LTC1562-2 [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： | LTC1562-2 |
| 厂家： | 
Linear |
| 描述： | Very Low Noise, Low Distortion Active RC Quad Universal Filter |
| 文件： | 总16页 (文件大小：182K) |
| 中文： | 中文翻译 | 下载： | 下载PDF数据表文档文件 |
LTC1562-2
Very Low Noise, Low Distortion
Active RC Quad Universal Filter
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FEATURES
DESCRIPTIO
The LTC®1562-2 is a low noise, low distortion continuous
time filter with rail-to-rail inputs and outputs, optimized for a
center frequency (fO) of 20kHz to 300kHz. 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, 20kHz to 300kHz
Center Frequency
■
Butterworth, Chebyshev, Elliptic or Equiripple
Delay Response
■
Lowpass, Bandpass, Highpass Responses
■
99dB Typical S/N, ±5V Supply (Q = 1)
■
93dB Typical S/N, Single 5V Supply (Q = 1)
■
Rail-to-Rail Input and Output Voltages
■
DC Accurate to 3mV (Typ)
■
±0.5% Typical Center Frequency Accuracy
■
“Zero-Power” Shutdown Mode
■
Single or Dual Supply, 5V to 10V Total
The LTC1562-2 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 90dB signal-to-noise
ratio from a single 5V power supply. Low level signals can
exploit the built-in gain capability of the LTC1562-2. Varying
the gain of a section can achieve a dynamic range as high as
114dB with a ±5V supply.
■
Resistor-Programmable fO, Q, Gain
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APPLICATIO S
■
High Resolution Systems (14 Bits to 18 Bits)
■
Antialiasing/Reconstruction Filters
■
Data Communications, Equalizers
■
Dual or I-and-Q Channels (Two Matched 4th Order
Filters in One Package)
Othercutofffrequencyrangescanbeprovideduponrequest.
Please contact LTC Marketing.
■
Linear Phase Filtering
■
Replacing LC Filter Modules
, LTC and LT are registered trademarks of Linear Technology Corporation.
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TYPICAL APPLICATIO
Amplitude Response
Dual 4th Order 200kHz Butterworth Lowpass Filter, SNR 96dB
10
0
R
7.87k
IN2
V
OUT1
R
7.87k
IN1
1
2
20
19
18
16
15
13
12
11
–10
–20
–30
–40
–50
–60
–70
–80
INV B
V1 B
V2 B
INV C
V
IN1
R
10.2k
R
4.22k
Q2
Q1
V1 C
V2 C
R21 7.87k
R22 7.87k
3
5
+
–
5V
–5V*
V
LTC1562-2
V
0.1µF
R23 7.87k
4.22k
0.1µF
6
SHDN
V2 A
AGND
V2 D
R24 7.87k
8
R
Q3
R
R
10.2k
7.87k
Q4
9
V1 A
V1 D
R
IN3
7.87k
10
INV A
INV D
V
IN2
–
1M
1.5M
50k
100k
V
OUT2
FREQUENCY (Hz)
*V ALSO AT PINS 4, 7, 14 & 17
ALL RESISTORS 1% METAL FILM
IN4
1562-2 TA01
1562-2 TA02
1
LTC1562-2
W W U W
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ABSOLUTE MAXIMUM RATINGS
PACKAGE/ORDER INFORMATION
(Note 1)
Total Supply Voltage (V+ to V–) .............................. 11V
Maximum Input Voltage
ORDER PART
NUMBER
TOP VIEW
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)
Storage Temperature Range ................. –65°C to 150°C
Operating Temperature Range
–*
V
–*
V
LTC1562CG-2
LTC1562IG-2
+
–
V
V
SHDN
15 AGND
14
13 V2 D
12 V1 D
11 INV D
–*
–*
LTC1562C-2 ............................................ 0°C to 70°C
LTC1562I-2 ........................................ –40°C to 85°C
Lead Temperature (Soldering, 10 sec).................. 300°C
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.
ELECTRICAL CHARACTERISTICS The ● denotes specifications that apply over the full operating temperature
range, otherwise specifications are at TA = 25°C. VS = ±5V, outputs unloaded, SHDN pin to logic “low”, unless otherwise noted. AC
specs are for a single 2nd order section, RIN = R2 = 10.4k ±0.1%, RQ = 9.09k ±0.1%, fO = 175kHz.
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
21
22.5
23.5
25
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
●
●
28
30
mA
mA
S
L
L
V = ±5V, R = 5k, C = 30pF, Outputs at 0V
S
L
L
Output Voltage Swing, V2 Outputs
Output Voltage Swing, V1 Outputs
DC Offset Magnitude, V2 Outputs
DC AGND Reference Point
V = ±2.375V, R = 5k, C = 30pF
●
●
4.2
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 = ±2.375V, R = 5k, C = 30pF, f = 250kHz
4.5
9.7
V
P-P
V
P-P
S
L
L
V = ±5V, R = 5k, C = 30pF, f = 250kHz
8.4
S
L
L
V
V = ±2.375V, Input at AGND Voltage
3
3
17
17
mV
mV
OS
S
V = ±5V, Input at AGND Voltage
S
V = Single 5V Supply
S
2.5
0.5
V
%
Center Frequency (f ) Error (Notes 2, 3) V = ±5V, V2 Output Has R = 5k, C = 30pF
1.7
O
S
L
L
H
L
Lowpass Passband Gain at V2 Output
V = ±2.375V, f = 10kHz,
●
0
+0.05 +0.1
dB
S
IN
V2 Output Has R = 5k, C = 30pF
L
L
Q Accuracy
V = ±2.375V, V2 Output Has R = 5k, C = 30pF
+2
%
S
L
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Wideband Output Noise
V = ±2.375V, BW = 400kHz, Input AC GND
V = ±5V, BW = 400kHz, Input AC GND
S
39
39
µV
RMS
µV
RMS
S
Input-Referred Noise, Gain = 100
BW = 400kHz, f = 200kHz, Q = 1, Input AC GND
7.3
µV
RMS
O
2
LTC1562-2
ELECTRICAL CHARACTERISTICS The ● denotes specifications that apply over the full operating temperature
range, otherwise specifications are at TA = 25°C. VS = ±5V, outputs unloaded, SHDN pin to logic “low”, unless otherwise noted. AC
specs are for a single 2nd order section, RIN = R2 = 10.4k ±0.1%, RQ = 9.09k ±0.1%, fO = 175kHz.
SYMBOL PARAMETER
THD Total Harmonic Distortion, V2 Output
CONDITIONS
= 20kHz, 2.8V , V1 and V2 Outputs Have
MIN
TYP
MAX UNITS
f
–100
dB
IN
P-P
R = 5k, C = 30pF
L
L
f
= 20kHz, 9V , V1 and V2 Outputs Have
–82
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
Note 1: Absolute Maximum Ratings are those values beyond which the life
of a device may be impaired.
Note 3: Tighter frequency tolerance is available, consult factory.
Note 2: f change from ±5V to ±2.375 supplies is –0.2% typical,
O
f temperature coefficient magnitude, 25°C to 85°C, is
O
50ppm/°C typical.
As with the LTC1562, f decreases with increasing temperature.
O
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TYPICAL PERFOR A CE CHARACTERISTICS
fO Error vs Nominal fO (VS = ±2.5V)
fO Error vs Nominal fO (VS = ±5V)
Q Error vs Nominal fO (VS = ±5V)
45
40
35
30
25
20
15
10
5
3.0
2.5
3.0
2.5
T
= 25°C
T
= 25°C
T
A
T
A
= 70°C
= 25°C
A
A
R
= R
R
= R
IN
Q
IN
Q
2.0
2.0
R
IN
= R
Q
Q = 5
1.5
1.5
1.0
1.0
Q = 5
Q = 2.5
Q = 1
0.5
0.5
Q = 5
0
0
Q = 2.5
Q = 1
–0.5
–1.0
–1.5
–2.0
–2.5
–3.0
–0.5
–1.0
–1.5
–2.0
–2.5
–3.0
Q = 2.5
Q = 1
0
–5
120 140 160 180 200 220 240 260 280
100 120 140 160 180 200 220 240 260 280 300
120 140 160 180 200 220 240 260 280
NOMINAL f (kHz)
O
NOMINAL f (kHz)
O
NOMINAL f (kHz)
O
1562-2 G02
1562-2 G03
1562-2 G01
3
LTC1562-2
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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)
55
50
45
40
35
30
25
20
15
10
5
3.00
2.75
2.50
2.25
2.00
1.75
1.50
1.25
1.00
0.75
0.50
0.25
0
4.00
3.75
3.50
3.25
3.00
2.75
2.50
2.25
2.00
1.75
1.50
1.25
1.00
0.75
0.50
0.25
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
R
IN
= R
R
= R
R
= R
IN
IN
Q = 5
Q = 5
Q = 5
Q = 2.5
Q = 2.5
Q = 1
Q = 2.5
Q = 1
Q = 1
0
–5
100 120 140 160 180 200 220
240 260
280
300
100 120 140 160
180
200
240 260 280 300
220
100 120 140 160 180 200 220 240 260 280 300
NOMINAL f (kHz)
NOMINAL f (kHz)
NOMINAL f (kHz)
O
O
O
1562-2 G04
1562-2 G5
1562-2 G6
LP Noise vs Nominal fO
(VS = ±5V, 25°C) (Figure 3,
V2 Output) (RIN = R2)
BP Noise vs Nominal fO
Distortion vs External Load
(VS = ±5V, 25°C) (Figure 3,
Resistance and Frequency
V1 Output) (RIN = RQ)
(VS = ±5V, 25°C) (Figure 8)
100
90
80
70
60
50
40
30
20
10
100
90
80
70
60
50
40
30
20
10
0
–10
–20
–30
–40
–50
–60
–70
–80
–90
–100
100
10
2nd ORDER LOWPASS
f
= 200kHz
O
Q = 0.7
OUTPUT LEVEL 1V
±5V SUPPLIES
(2.83V
)
P-P
RMS
Q = 5
1
Q = 5
Q = 2.5
Q = 1
Q = 2.5
Q = 1
0.1
0.01
0.001
f
f
f
= 100kHz
= 50kHz
IN
IN
IN
= 20kHz
220 280
240 260
120
200
140 160 180
220 280
240 260
10k
2k
EXTERNAL LOAD RESISTANCE (Ω)
1k
120
200
140 160 180
5k
NOMINAL f (kHz)
NOMINAL f (kHz)
O
O
1562-2 G08
1562-2 G07
1562-2 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. 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 supplies are not recommended as they will
lower the filter dynamic range.
Analog Ground (AGND): The AGND pin is the midpoint of
a resistive voltage divider, developing a potential halfway
between the V+ and V– pins, with an equivalent series
resistance nominally 7k. This serves as an internal 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
4
LTC1562-2
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PIN FUNCTIONS
should be connected to the ground plane (Figure 1). For
singlesupplyoperation,theAGNDpinshouldbebypassed
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 LTC1562-2 enters a “zero-power”
shutdown 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 effec-
tively disappear from the circuit. (If an input signal is
applied to a complete filter circuit while the LTC1562-2 is
in shutdown, some signal will normally flow to the output
throughpassivecomponentsaroundtheinactiveopamps.)
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
theLTC1562-2totheshutdownstateiftheSHDNpinisleft
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 LTC1562-2. (This
conventionpermitstrue“zero-power”shutdownsincenot
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
LTC1562-2
6
0.1µF
7
8
9
10
SINGLE-POINT
SYSTEM GROUND
DIGITAL
GROUND PLANE
(IF ANY)
1562-2 F01
1/4 LTC1562-2
*R1 AND C ARE PRECISION
INTERNAL COMPONENTS
Figure 1. Dual Supply Ground Plane Connection
(Including Substrate Pins 4, 7, 14, 17)
1
sR1C*
C
–
+
ANALOG
GROUND
PLANE
1
2
20
19
18
17
16
15
14
13
12
11
3
4
V2
INV
V1
5
+
R
R2
Q
V
LTC1562-2
6
0.1µF
7
Z
IN
1µF
8
+
V
IN
–
9
+
V /2
REFERENCE
IN EACH CASE,
10
7958Ω
f
= (200kHz)
RESPONSE RESPONSE
AT V1
BANDPASS LOWPASS
HIGHPASS BANDPASS
O
(
)
R2
Z
IN
TYPE
AT V2
RQ 200kHz
R2
R
C
Q =
(
)
SINGLE-POINT
SYSTEM GROUND
f
O
1562-2 F03
DIGITAL
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)
1562-2 F01
Figure 2. Single Supply Ground Plane Connection
(Including Substrate Pins 4, 7, 14, 17)
5
LTC1562-2
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PIN FUNCTIONS
INVA,INVB,INVC,INVD:EachoftheINVpinsisavirtual-
ground summing point for the corresponding 2nd order
section. For each section, all three external components
ZIN, R2, RQ connecttotheINVpinasshowninFigure3and
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.
also connects to the RQ resistor of the corresponding 2nd
orderfiltersection(seeFigure3andApplicationsInforma-
tion). Each output is designed to drive a nominal net load
of 4kΩ and 30pF, which includes the loading due to the
external RQ. Distortion performance improves when the
outputs are loaded as lightly as possible.
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 4kΩ and 30pF, which includes the loading due to the
external R2. Distortion performance improves when the
outputs are loaded as lightly as possible.
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
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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
1562-2 BD
INV
V1
V2
INV
V1
V2
6
LTC1562-2
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APPLICATIONS INFORMATION
The LTC1562-2 contains four matched, 2nd order,
3-terminal universal continuous-time filter blocks, each
with a virtual-ground input node (INV) and two rail-to-rail
outputs (V1, V2). In the most basic application, one such
block and three external resistors provide 2nd order
lowpassandbandpassresponsessimultaneously(Figure
3, with a resistor for ZIN). The three external resistors
program fO, Q and gain. A combination of internal preci-
sion components and external resistor R2 sets the center
frequency fO of each 2nd order block. The LTC1562-2 is
trimmed at manufacture so that fO will be 200kHz ±0.5%
iftheexternalresistorR2isexactly7958Ω.TheLTC1562-
2 is a higher frequency, pin compatible variant of the
LTC1562,withdifferentinternalRandCvaluesandhigher
speed amplifiers.
Setting fO and Q
Each of the four 2nd order sections in the LTC1562-2 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:
1
7958Ω
R2
fO =
Q =
=
200kHz
(
)
2πC (R1)R2
RQ
RQ
7958Ω R2
RQ 200kHz
R2
=
=
However, lowpass/bandpass filtering is only one specific
application for the 2nd order building blocks in the
LTC1562-2. Highpass response results if the external
impedanceZIN inFigure3becomesacapacitorCIN (whose
value sets only gain, not critical frequencies) as described
below. Responses with zeroes (e.g, elliptic or notch
responses) are available by feedforward connections with
multiple2ndorderblocks(seeTypicalApplicatons).More-
over, the virtual-ground input gives each 2nd order sec-
tion the built-in capability for analog operations such as
gain (preamplification), summing and weighting of mul-
tiple inputs, or accepting current or charge signals di-
rectly. These Operational FilterTM frequency-selective
building blocks are nearly as versatile as operational
amplifiers.
fO
(R1)R2
(
)
R1 = 7958Ω and C = 100pF are internal to the LTC1562-2
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
200kHz
R2 Ideal =
7958Ω
(
)
(
)
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 LTC1562-2,
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 (7958Ω)R2
The fO range is approximately 20kHz to 300kHz, 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
tolerance (by a factor of 2 incrementally), but it also
implies that R2 has a wider range than fO. (RQ and RIN also
Operational Filter and FilterCAD are trademarks of Linear Technology Corporation.
Windows is a registered trademark of Microsoft Corporation.
7
LTC1562-2
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APPLICATIONS INFORMATION
tend to scale with R2.) At high fO these resistors fall below
4k, heavily loading the outputs of the LTC1562-2 and
leading to increased THD and other effects. At the other
extreme, a lower fO limit of 20kHz reflects an arbitrary
upper resistor limit of 1MΩ. The LTC1562-2’s MOS input
circuitry can accommodate higher resistor values than
this,butjunctionleakagecurrentfromtheinputprotection
circuitry may cause DC errors.
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.”
Basic Lowpass
When ZIN of Figure 3 is a resistor of value RIN, a standard
2ndorderlowpasstransferfunctionresultsfromVIN toV2
(Figure 5):
V2(s)
–HLωO2
= HLP(s) =
s2 + ω /Q s + ω2
V (s)
IN
(
)
O
O
HL = R2/RIN is the DC gain magnitude. (Note that the
transfer function includes a sign inversion.) Parameters
R
IN
V
IN
R
Q
R2
V
OUT
INV
V1
2nd ORDER
1/4 LTC1562-2
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
B
P
L
L
P
H
0.707 H
0.707 H
0.707 H
H
1562-2 F04
f
f
f
f
f
f
f
P
L
O
H
P
C
C
f (LOG SCALE)
f (LOG SCALE)
f (LOG SCALE)
–1
fO
fH – fL
2
Q =
;fO = fLfH
2
1
2Q2
1
2Q2
fC = fO 1–
+
1–
+ 1
1
2Q2
1
2Q2
fC = fO
1–
+
1–
+ 1
2
2
–1
2Q
1
2Q
1
2Q2
fL = fO
+
+
+ 1
+ 1
fP = fO 1–
–1
1
2Q2
fP = fO 1–
1
1
f
H = fO
1
HP = HL
2Q
2Q
1
1
4Q2
1
1–
HP = HH
Q
1
Q
1
1–
4Q2
Figure 4. Characteristics of Standard 2nd Order Filter Responses
8
LTC1562-2
U
W U U
APPLICATIONS INFORMATION
HH = CIN/100pF is the highpass gain parameter. Param-
eters ωO = 2πfO and Q are set by R2 and RQ as above. 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.
ωO (=2πfO)andQaresetbyR2andRQ asabove. Fora2nd
orderlowpassresponsethegainmagnitudebecomesQHL
at frequency fO, and for Q > 0.707, a gain peak occurs at
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
IN
V
IN
R
R2
Q
–H ω /Q s
(
)
B
O
V
OUT
HBP(s) =
INV
V1
2nd ORDER
1/4 LTC1562-2
V2
s2 + ω /Q s + ω2
(
)
O
O
1562-2 F07
The value of the gain parameter HB depends on the circuit
configuration as follows. 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
thegainparameternowHB=(RQ/7958Ω)(CIN/100pF).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. ωO = 2πfO and Q are set
by R2 and RQ as described previously in Setting fO and Q.
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 LTC1562-2 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
For an LTC1562-2 section as in Figure 3, the magnitudes
of the two outputs V2 and V1, at a frequency ω = 2πf, have
the ratio,
INV
V1
2nd ORDER
1/4 LTC1562-2
V2
INV
V1
2nd ORDER
1/4 LTC1562-2
V2
1562-2 F06
(a) Resistive Input
(b) Capacitive Input
| V2(jω)| (200kHz)
=
| V1(jω)|
f
Figure 6. Basic Bandpass Configurations
regardless of the details of ZIN. Therefore, an input fre-
quency above or below 200kHz 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 LTC1562-2 section.
Basic Highpass
When ZIN of Figure 3 is a capacitor of value CIN, a highpass
response appears at the V1 output (Figure 7).
V1(s)
–HHs2
= HHP(s) =
s2 + ω /Q s + ω2
V (s)
IN
(
)
O
O
9
LTC1562-2
U
W U U
APPLICATIONS INFORMATION
Because 2nd order sections with Q ≥ 1 have response
peaks near fO, the gain ratio above implies some rules of
thumb:
require further dynamic range, reducing the value of ZIN
boosts the signal gain while reducing the input referred
noise. This feature can increase the SNR for low level
signals. Varying or switching ZIN is also an efficient way to
effect automatic gain control (AGC). From a system view-
point, this technique boosts the ratio of maximum signal
to minimum noise, for a typical 2nd order lowpass re-
sponse (Q = 1, fO = 200kHz), to 114dB.
fO < 200kHz V2 tends to have the larger swing
fO > 200kHz 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 < 200kHz
Bandpass response (capacitor input, V2 output, Figure
6b) with fO < 200kHz
Bandpass response (resistor input, V1 output, Figure
6a) with fO > 200kHz
Highpass response (capacitor input, V1 output, Figure
7) with fO > 200kHz
Properly used, the LTC1562-2 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
negativefeedback.TheLTC1562-2fundamentallyresponds
to input current and the external voltage VIN appears only
across the external impedance ZIN in Figure 3.
The LTC1562, a lower frequency variant of the LTC1562 -2,
has a design center fO of 100kHz compared to 200kHz in the
LTC1562-2. The rules summarized above apply to the
LTC1562 but with 100kHz replacing the 200kHz limits.
Thus, an LTC1562 highpass filter section with fO above
100kHzautomaticallysatisfiesthedesirableconditionofthe
unused output carrying the smaller signal swing.
To accept beyond-the-supply input voltages, it is impor-
tant to keep the LTC1562-2 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 LTC1562-2 could be damaged.
R
IN
7.87k
V
IN
R
R2
7.87k
Q
5.49k
V
OUT
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.
R
L
INV
V1
2nd ORDER
1/4 LTC1562-2
V2
C
L
(EXTERNAL
30pF
LOAD RESISTANCE)
1562-2 F08
Figure 8. 200kHz, 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 LTC1562-2 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
outputsignal99dBabovethenoiselevel.Iflowlevelinputs
10
LTC1562-2
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 frequency:
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
INA
R
INB
V
IN
V
IN
C
T
R
Q
R2
R
T
R
Q
R2
INV
V1
2nd ORDER
1/4 LTC1562-2
V2
INV
V1
2nd ORDER
1/4 LTC1562-2
V2
1562-2 F09
1562-2 F10
Figure 9. Lowpass “T” Input Circuit
Figure 10. Highpass “T” Input Circuit
Theprocedurethenistobeginwiththetargetcorner(pole)
frequency fP. Determine the series value CIN from the gain
requirement (for example, CIN = HH(100pF) for a high-
pass). Select a resistor value RT such that CP = 1/(2πRTfP)
is at least 4CIN, and select CINA and CINB that will simulta-
neously have the parallel value CP and the series value CIN.
Such CINA and CINB can be found directly from the
expression:
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:
1
2
1
2
RIN ±
RIN2 – 4R R
(
)
IN
P
2
1
2
1
2
CP ±
C – 4C C
P
IN P
11
LTC1562-2
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.
resistors and capacitors are provided to build application-
specific filters. Also provided are terminals for inputs,
outputs and power supplies.
Notches and Elliptic Responses
LTC1562/LTC1562-2 Demo Board
Further circuit techniques appear in the LTC1562 final
data sheet under the heading “Notches and Elliptic Re-
sponses.” These techniques are directly applicable to the
LTC1562-2withthesubstitutionofthedifferentvaluesfor
the internal components R1 and C. In the LTC1562-2, R1
is 7958Ω and C is 100pF.
The LTC demonstration board DC266 is assembled with
an LTC1562 or LTC1562-2 in a 20-pin SSOP package and
power supply decoupling capacitors. Jumpers on the
board configure the filter chip for dual or single supply
operation and power shutdown. Pads for surface mount
U
TYPICAL APPLICATIONS
175kHz 8th Order Elliptic Highpass Filter
C
IN2
82pF
R
20.5k
IN2
C
IN3
47pF
Amplitude Response
C
220pF
IN1
10
1
2
20
19
18
16
15
13
12
11
R
IN3
45.3k
INV B
V1 B
INV C
V1 C
V
IN
0
R
9.09k
R
26.7k
Q1
Q2
–10
–20
R21 7.15k
R22 10k
3
V2 B
+
V2 C
–
5
–30
–5V*
5V
V
LTC1562-2
V
AGND
V2 D
V1 D
INV D
0.1µF
0.1µF
R23 11.3k
R 59k
6
–40
–50
–60
–70
–80
–90
SHDN
V2 A
R24 4.02k
3.24k
8
R
Q4
Q3
9
V1 A
10
INV A
V
OUT
R
IN4
40.2k
200k
FREQUENCY (Hz)
50k
900k
C
IN4
100pF
1562-2 TA03b
–
1562-2 TA03a
*V ALSO AT PINS 4, 7, 14 & 17
ALL RESISTORS 1% METAL FILM
ALL CAPACITORS 5% STANDARD VALUES
12
LTC1562-2
U
TYPICAL APPLICATIONS
Dual 5th Order 170kHz Elliptic Highpass Filter, Single 5V Supply
C
220pF
IN2
R
15k
IN2
C
C
IN1
I1
100pF 82pF
V
OUT1
1
2
20
19
18
16
15
13
12
11
V
IN1
INV B
V1 B
INV C
V1 C
R
43.2k
R
7.68k
Q2
Q1
R
I1
2k
R22 6.34k
R21 11.5k
3
V2 B
+
V2 C
–
5
5V
V
*
LTC1562-2
V
0.1µF
R23 11.5k
1µF
6
+
SHDN
V2 A
V1 A
INV A
AGND
V2 D
V1 D
INV D
R24 6.34k
8
R
7.68k
R
43.2k
C
C
IN3
Q4
Q3
9
I3
100pF 82pF
10
V
IN2
V
OUT2
R
I3
2k
R
IN4
15k
C
220pF
IN4
1562-2 TA05a
*GROUND ALSO AT PINS 4, 7, 14 & 17
Amplitude Response
10
0
–10
–20
–30
–40
–50
–60
–70
–80
–90
10k
100k
1M
FREQUENCY (Hz)
1562-2 TA05b
13
LTC1562-2
TYPICAL APPLICATIONS
U
100kHz 8th Order Bandpass Linear Phase, –3dB BW = fCENTER/10
C
10pF
IN1
R
178k
IN2
1
2
20
19
18
16
15
13
12
11
INV B
V1 B
V2 B
INV C
V1 C
V
IN
R
76.8k
R
78.7k
Q2
Q1
R21 31.6k
R22 30.1k
3
V2 C
–
5
+
5V
–5V*
V
LTC1562-2
V
0.1µF
R24 28.7k
0.1µF
6
SHDN
V2 A
AGND
V2 D
V1 D
INV D
R23 35.7k
8
R
142k
R
118k
Q3
Q4
9
V1 A
10
INV A
C
10pF
IN3
V
OUT
R
221k
IN4
1562-2 TA6a
–
*V ALSO AT PINS 4, 7, 14 & 17
Frequency Response
10
0
AMPLITUDE
RESPONSE
–10
–20
–30
–40
–50
–60
–70
60
GROUP
DELAY
0
140k
80k
100k
FREQUENCY (Hz)
60k
120k
1562-2 TA06b
14
LTC1562-2
U
TYPICAL APPLICATIONS
LTC1562-2 9th Order 200kHz Lowpass Elliptic Filter
R
IN2
7.32k
C
IN2
27pF
R
R
IN1B
Amplitude Response
IN1A
4.02k 4.02k
1
2
20
19
18
16
15
13
12
11
V
IN
INVB
V1B
V2B
INVC
10
0
R
6.04k
R
13k
Q1
Q2
180pF
V1C
V2C
–10
–20
–30
–40
–50
–60
–70
–80
–90
–100
R21 8.06k
R22 6.04k
3
5
+
–
5V
–5V
V
LTC1562-2
V
0.1µF
0.1µF
6
SHDN
V2A
AGND
V2D
8
R24 6.04k
R23 12.4k
9
V1A
V1D
R
Q3
5.36k
R
Q4
14.3k
6.04k
R
10.2k
IN3
10
INVA
INVD
R
IN4
C
IN4
22pF
10
100
1000
V
OUT
FREQUENCY (kHz)
1562-2 TA07b
1562-2 TA07a
–
PINS 4, 7, 14, 17 (NOT SHOWN) ALSO CONNECT TO V
ALL RESISTORS ARE ±1%, ALL CAPACITORS ARE ±5%
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.
15
LTC1562-2
TYPICAL APPLICATIONS
U
256kHz Linear Phase 6th Order Lowpass Filter with a 2nd Order
Allpass Phase Equalizer, Single Supply
R
FF1
6.19k
V
IN
R
1.54k
B1
R
IN1
7.5k
1
2
20
19
18
16
15
13
12
11
INV B
V1 B
INV C
V1 C
R
3.24k
R
4.12k
Q2
Q1
R21 6.81k
R22 6.19k
3
V2 B
+
V2 C
–
5
V
LTC1562-2
V
AGND
V2 D
V1 D
INV D
5V
*
0.1µF
R23 4.12k
7.32k
1µF
6
+
SHDN
V2 A
R24 4.12k
8
R
7.32k
R
Q4
Q3
9
V1 A
10
INV A
V
OUT
R
IN3
C
22pF 5%
R
IN4
4.12k
IN4
4.12k
1562-2 TA04a
*GROUND ALSO AT PINS 4, 7, 14 & 17
ALL RESISTORS 1% METAL FILM
Amplitude Response
Group Delay Response
10
0
8
7
6
5
4
3
2
1
0
–10
–20
–30
–40
–50
–60
–70
–80
100k
10k
1M
50 100 150 200 250 300 350 400
FREQUENCY (Hz)
FREQUENCY (kHz)
1562-2 TA04b
1562-2 TA04c
RELATED PARTS
PART NUMBER
LTC1068-X
LTC1560-1
LTC1562
DESCRIPTION
COMMENTS
Quad 2-Pole Switched Capacitor Building Block
Clock Tuned
5-Pole Elliptic Lowpass, f = 1MHz/0.5MHz
No External Components, SO8
Same Pinout as LTC1562-2
C
Quad 2-Pole Active RC, 10kHz to 150kHz
15622f LT/TP 0599 4K • PRINTED IN USA
16 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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