0
R
R
IN1B
R
R
IN2A
f
= 100kHz
IN1A
IN2B
–3dB
20
19
18
16
15
13
12
11
1
2
3
5
6
8
9
V
INV B
V1 B
INV C
V1 C
V2 C
V
IN2
IN1
R
R
Q1
Q2
C
C
IN2
IN1
R21
R22
–10
–20
–30
–40
–50
–60
V2 B
+
–
LTC1562
–5V
5V
V
V
0.1µF
0.1µF
SHDN
V2 A
V1 A
INV A
AGND
V2 D
R23
R24
V1 D
R
R
R
IN4B
R
C
IN3A
IN3B
IN4A
R
Q3
R
Q4
10
V
INV D
V
IN4
IN3
C
IN3
IN4
V
V
OUT4
OUT3
10k
100k
FREQUENCY (Hz)
1M
1562 TA05a
SCHEMATIC INCLUDES PIN NUMBERS FOR 20-PIN PACKAGE.
–
PINS 4, 7, 14, 17 (NOT SHOWN) ALSO CONNECT TO V
1562 TA05b
Quad 3rd Order
Butterworth
Lowpass Filters
f
f
f
f
f
f
f
–3dB
140kHz
–3dB
–3dB
–3dB
–3dB
–3dB
–3dB
20kHz
40kHz
60kHz
80kHz
100kHz
120kHz
C
220pF
44.2k
205k
249k
249k
1000pF
4.32k
57.6k
61.9k
61.9k
1000pF
3.16k
24.3k
27.4k
27.4k
1000pF
2.43k
13.0k
15.4k
15.4k
1000pF
1.96k
8.06k
10.0k
10.0k
1000pF
1.87k
5.11k
6.98k
6.98k
1000pF
1.69k
3.4k
5.11k
5.11k
IN
R
R
R
INA
INB
Q
R2
All four sections have identical R , R and C values. All resistor values are ±1%
INA INB
IN
13
LTC1562
U
TYPICAL APPLICATIONS (Basic)
Dual 4th Order Lowpass Filters
Amplitude Response
10
0
R
IN2
BUTTERWORTH
= 100kHz
R
f
IN1
–3dB
20
19
18
16
15
13
12
11
1
2
INV C
V1 C
V
IN2
INV B
V1 B
R
R
Q2
–10
–20
–30
–40
–50
–60
–70
–80
Q1
R22
R21
3
V2 C
–
V2 B
+
V
OUT2
5
5V
LTC1562
–5V
V
V
0.1µF
0.1µF
6
AGND
V2 D
SHDN
V2 A
V1 A
INV A
V
OUT1
8
R23
R24
9
V1 D
R
IN3
R
R
Q3
10
Q4
INV D
V
IN1
1562 TA03a
R
IN4
10k
100k
FREQUENCY (Hz)
1M
SCHEMATIC INCLUDES PIN NUMBERS FOR 20-PIN PACKAGE.
PINS 4, 7, 14, 17 (NOT SHOWN) ALSO CONNECT TO V
1562 TA03b
–
Quick Design Formulas for Some Popular Response Types:
Butterworth
Chebyshev
Bessel
(Maximally Flat Passband)
(Equiripple Passband)
(Good Transient Response)
for f 10kHz to 140kHz
for f 20kHz to 120kHz
for f 10kHz to 70kHz
C
C
C
2
100kHz
2
100kHz
2
100kHz
R21, R23, R , R
=
=
=
=
10k
14.24k
3.951k
IN1 IN3
ƒ
ƒ
ƒ
C
C
C
100kHz
5.412k
100kHz
7.26k
100kHz
5.066k
R
, R
Q1 Q3
ƒ
ƒ
ƒ
C
C
C
2
100kHz
2
100kHz
2
100kHz
R22, R24, R , R
10k
7.097k
4.966k
IN2 IN4
ƒ
ƒ
ƒ
C
C
C
100kHz
13.07k
100kHz
17.53k
100kHz
3.679k
R
, R
Q2 Q4
ƒ
ƒ
ƒ
C
C
C
Notes: f is the cutoff frequency: For Butterworth and Bessel, response is 3dB down at f . For Chebyshev filters with
C
C
±0.1dB passband ripple up to 0.95 f , use LTC1562 “A” grade.
C
2
Example: Butterworth response, f = 50kHz. from the formulas above, R21 = R23 = R = R = 10k(100kHz/50kHz)
C
IN1
IN3
2
= 40k. R = R = 5.412k(100kHz/50kHz) = 10.82k. R22 = R24 = R = R = 10k(100kHz/50kHz) = 40k.
Q1
Q4
Q3
IN2
IN4
1562 TA03 TABLE
R
= R = 13.07k(100kHz/50kHz) = 26.14k. Use nearest 1% values.
Q2
14
LTC1562
U
TYPICAL APPLICATIONS (Basic)
8th Order Lowpass Filters
Amplitude Response
10
0
R
IN2
CHEBYSHEV
= 100kHz
R
f
IN1
C
20
19
18
16
15
13
12
11
1
2
INV C
V1 C
V
INV B
V1 B
IN
–10
–20
–30
–40
–50
–60
–70
–80
–90
R
R
Q1
Q2
R21
R22
3
V2 C
–
V2 B
+
5
5V
LTC1562
–5V
V
V
0.1µF
0.1µF
6
AGND
V2 D
SHDN
V2 A
V1 A
INV A
R23
R24
8
9
V1 D
R
R
10
Q3
Q4
INV D
R
IN4
10k
100k
FREQUENCY (Hz)
500k
V
OUT
R
IN3
1562 TA04b
1562 TA04a
SCHEMATIC INCLUDES PIN NUMBERS FOR 20-PIN PACKAGE.
–
PINS 4, 7, 14, 17 (NOT SHOWN) ALSO CONNECT TO V
Quick Design Formulas for Some Popular Response Types:
Butterworth
Chebyshev
Bessel
(Maximally Flat Passband)
(Equiripple Passband)
(Good Transient Response)
for f 10kHz to 140kHz
for f 20kHz to 120kHz
for f 10kHz to 70kHz
C
C
C
2
2
2
100kHz
100kHz
100kHz
R21 = R = 10k
IN1
R21 = 7.51k
, R = 2.2R21*
IN1
R21 = R = 2.61k
IN1
ƒ
ƒ
ƒ
C
C
C
100kHz
100kHz
100kHz
C
100kHz
R
Q1
= 6.01k
R
= 119.3k
R
= 3.63k
Q1
Q1
ƒ
ƒ
ƒ + 560kHz
ƒ
C
C
C
2
2
2
100kHz
100kHz
100kHz
R22 = R = 10k
IN2
R22 = R = 14.99k
IN2
R22 = R = 2.07k
IN2
ƒ
ƒ
ƒ
C
C
C
100kHz
100kHz
100kHz
100kHz
R
= 9k
R
= 279.9k
R
= 5.58k
Q2
Q2
Q2
ƒ
ƒ
ƒ + 2440kHz
ƒ
C
C
C
C
2
2
2
100kHz
100kHz
100kHz
R23 = R = 10k
IN3
R23 = R = 7.15k
IN3
R23 = R = 2.96k
IN3
ƒ
ƒ
ƒ
C
C
C
100kHz
100kHz
100kHz
C
100kHz
R
= 5.1k
R
= 118.1k
R
= 3.05k
Q3
Q3
Q3
ƒ
ƒ
ƒ + 530kHz
ƒ
C
C
C
2
2
2
100kHz
100kHz
R24*
2.2
100kHz
R24 = R = 10k
IN4
R24 = 26.7k
, R
IN4
=
R24 = R = 3.14k
IN4
ƒ
ƒ
ƒ
C
C
C
100kHz
100kHz
100kHz
R
= 25.63k
R
= 8.75k
R
= 2.84k
Q4
Q4
Q4
ƒ
ƒ
ƒ
C
C
C
Notes: f is the cutoff frequency: For Butterworth and Bessel, response is 3dB down at f . For Chebyshev filters with
C
C
±0.1dB passband ripple up to 0.95 f , use LTC1562 “A” grade. *The resistor values marked with an asterisk (*) in the
C
Chebyshev formulas (R21 and R24) should be rounded to the nearest standard finite-tolerance value before computing
the values dependent on them (R and R respectively).
IN1 IN4
Example: Chebyshev response, f = 100kHz. The formulas above give R21 = 7.51k, nearest standard 1% value 7.50k.
C
Using this 1% value gives R = 16.5k, already a standard 1% value. R = 18.075k, nearest 1% value 18.2k.
IN1 Q1
R22 = R = 14.99k, nearest 1% value 15k. R = 11.02k, nearest 1% value 11k. R23 = R = 7.15k, already a
IN2 Q2 IN3
standard 1% value. R = 18.75k, nearest 1% value 18.7k. R24 = 26.7k, already a standard 1% value. This gives
Q3
R
= 12.14k, nearest 1% value 12.1k. R = 8.75k, nearest 1% value 8.66k.
1562 TA04 TABLE
IN4
Q4
15
LTC1562
U
TYPICAL APPLICATIONS (Basic)
8th Order Bandpass Filter, Single 5V Supply,
Center Frequency
Amplitude Response
–3dB Bandwidth =
10
10
0
R
IN2
f
= 80kHz
CENTER
C
IN1
20
19
18
16
15
13
12
11
1
2
INV C
V1 C
V
IN
INV B
V1 B
–10
–20
–30
–40
–50
–60
–70
–80
–90
R
R
Q1
Q2
R21
R22
3
V2 C
–
V2 B
+
5
5V
LTC1562
V
V
1µF
0.1µF
6
AGND
V2 D
SHDN
V2 A
V1 A
INV A
R23
R24
8
9
V1 D
R
R
10
Q3
Q4
INV D
V
OUT
R
IN4
40 48 56 64 72 80 88 96 104 112 120
C
IN3
FREQUENCY (kHz)
1562 TA07a
1562 TA07b
SCHEMATIC INCLUDES PIN NUMBERS FOR 20-PIN PACKAGE.
–
PINS 4, 7, 14, 17 (NOT SHOWN) ALSO CONNECT TO V
Quick Design Formulas for Center Frequency f (Recommended Range 40kHz to 140kHz):
C
2
100kHz
100kHz
100kHz
C
R21 = R23 = 10.6k
R22 = R24 = 9.7k
R
R
= R = 164.6k
Q3
Q1
Q2
ƒ
ƒ
ƒ + 319kHz
C
C
2
100kHz
100kHz
100kHz
= R = 143.2k
Q4
ƒ
ƒ
ƒ + 294kHz
C
C
C
R22R
(10k)(10.6pF)
C
10k
Q1
100kHz
C
Q1 IN1
R
= R
=
IN4
C
= C = 159pF
IN3
IN2
IN1
R
ƒ + 286kHz
Notes: R , R22 and C should be rounded to the nearest standard finite-tolerance value before using these
Q1 IN1
values in the later formulas.
Example: Center frequency f of 80kHz. The formulas give R21 = R23 = 16.56k, nearest standard 1% value 16.5k.
C
R
Q1
= R = 51.56k, nearest 1% value 51.1k. R22 = R24 = 15.15k, nearest 1% value 15k. R = R = 47.86k,
Q3 Q2 Q4
nearest 1% value 47.5k. C = C = 31.11pF using 51.1k for R , nearest standard 5% capacitor value 33pF.
IN1 IN2 Q1
This and the 1% value R22 = 15k also go into the calculation for R = R = 65.20k, nearest 1% value 64.9k.
IN2 IN4
1562 TA07 TABLE
16
LTC1562
U
TYPICAL APPLICATIONS (Basic)
8th Order Bandpass Filter, Single 5V Supply,
Center Frequency
Amplitude Response
–1dB Bandwidth =
10
R
10
0
IN2
f
= 100kHz
CENTER
R
IN1
20
19
18
16
15
13
12
11
1
2
INV C
V1 C
V
INV B
V1 B
V2 B
IN
–10
–20
–30
–40
–50
–60
–70
–80
–90
R
R
Q2
Q1
R22
R21
3
V2 C
–
5
+
5V
LTC1562
V
AGND
V2 D
V1 D
INV D
V
1µF
0.1µF
6
SHDN
V2 A
R24
R23
8
9
V1 A
R
R
Q3
Q4
10
INV A
V
OUT
R
IN4
60 68 76 84 92 100 108 116 124 132 140
FREQUENCY (kHz)
R
IN3
1562 TA06a
1562 TA06b
SCHEMATIC INCLUDES PIN NUMBERS FOR 20-PIN PACKAGE.
–
PINS 4, 7, 14, 17 (NOT SHOWN) ALSO CONNECT TO V
Quick Design Formulas for a Center Frequency f (Recommended Range 50kHz to 120kHz):
C
2
ƒ + 1736kHz
100kHz
C
R21
2.56
100kHz
100kHz
C
R21 = R23 = 11.7k
R22 = R24 = 8.66k
R
R
= R
= R
=
=
R
R
= R = 215.5k
Q3
IN1
IN2
IN3
Q1
ƒ
100kHz
ƒ
ƒ + 229kHz
C
C
2
ƒ + 634kHz
R
100kHz
100kHz
100kHz
C
Q2
= R = 286.2k
Q4
Q2
IN4
ƒ
ƒ
ƒ + 351kHz
100kHz
14.36
C
C
C
Notes: R21 and R should be rounded to the nearest standard finite-tolerance value before using these values in
Q2
the later formulas. For f < 100kHz, the maximum peak-to-peak passband input level is (f /100kHz)5V. Use
C
C
LTC1562A for minimum variation of passband gain.
Example: Center frequency f of 100kHz. The formulas give R21 = R23 = 11.7k, nearest standard 1% value 11.5k.
C
This value gives R = R = 82.46k, nearest 1% value 82.5k. R = R = 65.5k, nearest 1% value 64.9k.
IN1 IN3 Q1 Q3
R22 = R24 = 8.66k, already a standard 1% value. This gives R = R = 32.4k (again already a standard 1% value).
IN2 IN4
R
= R = 63.45k, nearest 1% value 63.4k. If LTC1562A is used, resistor tolerances tighter than 1% will further
Q2
improve the passband gain accuracy.
Q4
1562 TA06 TABLE
17
LTC1562
U
TYPICAL APPLICATIONS (Basic)
8th Order Bandpass (High Frequency) Filter
Center Frequency
Amplitude Response
–3dB Bandwidth =
, Gain = 10
10
R
30
20
IN2
f
= 100kHz
CENTER
R
IN1
20
19
18
16
15
13
12
11
1
INV C
V1 C
V
IN
INV B
10
R
R
Q2
Q1
2
3
V1 B
V2 B
0
R22
R21
V2 C
–
–10
–20
–30
–40
–50
–60
–70
5
+
–
+
V
LTC1562
V
V
V
0.1µF
0.1µF
6
AGND
V2 D
SHDN
V2 A
R24
R23
8
9
V1 D
V1 A
R
Q3
R
Q4
10
INV D
INV A
R
IN4
40
60
80 100 120 140 160 180
FREQUENCY (kHz)
V
OUT
R
IN3
1562 TA08a
1562 TA08b
SCHEMATIC INCLUDES PIN NUMBERS FOR 20-PIN PACKAGE.
–
PINS 4, 7, 14, 17 (NOT SHOWN) ALSO CONNECT TO V
8th Order Bandpass Filter
f
f
f
f
f
f
f
f
CENTER
140kHz
CENTER
10
CENTER
CENTER
CENTER
CENTER
CENTER
CENTER
–3dB BW =
Side B
, Gain = 10
80kHz
90kHz
100kHz
110kHz
120kHz
130kHz
R
R
R21
4.64k
46.4k
12.4k
5.23k
52.3k
15.4k
6.34k
42.2k
10.0k
5.11k
38.3k
8.25k
5.11k
34.8k
6.98k
5.49k
32.4k
5.9k
5.62k
30.1k
5.11k
IN1
Q1
Sides A, C, D
R
R
, R , R
, R , R
Q2 Q3 Q4
46.4k
46.4k
12.4k
52.3k
52.3k
15.4k
42.2k
42.2k
10.0k
38.3k
38.3k
8.25k
34.8k
34.8k
6.98k
32.4k
32.4k
5.90k
30.1k
30.1k
5.11k
IN2 IN3 IN4
R22, R23, R24
All resistor values are ±1%
18
LTC1562
U
TYPICAL APPLICATIONS (Basic)
8th Order Wideband Bandpass Filter
fCENTER = 50kHz, –3dB BW 40kHz to 60kHz
Amplitude Response
R
10
0
IN2
69.8k
C
IN1
22pF
20
19
18
16
15
13
12
11
1
2
INV C
V1 C
V
INV B
V1 B
V2 B
IN
–10
–20
–30
–40
–50
–60
R
59k
R
48.7k
Q2
Q1
R21 56.2k
R22 34.8k
3
V2 C
–
5
+
–
+
V
LTC1562
V
AGND
V2 D
V1 D
INV D
V
V
1µF
0.1µF
6
SHDN
V2 A
8
R23 63.4k
R24 28.7k
9
V1 A
R
Q3
82.5k
R
100k
10
Q4
INV A
20
100
C
IN3
27pF
FREQUENCY (kHz)
V
OUT
1562 TA09b
C
47pF
IN4
1562 TA09a
SCHEMATIC INCLUDES PIN NUMBERS FOR 20-PIN PACKAGE.
–
PINS 4, 7, 14, 17 (NOT SHOWN) ALSO CONNECT TO V
8th Order Highpass 0.05dB Ripple Chebyshev Filter fCUTOFF = 30kHz
Amplitude Response
10
0
C
IN1
150pF
20
19
18
16
15
13
12
11
1
2
INV C
V1 C
C
IN
INV B
V1 B
V2 B
–10
–20
–30
–40
–50
–60
–70
–80
–90
R
, 22.1k
Q2
R
, 10.2k
Q1
C
IN2
R22, 66.5k 150pF
R21, 35.7k
3
V2 C
–
5
+
LTC1562
–5V
5V
V
AGND
V2 D
V1 D
INV D
V
0.1µF
0.1µF
6
SHDN
V2 A
8
R23, 107k
R24, 127k
C
C
IN3
150pF
9
IN4
V1 A
150pF
R
, 98.9k
Q4
R
, 54.9k
Q3
10
INV A
1562 TA10a
1k
10k
100k
1M
V
OUT
FREQUENCY (Hz)
SCHEMATIC INCLUDES PIN NUMBERS FOR 20-PIN PACKAGE.
–
1562 TA10b
PINS 4, 7, 14, 17 (NOT SHOWN) ALSO CONNECT TO V
TOTAL OUTPUT NOISE = 40µV
RMS
Amplitude Response
2nd Order 30kHz Highpass Cascaded with 6th Order 138kHz Lowpass
R
, 5.23k
IN2
C
IN1
150pF
20
10
20
19
18
16
15
13
12
11
1
2
INV C
V
IN
INV B
V1 B
R
, 30.1k
R
, 5.11k
Q2
Q1
0
V1 C
–10
–20
–30
–40
–50
–60
–70
–80
R22, 5.23k
R21, 110k
3
V2 C
V2 B
+
5
–
5V
–5V
LTC1562
V
V
0.1µF
0.1µF
6
AGND
V2 D
SHDN
V2 A
V1 A
INV A
8
R24, 5.23k
R23, 5.23k
9
V1 D
R
, 3.74k
Q4
R
, 14k
Q3
10
INV D
V
OUT
R
IN3
, 8.06k
R
, 3.4k
IN4
10
100
400
1562 TA11a
FREQUENCY (kHz)
1562 TA11b
SCHEMATIC INCLUDES PIN NUMBERS FOR 20-PIN PACKAGE.
–
PINS 4, 7, 14, 17 (NOT SHOWN) ALSO CONNECT TO V
ALL RESISTORS = 1% METAL FILM
19
LTC1562
U
W U U
APPLICATIONS INFORMATION
Notches and Elliptic Responses
fN. The two signals then cancel out at frequency fN. The
notch depth (the completeness of cancellation) will be
infinite to the extent that the two paths have matching
gains. Three practical circuit methods are presented here,
with different features and advantages.
The basic (essentially all-pole) LTC1562 circuit tech-
niques described so far will serve many applications.
However, the sharpest-cutoff lowpass, highpass and
bandpass filters include notches (imaginary zero pairs) in
the stopbands. A notch, or band-reject, filter has zero gain
at a frequency fN. Notches are also occasionally used by
themselves to reject a narrow band of frequencies. A
number of circuit methods will give notch responses from
anOperationalFilterblock. Eachmethodexhibitsaninput-
outputtransferfunctionthatisastandard2ndorderband-
reject response:
Examplesanddesignproceduresforpracticalfiltersusing
these techniques appear in a series of articles1 attached to
this data sheet on the Linear Technology web site
(www.linear-tech.com). Also available free is the analog
filter design software, FilterCAD for Windows, recom-
mended for designing filters not shown in the Typical
Applications schematics in this data sheet.
–HN s2 + ωN2
HBR(s) =
Elementary Feedforward Notches
A “textbook” method to get a 180° phase difference at
frequency fN for a notch is to dedicate a bandpass 2nd
order section (described earlier under Basic Bandpass),
which gives 180° phase shift at the section’s center
frequency fO (Figure 11, with CIN1 = 0), so that fN = fO. The
bandpass section of Figure 6a, at its center frequency fO,
has a phase shift of 180° and a gain magnitude of HB =
RQ/RIN. A notch results in Figure 11 if the paths summed
into virtual ground have the same gains at the 180°
frequency (then IO = 0). This requires a constraint on the
resistor values:
s2 + ω /Q s + ω2
(
)
O
O
with parameters ωN = 2πfN and HN set by component
values as described below. (ω0 = 2πf0 and Q are set for the
Operational Filter block by its R2 and RQ resistors as
described earlier in Setting f0 and Q). Characteristically,
thegainmagnitude|HBR(j2πf)|hasthevalueHN(fN /f0 )at
DC (f = 0) and HN at high frequencies (f >> fN), so in
addition to the notch, the gain changes by a factor:
2
2
2
O
ƒ
HighFrequency Gain
DC Gain
=
ƒN2
RIN2
RFF2
RQ1
RIN1
=
The common principle in the following circuit methods is
toaddasignaltoafilteredreplicaofitselfhavingequalgain
and 180° phase difference at the desired notch frequency
1Nello Sevastopoulos, et al. “How to Design High Order Filters with Stopband Notches Using the
LTC1562 Quad Operational Filter.” Attached to this data sheet, available on the LTC web site
(www.linear-tech.com).
C
IN1
R
IN1
V
IN
R
R21
Q1
I
O
R
R
R
GAIN
IN2
INV
V1
2nd ORDER
1/4 LTC1562
V2
–
+
VIRTUAL
GROUND
V
OUT
FF2
1562 F11
Figure 11. Feedforward Notch Configuration for fN ≥ fO
20
LTC1562
U
W U U
APPLICATIONS INFORMATION
Note that the depth of the notch depends on the accuracy
of this resistor ratioing. The virtual-ground summing
point in Figure 11 may be from an op amp as shown, or in
a practical cascaded filter, the INV input of another Opera-
tional Filter block. The transfer function in Figure 11 with
Feedforward Notches for fN > f0
WhenCIN1 ≠ 0inFigure11,thenotchfrequencyfN isabove
the center frequency f0 and the response has a lowpass
shape as well as a notch (Figure 13). CIN1 contributes
phase lead, which increases the notch frequency above
the center frequency of the 2nd order Operational Filter
section. The resistor constraint from the previous section
also applies here and the HBR(s) parameters become:
C
IN1 =0isa“pure”notch(fN =f0)oftheHBR(s)formabove,
and the parameters are:
ƒN = ƒO
RGAIN
RFF2
HN =
1
ƒN = ƒO
RIN1CIN1
RQ1C
RGAIN ƒ2O
1–
Because fN = f0 in this case, the gain magnitude both at DC
andathighfrequencies(f>>fN)isthesame,HN (assuming
that the op amp in Figure 11 adds no significant frequency
response). Figure 12 shows this. Such a notch is ineffi-
cientasacascadedpartofahighpass,lowpassorbandpass
filter (the most common uses for notches). Variations of
Figure 11 can add a highpass or lowpass shape to the
notch, without using more Operational Filter blocks. The
key to doing so is to decouple the notch frequency fN from
the center frequency f0 of the Operational Filter (this is
shown in Figures 13 and 15). The next two sections
summarize two variations of Figure 11 with this highpass/
lowpass shaping, and the remaining section shows a
different approach to building notches.
HN =
ƒN2
RFF2
C is the internal capacitor value in the Operational Filter (in
the LTC1562, 159pF).
TheconfigurationofFigure11ismostusefulforastopband
notch in a lowpass filter or as an upper stopband notch in
abandpassfilter, sincethetworesistorsRIN2 andRFF2 can
replace the input resistor RIN of either a lowpass section
(Figure 5) or a resistor-input bandpass section (Figure 6a)
builtfroma second Operational Filter. The configurationis
0
–20
–40
–60
–80
20
2
f
f
N
O
DC GAIN = H
N
(
)
2
0
–20
–40
–60
HIGH FREQ
GAIN = H
N
f
f
= 100kHz
= 200kHz
O
N
f
= f = 100kHz
O
N
N
Q = 1
H
= 1
DC GAIN = 0dB
Q = 1
–100
10
100
1000
10
100
FREQUENCY (kHz)
1000
FREQUENCY (kHz)
AN54 • TA18
1562 F13
Figure 12. Notch Response with fN = fO
Figure 13. Notch Response with fN > fO
21
LTC1562
U
W U U
APPLICATIONS INFORMATION
robust against tolerances in the CIN1 value when fN ap-
proaches f0 (for fN/f0 ≤ 1.4, as a rule of thumb) which is
attractive in narrow transition-band filters, because of the
relative cost of high accuracy capacitors. Further applica-
tion details appear in Part 1 of the series of articles.1
RIN2
RFF2
RQ1CIN1
R1C
=
R1 and C are the internal precision components (in the
LTC1562,10kand159pFrespectively)asdescribedabove
in Setting f0 and Q.
Feedforward Notches for fN < f0
The configuration of Figure 14 is most useful as a lower
stopband notch in a bandpass filter, because the resistors
RIN2 and RFF2 can replace the input resistor RIN of a
bandpass section made from a second Operational Filter,
as in Figure 6a. The configuration is robust against toler-
ances in the CIN1 value when fN approaches f0 (for f0/fN ≤
1.4, as a rule of thumb) which is attractive in narrow
transition-band filters, because of the relative cost of high
accuracy capacitors. Further application details appear in
Part 2 of the series of articles.1
Just as feedforward around an inverting bandpass section
yields a notch at the section’s f0 (Figure 11 with CIN1 = 0),
feedforward around an inverting lowpass section causes
a notch at zero frequency (which is to say, a highpass
response). Moreover, and this is what makes it useful,
introducing a capacitor for phase lead moves the notch
frequency up from DC, exactly as CIN1 in Figure 11 moves
the notch frequency up from the center frequency f0. In
Figure 14, the inverting lowpass output (V2) of the Opera-
tional Filter is summed, at a virtual ground, with a fed-
forward input signal. Capacitor CIN1 shifts the resulting
notch frequency, fN, up from zero, giving a low frequency
notch with a highpass shape (Figure 15). The HBR(s)
response parameters are now:
20
HIGH FREQ
GAIN = H
N
2
2
f
f
0
–20
–40
–60
N
O
DC GAIN = H
N
(
)
R1
C
R21
ƒN = ƒO 1–
RQ1 CIN1 RIN1
f
f
= 100kHz
= 50kHz
O
N
Q = 1
RGAIN
RFF2
HIGH FREQ GAIN = 0dB
HN =
10k
100k 1M
FREQUENCY (Hz)
1562 F15
The constraint required for exact cancellation of the two
paths (i.e., for infinite notch depth) becomes:
Figure 15. Notch Response with fN < f0
C
IN1
IN1
R
V
IN
R
R21
Q1
I
O
R
R
R
GAIN
IN2
INV
V1
2nd ORDER
1/4 LTC1562
V2
–
+
VIRTUAL
GROUND
V
OUT
FF2
1562 F14
Figure 14. Feedforward Notch Configuration for fN < fO
22
LTC1562
U
W U U
APPLICATIONS INFORMATION
R-C Universal Notches
RGAIN R21
DC Gain =
A different way to get 180° phase shift for a notch is to use
the built-in 90° phase difference between the two Opera-
tional Filter outputs along with a further 90° from an
external capacitor. This method achieves deep notches
independent of component matching, unlike the previous
techniques, and it is convenient for cascaded highpass as
well as lowpass and bandpass filters.
RIN1
RN
ƒ2O
ƒN2
High Frequency Gain RNCN
=
=
DC Gain
R21C
R1 and C are the internal precision components (in the
LTC1562,10kand159pFrespectively)asdescribedabove
in Setting f0 and Q.
The V2 output of an Operational Filter is a time-integrated
version of V1 (see Figure 3), and therefore lags V1 by 90°
over a wide range of frequencies. In Figure 16, a notch
responseoccurswhena2ndordersectiondrivesavirtual-
ground input through two paths, one through a capacitor
and one through a resistor. Again, the virtual ground may
come from an op amp as shown, or from another Opera-
tional Filter’s INV input. Capacitor CN adds a further 90° to
the 90° difference between V1 and V2, producing a
wideband 180° phase difference, but frequency-depen-
dent amplitude ratio, between currents IR and IC. At the
frequency where IR and IC have equal magnitude, IO
becomeszeroandanotchoccurs. Thisgivesanettransfer
function from VIN to VOUT in the form of HBR(s) as above,
with parameters:
Unlike the notch methods of Figures 11 and 14, notch
depthfromFigure16isinherent, notderivedfromcompo-
nentmatching.ErrorsintheRN orCN valuesalterthenotch
frequency, fN, rather than the degree of cancellation at fN.
Also,thenotchfrequency,fN,isindependentofthesection’s
center frequency f0, so fN can freely be equal to, higher
than or lower than f0 (Figures 12, 13 or 15, respectively)
withoutchangingtheconfiguration. Thechiefdrawbackof
Figure 16 compared to the previous methods is a very
practical one—the CN capacitor value directly scales HN
(and therefore the high frequency gain). Capacitor values
are generally not available in increments or tolerances as
fine as those of resistors, and this configuration lacks the
property of the previous two configurations that sensitiv-
ity to the capacitor value falls as fN approaches f0.
1
ƒN =
2π RNCNR1C
RGAIN CN
HN =
RIN1
C
R
IN1
V
IN
R
R21
Q1
I
I
O
R
R
C
R
GAIN
N
–
+
VIRTUAL
GROUND
INV
V1
2nd ORDER
1/4 LTC1562
V2
V
OUT
N
I
C
1562 F16
Figure 16. The R-C Universal Notch Configuration for an Operational Filter Block
23
LTC1562
U
TYPICAL APPLICATIONS (Advanced)
8th Order 50kHz Lowpass Elliptic Filter
with 100dB Stopband Attenuation
C
24pF
IN2
Amplitude Response
R
IN2
37.4k
R
IN1
48.7k
20
0
1
2
20
V
IN
INVB
V1B
V2B
INVC
R
30.1k
R
13k
Q1
Q2
19
V1C
V2C
R21 31.6k
R22 57.6k
–20
–40
–60
–80
–100
–120
3
18
16
15
13
12
11
5
+
–
5V
–5V
V
LTC1562
V
0.1µF
0.1µF
6
SHDN
V2A
AGND
V2D
8
R24 32.4k
R23 31.6k
9
V1A
V1D
R
34k
R
Q4
11.5k
32.4k
Q3
10
INVA
INVD
V
OUT
R
10
500
100
IN4
R
31.6k
18pF
IN3
FREQUENCY (kHz)
1562 TA12b
C
IN3
C
IN4
10pF
1562 TA12a
USES THREE R-C UNIVERSAL NOTCHES AT f = 133kHz, 167kHz, 222kHz.
N
SCHEMATIC INCLUDES PIN NUMBERS FOR 20-PIN PACKAGE.
PINS 4, 7, 14, 17 (NOT SHOWN) ALSO CONNECT TO V
DETAILED DESCRIPTION IN LINEAR TECHNOLOGY DESIGN NOTE 195.
–
WIDEBAND OUTPUT NOISE 60µV
RMS
8th Order 100kHz Elliptic Bandpass Filter
R
301k
FF2
R
93.1k
IN2
Amplitude Response
R
IN1
95.3k
1
2
20
19
18
16
15
13
12
11
10
0
V
INVB
V1B
V2B
INVC
V1C
V2C
IN
R
86.6k
R
84.5k
Q2
Q1
C
IN1
5.6pF
–10
–20
–30
–40
–50
–60
–70
–80
–90
R21 10.7k
R22 10k
3
5
+
–
5V
–5V
V
LTC1562
V
0.1µF
0.1µF
6
SHDN
V2A
AGND
V2D
R23 10k
8
R
71.5k
R24 9.53k
Q3
9
V1A
V1D
R
82.5k
R
294k
Q4
IN3
10
INVA
INVD
C
18pF
IN3
R
95.3k
IN4
25
100
175
V
OUT
FREQUENCY (kHz)
R
332k
FF4
1562 TA13b
1562 F13a
SCHEMATIC INCLUDES PIN NUMBERS FOR 20-PIN PACKAGE.
–
PINS 4, 7, 14, 17 (NOT SHOWN) ALSO CONNECT TO V
24
LTC1562
U
TYPICAL APPLICATIONS (Advanced)
9th Order 22kHz Lowpass Elliptic Filter
R
IN2
249k
C
IN2
33pF
C
IN3
27pF
R
R
IN1B
IN1A
TO
140k
69.8k
1
2
20
19
18
16
15
13
12
11
PIN 10
V
INVB
V1B
V2B
INVC
V1C
V2C
IN
R
95.3k
R
182k
Q1
Q2
C
IN1
390pF
R
R21 324k
R22 226k
IN3
3
536k
5
+
–
+
–
V
V
V
LTC1562
V
0.1µF
0.1µF
6
–
V
SHDN
V2A
AGND
V2D
R23 196k
R24 649k
8
R
392k
R
Q4
66.5k
Q3
9
V1A
V1D
10
INVA
INVD
R
IN4
301k
C
56pF
IN4
V
OUT
1562 F14a
SCHEMATIC INCLUDES PIN NUMBERS FOR 20-PIN PACKAGE.
–
PINS 4, 7, 14, 17 (NOT SHOWN) ALSO CONNECT TO V
Noise + THD vs Frequency
Amplitude Response
10
0
–40
V
IN
V
S
= 1.65V
= 4.6V
RMS P-P
= ±5V
–45
–50
–10
–20
–30
–40
–50
–60
–70
–80
–90
–55
–60
–65
–70
–75
–80
–85
–90
5
10
50
1
10
20
FREQUENCY (kHz)
FREQUENCY (kHz)
1562 TA14b
1562 TA14c
25
LTC1562
U
TYPICAL APPLICATIONS (Advanced)
Dual 5th Order Lowpass “Elliptic” Filter
R
IN2
C
IN2
Amplitude Response
R
R
IN1B
IN1A
V
OUT1
1
2
20
19
18
16
15
13
12
11
20
0
V
IN1
INVB
V1B
V2B
INVC
V1C
V2C
f
= 100kHz
C
R
R
Q1
Q2
C
IN1
R21
R22
R22
3
–20
–40
–60
–80
–100
–120
5
+
–
5V
–5V
V
LTC1562
V
0.1µF
0.1µF
6
SHDN
V2A
AGND
V2D
R21
8
R
R
Q1
Q2
9
V1A
V1D
R
R
IN1B
IN1A
10
V
IN2
INVA
INVD
C
IN1
V
C
OUT2
IN2
10
1000
100
FREQUENCY (kHz)
R
IN2
1562 TA15b
1562 TA15a
SCHEMATIC INCLUDES PIN NUMBERS FOR 20-PIN PACKAGE.
–
PINS 4, 7, 14, 17 (NOT SHOWN) ALSO CONNECT TO V
f (Hz)
R
R
C
R
R21
R
C
R
Q2
R22
11.3k
20k
C
IN1A
IN1B
IN1
Q1
IN2
IN2
100k
75k
5.9k
8.06k
16.9k
7.5k
15.4k
35.7k
680pF
560pF
390pF
28k
7.5k
13.3k
30.1k
6.34k
11.3k
25.5k
68pF
68pF
68pF
9.31k
12.7k
18.7k
36.5k
56.2k
50k
44.2k
Construction and Instrumentation Cautions
100dB rejections at hundreds of kilohertz require electri-
cally clean, compact construction, with good grounding
and supply decoupling, and minimal parasitic capaci-
tances in critical paths (such as Operational Filter INV
inputs). In a circuit with 5k resistances trying for 100dB
rejection at 100kHz, a stray coupling of 0.003pF around
the signal path can preclude the 100dB. (By comparison,
the stray capacitance between two adjacent pins of an IC
can be 1pF or more.) Also, high quality supply bypass
capacitorsof0.1µFnear the chip providegooddecoupling
from a clean, low inductance power source. But several
inchesofwire(i.e., afewmicrohenrysofinductance)from
the power supplies, unless decoupled by substantial
capacitance (≥10µF) near the chip, can cause a high-Q LC
resonance in the hundreds of kHz in the chip’s supplies or
ground reference, impairing stopband rejection and other
specifications at those frequencies. In demanding filter
circuits we have often found that a compact, carefully laid
out printed circuit board with good ground plane makes a
difference of 20dB in both stopband rejection and distor-
tion performance. Highly selective circuits can even ex-
hibit these issues at frequencies well below 100kHz.
Finally, equipmenttomeasurefilterperformancecanitself
introduce distortion or noise floors; checking for these
limits with a wire replacing the filter is a prudent routine
procedure.
26
LTC1562
U
PACKAGE DESCRIPTION
Dimensions in inches (millimeters) unless otherwise noted.
G Package
20-Lead Plastic SSOP (0.209)
(LTC DWG # 05-08-1640)
0.278 – 0.289*
(7.07 – 7.33)
20 19 18 17 16 15 14 13 12 11
0.301 – 0.311
(7.65 – 7.90)
5
7
8
1
2
3
4
6
9 10
0.205 – 0.212**
(5.20 – 5.38)
0.068 – 0.078
(1.73 – 1.99)
0° – 8°
0.0256
(0.65)
BSC
0.005 – 0.009
(0.13 – 0.22)
0.022 – 0.037
(0.55 – 0.95)
0.002 – 0.008
(0.05 – 0.21)
0.010 – 0.015
(0.25 – 0.38)
*DIMENSIONS DO NOT INCLUDE MOLD FLASH. MOLD FLASH
SHALL NOT EXCEED 0.006" (0.152mm) PER SIDE
**DIMENSIONS DO NOT INCLUDE INTERLEAD FLASH. INTERLEAD
FLASH SHALL NOT EXCEED 0.010" (0.254mm) PER SIDE
G20 SSOP 0595
Information furnished by Linear Technology Corporation is believed to be accurate and reliable.
However, no responsibility is assumed for its use. Linear Technology Corporation makes no represen-
tationthattheinterconnectionofitscircuitsasdescribedhereinwillnotinfringeonexistingpatentrights.
27
LTC1562
U
TYPICAL APPLICATION
Amplitude Response
20
0
–20
–40
–60
–80
Dual 4th Order 12dB Gaussian Lowpass Filter
f
= 64kHz
C
f
C
= 32kHz
R
IN2
f
= 16kHz
R
C
IN1
20
19
18
16
15
13
12
11
1
2
INV C
V1 C
V
INV B
V1 B
IN2
5V
R
R
Q2
Q1
R22
1µF
R24
R21
3
V2 C
–
V2 B
+
V
V
OUT2
1
10
FREQUENCY (kHz)
100
300
5
LTC1562
V
V
0.1µF
6
AGND
V2 D
SHDN
V2 A
V1 A
INV A
1562 TA16b
OUT1
8
R23
9
4-Level Eye Diagram
fC = 16kHz, Data Clock = 32kHz
V1 D
R
IN3
R
R
Q3
10
Q4
INV D
V
IN1
1562 TA16a
R
IN4
SCHEMATIC INCLUDES PIN NUMBERS FOR 20-PIN PACKAGE.
–
PINS 4, 7, 14, 17 (NOT SHOWN) ALSO CONNECT TO V
1V/DIV
1562 TA16c
10µs/DIV
f (Hz)
R
= R
R21 = R23
105k
R
= R
R
= R
R22 = R24
340k
R
= R
C
IN1
IN3
Q1
Q3
IN2
IN4
Q2 Q4
16k
32k
64k
105k
26.1k
8.45k
34k
340k
84.5k
16.2k
34k
26.1k
16.9k
8.45k
84.5k
16.9k
8.45k
6.49k
21k
RELATED PARTS
PART NUMBER
LTC1068, LTC1068-X
LTC1560-1
DESCRIPTION
COMMENTS
Quad 2-Pole Switched Capacitor Building Block Family
Clock-Tuned
5-Pole Elliptic Lowpass, f = 1MHz/0.5MHz
No External Components, SO8
Same Pinout as the LTC1562
C
LTC1562-2
Quad 2-Pole Active RC, 20kHz to 300kHz
1562f LT/TP 0199 4K • PRINTED IN USA
28 LinearTechnology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
●
●
(408)432-1900 FAX:(408)434-0507 www.linear-tech.com
LINEAR TECHNOLOGY CORPORATION 1998
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