LT6600CS8-20#TR [Linear]
LT6600-20 - Very Low Noise, Differential Amplifier and 20MHz Lowpass Filter; Package: SO; Pins: 8; Temperature Range: 0°C to 70°C;
| 型号： | LT6600CS8-20#TR |
| 厂家： | 
Linear |
| 描述： | LT6600-20 - Very Low Noise, Differential Amplifier and 20MHz Lowpass Filter; Package: SO; Pins: 8; Temperature Range: 0°C to 70°C 运算放大器 放大器电路 光电二极管 |
| 文件： | 总12页 (文件大小：220K) |
| 中文： | 中文翻译 | 下载： | 下载PDF数据表文档文件 |
LT6600-20
Very Low Noise, Differential
Amplifier and 20MHz Lowpass Filter
U
FEATURES
DESCRIPTIO
■
Programmable Differential Gain via Two External
The LT®6600-20 combines a fully differential amplifier
with a 4th order 20MHz lowpass filter approximating a
Chebyshev frequency response. Most differential amplifi-
ers require many precision external components to tailor
gain and bandwidth. In contrast, with the LT6600-20, two
externalresistorsprogramdifferentialgain, andthefilter’s
20MHzcutofffrequencyandpassbandrippleareinternally
set. The LT6600-20 also provides the necessary level
shifting to set its output common mode voltage to accom-
modate the reference voltage requirements of A/Ds.
Resistors
■
Adjustable Output Common Mode Voltage
■
Operates and Specified with 3V, 5V, ±5V Supplies
■
0.5dB Ripple 4th Order Lowpass Filter with 20MHz
Cutoff
■
76dB S/N with 3V Supply and 2VP-P Output
■
Low Distortion, 2VP-P, 800Ω Load
2.5MHz: 83dBc 2nd, 88dBc 3rd
20MHz: 63dBc 2nd, 64dBc 3rd
■
Fully Differential Inputs and Outputs
Using a proprietary internal architecture, the LT6600-20
integrates an antialiasing filter and a differential amplifier/
driver without compromising distortion or low noise
performance. At unity gain the measured in band
signal-to-noise ratio is an impressive 76dB. At higher
gains the input referred noise decreases so the part can
process smaller input differential signals without signifi-
cantly degrading the output signal-to-noise ratio.
■
SO-8 Package
■
Compatible with Popular Differential Amplifier
Pinouts
U
APPLICATIO S
■
High Speed ADC Antialiasing and DAC Smoothing in
Networking or Cellular Base Station Applications
The LT6600-20 also features low voltage operation. The
differential design provides outstanding performance for
a2VP-P signallevelwhilethepartoperateswithasingle3V
supply.
■
High Speed Test and Measurement Equipment
■
Medical Imaging
■
Drop-in Replacement for Differential Amplifiers
, LTC and LT are registered trademarks of Linear Technology Corporation.
The LT6600-20 is packaged in an SO-8 and is pin compat-
ible with stand alone differential amplifiers.
U
TYPICAL APPLICATIO
An 8192 Point FFT Spectrum
0
INPUT 5.9MHz
–10
A/D
LTC1748
LT6600-20
5V
2V
f
P-P
SAMPLE
–20
= 80MHz
–30
–40
–50
0.1µF
5V
R
402Ω
IN
3
+
1
7
2
8
V
49.9Ω
49.9Ω
–
4
–60
–70
+
+
–
V
MID
D
A
IN
18pF
OUT
–
V
–80
0.01µF
OCM
–
5
–90
V
IN
+
V
CM
V
R
6
IN
402Ω
–100
–110
–120
1µF
GAIN = 402Ω/R
IN
20
FREQUENCY (MHz)
0
10
30
40
66002 TA01a
66002 TA01b
66002f
1
LT6600-20
W W U W
U
W
U
ABSOLUTE AXI U RATI GS
(Note 1)
PACKAGE/ORDER I FOR ATIO
Total Supply Voltage................................................ 11V
Operating Temperature Range (Note 6) ...–40°C to 85°C
Specified Temperature Range (Note 7)....–40°C to 85°C
Junction Temperature........................................... 150°C
Storage Temperature Range ................. –65°C to 150°C
Lead Temperature (Soldering, 10 sec).................. 300°C
ORDER PART
TOP VIEW
NUMBER
–
+
IN
1
2
3
4
8
7
6
5
IN
LT6600CS8-20
LT6600IS8-20
V
V
OCM
MID
–
+
V
V
+
–
OUT
OUT
S8 PART MARKING
S8 PACKAGE
8-LEAD PLASTIC SO
660020
600I20
TJMAX = 150°C, θJA = 100°C/W
Consult LTC Marketing for parts specified with wider operating temperature ranges.
ELECTRICAL CHARACTERISTICS
The ● denotes specifications that apply over the full operating temperature
range, otherwise specifications are at TA = 25°C. Unless otherwise specified VS = 5V (V+ = 5V, V– = 0V), RIN = 402Ω, and RLOAD = 1k.
PARAMETER
Filter Gain, V = 3V
CONDITIONS
MIN
– 0.4
– 0.1
– 0.2
– 0.1
–0.8
TYP
0.1
0
MAX
0.5
0.1
0.5
1.9
1
UNITS
dB
dB
dB
dB
dB
dB
dB
dB
dB
dB
dB
dB
dB
dB
dB
V
V
V
V
V
V
V
V
V
V
V
V
V
V
V
= 2V , f = DC to 260kHz
P-P IN
S
IN
IN
IN
IN
IN
IN
IN
IN
IN
IN
IN
IN
IN
IN
IN
= 2V , f = 2MHz (Gain Relative to 260kHz)
●
●
●
●
●
●
P-P IN
= 2V , f = 10MHz (Gain Relative to 260kHz)
0.1
0.5
0
P-P IN
= 2V , f = 16MHz (Gain Relative to 260kHz)
P-P IN
= 2V , f = 20MHz (Gain Relative to 260kHz)
P-P IN
= 2V , f = 60MHz (Gain Relative to 260kHz)
– 33
– 50
0
– 28
P-P IN
= 2V , f = 100MHz (Gain Relative to 260kHz)
P-P IN
Filter Gain, V = 5V
= 2V , f = DC to 260kHz
–0.5
– 0.1
– 0.2
– 0.3
– 1.3
0.5
0.1
0.4
1.6
0.6
–28
S
P-P IN
= 2V , f = 2MHz (Gain Relative to 260kHz)
●
●
●
●
●
●
0
P-P IN
= 2V , f = 10MHz (Gain Relative to 260kHz)
0.1
0.4
–0.4
–33
–50
–0.1
P-P IN
= 2V , f = 16MHz (Gain Relative to 260kHz)
P-P IN
= 2V , f = 20MHz (Gain Relative to 260kHz)
P-P IN
= 2V , f = 60MHz (Gain Relative to 260kHz)
P-P IN
= 2V , f = 100MHz (Gain Relative to 260kHz)
P-P IN
Filter Gain, V = ±5V
= 2V , f = DC to 260kHz
–0.6
0.4
S
P-P IN
Filter Gain, R = 100Ω
V
V
V
= 2V , f = DC to 260kHz, V = 3V
11.6
11.5
11.4
12.1
12.0
11.9
12.6
12.5
12.4
dB
dB
dB
IN
IN
IN
IN
P-P IN
S
= 2V , f = DC to 260kHz, V = 5V
P-P IN
S
= 2V , f = DC to 260kHz, V = ±5V
P-P IN
S
Filter Gain Temperature Coefficient (Note 2)
f
= 250kHz, V = 2V
P-P
780
118
ppm/C
IN
IN
Noise
Noise BW = 10kHz to 20MHz
2.5MHz, 2V , R = 800Ω
µV
RMS
Distortion (Note 4)
2nd Harmonic
3rd Harmonic
83
88
dBc
dBc
P-P
L
20MHz, 2V , R = 800Ω
2nd Harmonic
3rd Harmonic
63
64
dBc
dBc
P-P
L
Differential Output Swing
Input Bias Current
Measured Between Pins 4 and 5
Average of Pin 1 and Pin 8
V = 5V
S
●
●
3.80
3.75
4.75
4.50
V
V
S
P-P DIFF
P-P DIFF
V = 3V
●
– 95
–50
µA
66002f
2
LT6600-20
ELECTRICAL CHARACTERISTICS
The ● denotes specifications that apply over the full operating temperature
range, otherwise specifications are at TA = 25°C. Unless otherwise specified VS = 5V (V+ = 5V, V– = 0V), RIN = 402Ω, and RLOAD = 1k.
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
Input Referred Differential Offset
R
IN
= 402Ω
V = 3V
●
●
●
5
10
10
25
30
35
mV
mV
mV
S
V = 5V
S
V = ±5V
S
R
IN
= 100Ω
V = 3V
●
●
●
5
5
5
15
17
20
mV
mV
mV
S
V = 5V
S
V = ±5V
S
Differential Offset Drift
10
µV/°C
Input Common Mode Voltage (Note 3)
Differential Input = 500mV
IN
,
V = 3V
●
●
●
0.0
0.0
–2.5
1.5
3.0
1.0
V
V
V
P-P
S
R
= 100Ω
V = 5V
S
V = ±5V
S
Output Common Mode Voltage (Note 5)
Differential Input = 2V
Pin 7 at Mid-Supply
,
V = 3V
●
●
●
1.0
1.5
–1.0
1.5
3.0
2.0
V
V
V
P-P
S
V = 5V
S
Common Mode Voltage at Pin 2
V = ±5V
S
Output Common Mode Offset
(with Respect to Pin 2)
V = 3V
●
●
●
–35
–40
–55
5
0
–5
40
40
35
mV
mV
mV
S
V = 5V
S
V = ±5V
S
Common Mode Rejection Ratio
66
dB
Voltage at V
(Pin 7)
V = 5
S
●
●
2.46
4.35
2.51
1.5
2.55
7.65
V
V
MID
S
V = 3
V
V
Input Resistance
Bias Current
5.7
kΩ
MID
V
= V = V /2
V = 5V
●
●
–15
–10
–3
–3
µA
µA
OCM
OCM
MID
S
S
V = 3V
S
Power Supply Current
V = 3V, V = 5
42
46
53
56
mA
mA
mA
S
S
V = 3V, V = 5
●
●
S
S
V = ±5V
46
S
Note 1: Absolute Maximum Ratings are those values beyond which the life
of a device may be impaired.
Note 2: This is the temperature coefficient of the internal feedback
Note 5: Output common mode voltage is the average of the voltages at
Pins 4 and 5. The output common mode voltage is equal to the voltage
applied to Pin 2.
resistors assuming a temperature independent external resistor (R ).
Note 3: The input common mode voltage is the average of the voltages
Note 6: The LT6600C-20 is guaranteed functional over the operating
temperature range –40°C to 85°C.
IN
applied to the external resistors (R ). Specification guaranteed for
Note 7: The LT6600C-20 is guaranteed to meet 0°C to 70°C specifications
and is designed, characterized and expected to meet the extended
temperature limits, but is not tested at –40°C and 85°C. The LT6600I-20
is guaranteed to meet specified performance from –40°C to 85°C.
IN
R
IN
≥ 100Ω.
Note 4: Distortion is measured differentially using a differential stimulus,
The input common mode voltage, the voltage at Pin 2, and the voltage at
Pin 7 are equal to one half of the total power supply voltage.
66002f
3
LT6600-20
U W
TYPICAL PERFOR A CE CHARACTERISTICS
Amplitude Response
Passband Gain and Phase
Passband Gain and Group Delay
10
0
2
0
50
45
40
35
30
25
20
15
10
5
2
0
95
V
= 5V
V
= 5V
GAIN
GAIN
S
S
GAIN = 1
= 25°C
50
GAIN = 1
= 25°C
T
A
T
A
–10
–20
–30
–40
–50
–60
–70
–80
–90
–2
–2
5
–4
–4
–40
–85
–130
–175
–220
–265
–310
–355
PHASE
GROUP
DELAY
–6
–6
–8
–8
–10
–12
–14
–16
–18
–10
–12
–14
–16
–18
V
S
= 5V
GAIN = 1
= 25°C
T
A
0
0.1
1
10
100
0.5
6.5
12.5
18.5
24.5
30.5
0.5
6.5
12.5
18.5
24.5
30.5
FREQUENCY (MHz)
FREQUENCY (MHz)
FREQUENCY (MHz)
66002 G01
66002 G03
66002 G02
Passband Gain and Group Delay
Output Impedance
Common Mode Rejection Ratio
100
80
75
70
65
60
55
50
45
40
35
30
14
12
10
8
50
45
40
35
30
25
20
15
10
5
INPUT = 1V
P-P
V
S
= 5V
V
= 5V
S
GAIN
V
= 5V
GAIN = 1
= 25°C
GAIN = 4
= 25°C
S
GAIN = 1
= 25°C
T
A
T
A
T
A
10
1
GROUP
DELAY
6
4
2
0
–2
–4
–6
0.1
0
0.1
1
10
100
0.5
6.5
12.5
18.5
24.5
30.5
0.1
1
10
100
FREQUENCY (MHz)
FREQUENCY (MHz)
FREQUENCY (MHz)
66002 G05
66002 G06
66002 G04
Distortion vs Signal Level,
VS = 3V
Power Supply Rejection Ratio
Distortion vs Frequency
–40
–50
–60
–70
–80
–90
–40
–50
100
90
+
DIFFERENTIAL INPUT,
2ND HARMONIC
DIFFERENTIAL INPUT,
3RD HARMONIC
SINGLE-ENDED INPUT,
2ND HARMONIC
SINGLE-ENDED INPUT,
3RD HARMONIC
3RD HARMONIC
10MHz INPUT
V
V
TO DIFFOUT
= 3V
= 25°C
V
= 3V
S
R
= 800Ω AT
L
S
A
T
EACH OUTPUT
GAIN = 1
80
2ND
HARMONIC
10MHz INPUT
T
= 25°C
70
A
–60
–70
60
50
3RD
40
30
20
10
0
HARMONIC
V
= 2V
= 3V
IN
S
L
P-P
–80
–90
1MHz INPUT
V
2ND HARMONIC
1MHz INPUT
R
= 800Ω AT
EACH OUTPUT
GAIN = 1
T
= 25°C
A
–100
–100
0.1
1
10
100
0
1
2
3
4
5
0.001
0.01
0.1
1
10
100
FREQUENCY (MHz)
INPUT LEVEL (V
)
FREQUENCY (MHz)
P-P
66002 G07
66002 G08
66002 G09
66002f
4
LT6600-20
U W
TYPICAL PERFOR A CE CHARACTERISTICS
Distortion vs Signal Level,
VS = ±5V
Distortion
vs Input Common Mode Level
Distortion
vs Input Common Mode Level
–40
–50
–40
–50
–40
–50
2V 1MHz INPUT
2ND HARMONIC,
10MHz INPUT
3RD HARMONIC,
10MHz INPUT
2ND HARMONIC,
1MHz INPUT
2ND HARMONIC,
= 3V
2ND HARMONIC,
P-P
R
= 800Ω AT
V
S
V
= 3V
L
S
EACH OUTPUT
GAIN = 1
3RD HARMONIC,
= 3V
3RD HARMONIC,
= 3V
V
V
S
S
T
A
= 25°C
2ND HARMONIC,
= 5V
2ND HARMONIC,
= 5V
–60
–70
–60
–70
V
–60
–70
V
S
S
3RD HARMONIC,
1MHz INPUT
3RD HARMONIC,
= 5V
3RD HARMONIC,
= 5V
V
V
S
S
–80
–90
–80
–90
–80
–90
V
= ±5V
S
L
R
= 800Ω AT
EACH OUTPUT
GAIN = 1
500mV 1MHz INPUT, GAIN = 4,
L
P-P
T
A
= 25°C
R
= 800Ω AT EACH OUTPUT
–100
–100
–100
0
1
2
3
4
5
–3
–2
–1
0
1
2
3
–3
–2
–1
0
1
2
3
INPUT LEVEL (V
)
INPUT COMMON MODE VOTLAGE RELATIVE TO PIN 7 (V)
INPUT COMMON MODE VOTLAGE RELATIVE TO PIN 7 (V)
P-P
66002 G10
66002 G11
66002 G12
Distortion
vs Output Common Mode
–40
2ND HARMONIC,
V
= 3V
S
–50
–60
3RD HARMONIC,
= 3V
V
S
2ND HARMONIC,
= 5V
V
S
3RD HARMONIC,
= 5V
–70
V
S
2ND HARMONIC,
= ±5V
–80
V
S
3RD HARMONIC,
= ±5V
V
S
–90
–100
–110
2V 1MHz INPUT, GAIN = 1,
L
P-P
R
= 800Ω AT EACH OUTPUT
–1.5 –1 –0.5
0
2
–2
0.5
1
1.5
VOLTAGE PIN 2 TO PIN 7 (V)
66002 G13
Total Supply Current
vs Total Supply Voltage
Transient Response, Gain = 1
60
50
+
T
T
= 85°C
A
A
VOUT
50mV/DIV
40
30
= 25°C
= –40°C
T
DIFFERENTIAL
INPUT
200mV/DIV
A
20
10
0
100ns/DIV
66002 G15
0
1
2
3
4
5
6
7
8
9
10
TOTAL SUPPLY VOLTAGE (V)
66002 G14
66002f
5
LT6600-20
U
U
U
PI FU CTIO S
IN– and IN+ (Pins 1, 8): Input Pins. Signals can be applied
to either or both input pins through identical external
resistors, RIN. The DC gain from differential inputs to the
differential outputs is 402Ω/RIN.
supply applications, bypass Pin 3 to ground and Pin 6 to
ground with a quality 0.1µF ceramic capacitor.
OUT+ and OUT– (Pins 4, 5): Output Pins. Pins 4 and 5 are
the filter differential outputs. Each pin can drive a 100Ω
and/or 50pF load.
V
OCM (Pin 2): Is the DC Common Mode Reference Voltage
for the 2nd Filter Stage. Its value programs the common
mode voltage of the differential output of the filter. Pin 2 is
a high impedance input, which can be driven from an
external voltage reference, or Pin 2 can be tied to Pin 7 on
the PC board. Pin 2 should be bypassed with a 0.01µF
ceramiccapacitorunlessitisconnectedtoagroundplane.
V+ and V– (Pins 3, 6): Power Supply Pins. For a single
3.3V or 5V supply (Pin 6 grounded) a quality 0.1µF
ceramic bypass capacitor is required from the positive
supply pin (Pin 3) to the negative supply pin (Pin 6). The
bypass should be as close as possible to the IC. For dual
V
MID (Pin 7): The VMID pin is internally biased at mid-
supply, see block diagram. For single supply operation,
the VMID pin should be bypassed with a quality 0.01µF
ceramic capacitor to Pin 6. For dual supply operation, Pin
7 can be bypassed or connected to a high quality DC
ground. A ground plane should be used. A poor ground
will increase noise and distortion. Pin 7 sets the output
common mode voltage of the 1st stage of the filter. It has
a 5.5kΩ impedance, and it can be overridden with an
external low impedance voltage source.
W
BLOCK DIAGRA
R
IN
+
+
–
–
V
IN
IN
V
OUT
5
V
MID
8
7
6
+
V
11k
11k
PROPRIETARY
LOWPASS
402Ω
FILTER STAGE
200Ω
–
V
OP AMP
200Ω
200Ω
+
+
OCM
–
–
V
V
OCM
+
+
–
–
200Ω
402Ω
1
2
3
4
66002 BD
+
–
–
+
V
V
IN
IN
OCM
V
OUT
R
IN
66002f
6
LT6600-20
W U U
APPLICATIO S I FOR ATIO
U
Interfacing to the LT6600-20
output voltage is 1.65V, and the differential output voltage
is2VP-P forfrequenciesbelow20MHz.Thecommonmode
output voltage is determined by the voltage at pin 2. Since
pin 2 is shorted to pin 7, the output common mode is the
mid-supply voltage. In addition, the common mode input
voltage can be equal to the mid-supply voltage of Pin 7
(see the Distortion vs Input Common Mode Level graphs
in the Typical Performance Characteristics).
The LT6600-20 requires two equal external resistors, RIN,
to set the differential gain to 402Ω/RIN. The inputs to the
+
filter are the voltages VIN and VIN– presented to these
external components, Figure 1. The difference between
VIN+ and VIN– is the differential input voltage. The average
+
–
of VIN and VIN is the common mode input voltage.
Similarly, the voltages VOUT+ and VOUT– appearing at Pins
4 and 5 of the LT6600-20 are the filter outputs. The
Figure 2 shows how to AC couple signals into the
LT6600-20. In this instance, the input is a single-ended
signal. AC coupling allows the processing of single-ended
or differential signals with arbitrary common mode levels.
The 0.1µF coupling capacitor and the 402Ω gain setting
resistor form a high pass filter, attenuating signals below
4kHz. Larger values of coupling capacitors will propor-
tionally reduce this highpass 3dB frequency.
+
–
+
difference between VOUT and VOUT is the differential
–
output voltage. The average of VOUT and VOUT is the
common mode output voltage.
Figure 1 illustrates the LT6600-20 operating with a single
3.3V supply and unity passband gain; the input signal is
DCcoupled. Thecommonmodeinputvoltageis0.5V, and
the differential input voltage is 2VP-P. The common mode
3.3V
0.1µF
V
V
3
–
402Ω
3
2
1
0
3
2
1
0
1
7
2
8
–
+
V
V
4
IN
IN
+
–
+
–
V
V
+
OUT
OUT
V
OUT
LT6600-20
+
V
IN
0.01µF
V
OUT
–
5
+
402Ω
6
t
t
–
66002 F01
V
IN
Figure 1
3.3V
3
0.1µF
V
V
0.1µF
0.1µF
402Ω
402Ω
1
3
2
1
0
2
1
–
4
+
+
V
V
+
7
2
8
OUT
OUT
V
V
OUT
LT6600-20
+
–
V
0.01µF
IN
–
OUT
–
0
t
+
5
V
+
IN
6
–1
66002 F02
Figure 2
62pF
5V
0.1µF
V
V
3
100Ω
1
–
+
3
3
V
–
4
IN
+
–
+
V
V
OUT
+
7
OUT
OUT
LT6600-20
2
1
0
2
1
2
8
–
V
0.01µF
OUT
V
500mV (DIFF)
P-P
–
5
V
+
IN
+
V
V
100Ω
IN
6
t
t
0
–
+
66002 F03
2V
IN
–
62pF
Figure 3
66002f
7
LT6600-20
U
W U U
APPLICATIO S I FOR ATIO
In Figure 3 the LT6600-20 is providing 12dB of gain. The
gain resistor has an optional 62pF in parallel to improve
the passband flatness near 20MHz. The common mode
output voltage is set to 2V.
examples where impedance must be considered is the
evaluation of the LT6600-20 with a network analyzer.
Figure 5 is a laboratory setup that can be used to charac-
terizetheLT6600-20usingsingle-endedinstrumentswith
50Ω source impedance and 50Ω input impedance. For a
unity gain configuration the LT6600-20 requires a 402Ω
source resistance yet the network analyzer output is
calibrated for a 50Ω load resistance. The 1:1 transformer,
53.6Ω and 388Ω resistors satisfy the two constraints
above. The transformer converts the single-ended source
into a differential stimulus. Similarly, the output of the
LT6600-20 will have lower distortion with larger load
resistance yet the analyzer input is typically 50Ω. The 4:1
turns (16:1 impedance) transformer and the two 402Ω
resistors of Figure 5, present the output of the LT6600-20
with a 1600Ω differential load, or the equivalent of 800Ω
to ground at each output. The impedance seen by the
network analyzer input is still 50Ω, reducing reflections in
the cabling between the transformer and analyzer input.
Use Figure 4 to determine the interface between the
LT6600-20 and a current output DAC. The gain, or “trans-
impedance,” is defined as A = VOUT/IIN. To compute the
transimpedance, use the following equation:
402 •R1
A =
Ω
( )
R1+R2
(
)
By setting R1 + R2 = 402Ω, the gain equation reduces to
A = R1(Ω).
The voltage at the pins of the DAC is determined by R1,
R2, the voltage on Pin 7 and the DAC output current.
Consider Figure 4 with R1 = 49.9Ω and R2 = 348Ω. The
voltage at Pin 7 is 1.65V. The voltage at the DAC pins is
given by:
Differential and Common Mode Voltage Ranges
R1
R1•R2
R1+R2
V
DAC = VPIN7
•
+I •
IN
The differential amplifiers inside the LT6600-20 contain
circuitry to limit the maximum peak-to-peak differential
voltage through the filter. This limiting function prevents
excessive power dissipation in the internal circuitry
and provides output short-circuit protection. The limiting
function begins to take effect at output signal levels above
2VP-P and it becomes noticeable above 3.5VP-P. This is
illustrated in Figure 6; the LT6600-20 was configured with
unity passband gain and the input of the filter was driven
with a 1MHz signal. Because this voltage limiting takes
place well before the output stage of the filter reaches the
R1+R2 + 402
= 26mV +I • 48.3Ω
IN
IIN is IIN+ or IIN–. The transimpedance in this example is
50.4Ω.
Evaluating the LT6600-20
The low impedance levels and high frequency operation of
the LT6600-20 require some attention to the matching
networks between the LT6600-20 and other devices. The
previous examples assume an ideal (0Ω) source imped-
ance and a large (1kΩ) load resistance. Among practical
2.5V
0.1µF
CURRENT
OUTPUT
DAC
3.3V
COILCRAFT
TTWB-16A
4:1
COILCRAFT
TTWB-1010
NETWORK
ANALYZER
SOURCE
NETWORK
ANALYZER
INPUT
0.1µF
3
388Ω
1:1
1
7
2
8
402Ω
–
4
+
–
3
R2
R2
I
I
50Ω
IN
1
7
2
8
LT6600-20
–
4
53.6Ω
50Ω
+
–
V
V
+
OUT
R1
+
–
5
+
0.01µF
LT6600-20
402Ω
388Ω
6
0.1µF
–
IN
OUT
66002 F05
5
+
6
R1
66002 F04
–2.5V
Figure 5
Figure 4
66002f
8
LT6600-20
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APPLICATIO S I FOR ATIO
20
The LT6600-20 was designed to process a variety of input
signals including signals centered around the mid-supply
voltage and signals that swing between ground and a
positive voltage in a single supply system (Figure 1). The
range of allowable input common mode voltage (the
average of VIN and VIN in Figure 1) is determined by
the power supply level and gain setting (see Distortion vs
Input Common Mode Level in the Typical Performance
Characteristics).
1dB PASSBAND GAIN
COMPRESSION POINTS
1MHz 25°C
0
1MHz 85°C
–20
3RD HARMONIC
85°C
–40
3RD HARMONIC
+
–
25°C
–60
2ND HARMONIC
–80
25°C
2ND HARMONIC
85°C
–100
–120
4
6
7
0
1
2
3
5
Common Mode DC Currents
1MHz INPUT LEVEL (V
)
P-P
66002 F06
In applications like Figure 1 and Figure 3 where the
LT6600-20 not only provides lowpass filtering but also
level shifts the common mode voltage of the input signal,
DC currents will be generated through the DC path be-
tweeninputandoutputterminals.Minimizethesecurrents
to decrease power dissipation and distortion.
Figure 6. Output Level vs Input Level,
Differential 1MHz Input, Gain = 1
supply rails, the input/output behavior of the IC shown in
Figure 6 is relatively independent of the power supply
voltage.
Consider the application in Figure 3. Pin 7 sets the output
common mode voltage of the 1st differential amplifier
insidetheLT6600-20(seethe“BlockDiagram”section)at
2.5V. Since the input common mode voltage is near 0V,
there will be approximately a total of 2.5V drop across the
series combination of the internal 402Ω feedback resistor
and the external 100Ω input resistor. The resulting 5mA
common mode DC current in each input path, must be
The two amplifiers inside the LT6600-20 have indepen-
dent control of their output common mode voltage (see
the“blockdiagram”section). Thefollowingguidelineswill
optimize the performance of the filter.
Pin 7 must be bypassed to an AC ground with a 0.01µF or
largercapacitor. Pin7canbedrivenfromalowimpedance
source, provided it remains at least 1.5V above V– and at
least 1.5V below V+. An internal resistor divider sets the
voltage of Pin 7. While the internal 11k resistors are well
matched, their absolute value can vary by ±20%. This
should be taken into consideration when connecting an
external resistor network to alter the voltage of Pin 7.
+
absorbed by the sources VIN and VIN–. Pin 2 sets the
common mode output voltage of the 2nd differential
amplifier inside the LT6600-20, and therefore sets the
common mode output voltage of the filter. Since, in the
example of Figure 3, Pin 2 differs from Pin 7 by 0.5V, an
additional2.5mA(1.25mAperside) ofDCcurrentwillflow
in the resistors coupling the 1st differential amplifier
output stage to filter output. Thus, a total of 12.5mA is
used to translate the common mode voltages.
Pin 2 can be shorted to Pin 7 for simplicity. If a different
common mode output voltage is required, connect Pin 2
to a voltage source or resistor network. For 3V and 3.3V
supplies the voltage at Pin 2 must be less than or equal to
the mid supply level. For example, voltage (Pin 2) ≤ 1.65V
on a single 3.3V supply. For power supply voltages higher
than 3.3V the voltage at Pin 2 should be within the voltage
of Pin 7 – 1V to the voltage of Pin 7 + 2V. Pin 2 is a high
impedance input.
A simple modification to Figure 3 will reduce the DC
common mode currents by 36%. If Pin 7 is shorted to
Pin 2 the common mode output voltage of both op amp
stages will be 2V and the resulting DC current will be 8mA.
Of course, by AC coupling the inputs of Figure 3, the
common mode DC current can be reduced to 2.5mA.
66002f
9
LT6600-20
U
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APPLICATIO S I FOR ATIO
2.5V
Noise
0.1µF
SPECTRUM
ANALYZER
INPUT
ThenoiseperformanceoftheLT6600-20canbeevaluated
with the circuit of Figure 7.
COILCRAFT
TTWB-1010
1:1
R
R
3
IN
IN
V
IN
1
7
2
8
25Ω
–
4
+
LT6600-20
Given the low noise output of the LT6600-20 and the 6dB
attenuation of the transformer coupling network, it is
necessary to measure the noise floor of the spectrum
analyzer and subtract the instrument noise from the filter
noise measurement.
50Ω
–
5
+
25Ω
0.1µF
6
66002 F07
–2.5V
Example: With the IC removed and the 25Ω resistors
grounded, Figure 7, measure the total integrated noise
(eS) of the spectrum analyzer from 10kHz to 20MHz. With
the IC inserted, the signal source (VIN) disconnected, and
the input resistors grounded, measure the total integrated
noise out of the filter (eO). With the signal source con-
nected, set the frequency to 1MHz and adjust the ampli-
tude until VIN measures 100mVP-P. Measure the output
amplitude, VOUT, and compute the passband gain
A = VOUT/VIN. Now compute the input referred integrated
noise (eIN) as:
Figure 7
50
250
V
= 5V
S
40
30
20
10
0
200
150
100
50
SPECTRAL DENSITY
INTEGRATED
0
0.1
1
10
100
(eO)2 –(eS)2
FREQUENCY (MHz)
66002 F08
eIN =
A
Figure 8. Input Referred Noise, Gain = 1
Table 1 lists the typical input referred integrated noise for
various values of RIN.
noisepoweraddedtogether,theresultingcalculatednoise
level will be higher than the true differential noise.
Figure 8 is plot of the noise spectral density as a function
of frequency for an LT6600-20 with RIN = 402Ω using the
fixture of Figure 7 (the instrument noise has been sub-
tracted from the results).
Power Dissipation
The LT6600-20 amplifiers combine high speed with large-
signal currents in a small package. There is a need to
ensure that the die junction temperature does not exceed
150°C. TheLT6600-20packagehasPin6fusedtothelead
frame to enhance thermal conduction when connecting to
a ground plane or a large metal trace. Metal trace and
plated through-holes can be used to spread the heat
generated by the device to the backside of the PC board.
Forexample,ona3/32"FR-4boardwith2ozcopper,atotal
of 660 square millimeters connected to Pin 6 of the
LT6600-20(330squaremillimetersoneachsideofthePC
board) will result in a thermal resistance, θJA, of about
85°C/W. WithouttheextrametaltraceconnectedtotheV–
pin to provide a heat sink, the thermal resistance will be
around 105°C/W. Table 2 can be used as a guide when
Table 1. Noise Performance
INPUT REFERRED
PASSBAND
GAIN (V/V)
INTEGRATED NOISE
10kHz TO 20MHz
INPUT REFERRED
NOISE dBm/Hz
R
IN
4
2
1
100Ω
200Ω
402Ω
42µV
–148
–143
–139
RMS
67µV
RMS
118µV
RMS
The noise at each output is comprised of a differential
component and a common mode component. Using a
transformerorcombinertoconvertthedifferentialoutputs
tosingle-endedsignalrejectsthecommonmodenoiseand
gives a true measure of the S/N achievable in the system.
Conversely,ifeachoutputismeasuredindividuallyandthe
considering thermal resistance.
66002f
10
LT6600-20
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APPLICATIO S I FOR ATIO
Table 2. LT6600-20 SO-8 Package Thermal Resistance
For a given supply voltage, the worst-case power dissi-
pation occurs when the differential input signal is maxi-
mum, the common mode currents are maximum (see
Applications Information regarding common mode DC
currents), the load impedance is small and the ambient
temperature is maximum. To compute the junction tem-
perature, measure the supply current under these worst-
case conditions, estimate the thermal resistance from
Table2, thenapplytheequationforTJ. Forexample, using
the circuit in Figure 3 with a DC differential input voltage
of 250mV, a differential output voltage of 1V, no load
resistance and an ambient temperature of 85°C, the
supply current (current into Pin 3) measures 55.5mA.
Assuming a PC board layout with a 35mm2 copper trace,
theθJA is100°C/W. Theresultingjunctiontemperatureis:
COPPER AREA
TOPSIDE BACKSIDE BOARD AREA
THERMAL RESISTANCE
(JUNCTION-TO-AMBIENT)
2
2
2
(mm )
(mm )
(mm )
1100
330
35
1100
330
35
2500
2500
2500
2500
2500
65°C/W
85°C/W
95°C/W
35
0
100°C/W
0
0
105°C/W
Junction temperature, TJ, is calculated from the ambient
temperature, TA, and power dissipation, PD. The power
dissipation is the product of supply voltage, VS, and
supply current, IS. Therefore, the junction temperature is
given by:
TJ = TA + (PD • θJA) = TA + (VS • IS • θJA)
TJ = TA + (PD • θJA) = 85 + (5 • 0.0555 • 100) = 113°C
where the supply current, IS, is a function of signal level,
load impedance, temperature and common mode
voltages.
When using higher supply voltages or when driving small
impedances, more copper may be necessary to keep TJ
below 150°C.
U
PACKAGE DESCRIPTIO
S8 Package
8-Lead Plastic Small Outline (Narrow .150 Inch)
(Reference LTC DWG # 05-08-1610)
.189 – .197
(4.801 – 5.004)
.045 ±.005
NOTE 3
.050 BSC
7
5
8
6
.245
MIN
.160 ±.005
.150 – .157
(3.810 – 3.988)
NOTE 3
.228 – .244
(5.791 – 6.197)
.030 ±.005
TYP
1
3
4
2
RECOMMENDED SOLDER PAD LAYOUT
.010 – .020
(0.254 – 0.508)
× 45°
.053 – .069
(1.346 – 1.752)
.004 – .010
(0.101 – 0.254)
.008 – .010
(0.203 – 0.254)
0°– 8° TYP
.016 – .050
(0.406 – 1.270)
.050
(1.270)
BSC
.014 – .019
(0.355 – 0.483)
TYP
NOTE:
INCHES
1. DIMENSIONS IN
(MILLIMETERS)
2. DRAWING NOT TO SCALE
3. THESE DIMENSIONS DO NOT INCLUDE MOLD FLASH OR PROTRUSIONS.
MOLD FLASH OR PROTRUSIONS SHALL NOT EXCEED .006" (0.15mm)
SO8 0303
66002f
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.
11
LT6600-20
U
TYPICAL APPLICATIO
A 5th Order, 20MHz Lowpass Filter
+
V
0.1µF
3
R
R
R
1
7
2
8
–
+
V
V
–
IN
IN
4
+
–
V
V
+
OUT
OUT
LT6600-20
C
R
–
5
+
0.1µF
6
1
C =
2π • R • 20MHz
66002 TA02a
402Ω
2R
–
GAIN =
, MAXIMUM GAIN = 4
V
Amplitude Response
Transient Response, Gain = 1
10
0
–10
–20
–30
–40
–50
–60
–70
–80
–90
+
VOUT
50mV/DIV
DIFFERENTIAL
INPUT
200mV/DIV
V
= ±2.5V
S
GAIN = 1
C = 39pF
R = 200Ω
T
A
= 25°C
100ns/DIV
66002 TA03
0.1
1
10
100
FREQUENCY (MHz)
66002 TA04
RELATED PARTS
PART NUMBER
LTC®1565-31
LTC1566-1
LT1567
DESCRIPTION
COMMENTS
650kHz Linear Phase Lowpass Filter
Low Noise, 2.3MHz Lowpass Filter
Continuous Time, SO8 Package, Fully Differential
Continuous Time, SO8 Package
Very Low Noise, High Frequency Filter Building Block
Very Low Noise, 4th Order Building Block
1.4nV/√Hz Op Amp, MSOP Package, Fully Differential
LT1568
Lowpass and Bandpass Filter Designs Up to 10MHz,
Differential Outputs
LT6600-2.5
LT6600-10
Very Low Noise Differential Amplifier and 2.5MHz
Lowpass Filter
86dB S/N with 3V Supply, SO-8
Very Low Noise Differential Amplifier and 10MHz
Lowpass Filter
82dB S/N with 3V Supply, SO-8
66002f
LT/TP 0503 1K • PRINTED IN USA
12 LinearTechnology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
●
●
(408) 432-1900 FAX: (408) 434-0507 www.linear.com
LINEAR TECHNOLOGY CORPORATION 2003
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