LT6600-20
Very Low Noise, Differential
Amplifier and 20MHz Lowpass Filter
FEATURES
DESCRIPTION
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The LT®6600-20 combines a fully differential amplifier with a
4th order 20MHz lowpass filter approximating a Chebyshev
frequency response. Most differential amplifiers require
many precision external components to tailor gain and
bandwidth. In contrast, with the LT6600-20, two external
resistors program differential gain, and the filter’s 20MHz
cutoff frequency and passband ripple are internally set.
The LT6600-20 also provides the necessary level shifting
to set its output common mode voltage to accommodate
the reference voltage requirements of A/Ds.
n
n
n
n
n
n
n
n
Programmable Differential Gain via Two External
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
SO-8 Package
Compatible with Popular Differential Amplifier
Pinouts
APPLICATIONS
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High Speed ADC Antialiasing and DAC Smoothing in
Networking or Cellular Base Station Applications
High Speed Test and Measurement Equipment
Medical Imaging
Drop-in Replacement for Differential Amplifiers
L, LT, LTC and LTM are registered trademarks of Linear Technology Corporation.
All other trademarks are the property of their respective owners.
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 signalto-noise ratio is an impressive 76dB. At higher gains the
input referred noise decreases so the part can process
smaller input differential signals without significantly
degrading the output signal-to-noise ratio.
The LT6600-20 also features low voltage operation. The
differential design provides outstanding performance for
a 2VP-P signal level while the part operates with a single
3V supply.
The LT6600-20 is packaged in an SO-8 and is pin compatible with stand alone differential amplifiers.
TYPICAL APPLICATION
An 8192 Point FFT Spectrum
5V
0.1μF
RIN
402Ω 1
7
0.01μF
VIN
RIN
402Ω
2
8
3
–
VMID
VOCM
+
+
–
4
49.9Ω
49.9Ω
5
6
5V
DOUT
AIN
–
–20
–30
–40
V+
+
18pF
0
–10
A/D
LTC1748
INPUT 5.9MHz
2VP-P
fSAMPLE = 80MHz
–50
–60
–70
–80
–90
V–
VCM
AMPLITUDE (dB)
LT6600-20
–100
–110
1μF
–120
GAIN = 402Ω/RIN
66002 TA01a
0
10
20
30
40
FREQUENCY (MHz)
66002 TA01b
66002fb
1
LT6600-20
ABSOLUTE MAXIMUM RATINGS
PIN CONFIGURATION
(Note 1)
TOP VIEW
Total Supply Voltage .................................................11V
Input Current (Note 8)..........................................±10mA
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
IN– 1
8
IN+
VOCM 2
7
VMID
V+ 3
6
V–
OUT+ 4
5
OUT–
S8 PACKAGE
8-LEAD PLASTIC SO
TJMAX = 150°C, θJA = 100°C/W
ORDER INFORMATION
LEAD FREE FINISH
TAPE AND REEL
PART MARKING
PACKAGE DESCRIPTION
TEMPERATURE RANGE
LT6600CS8-20#PBF
LT6600CS8-20#TRPBF
660020
8-Lead Plastic SO
–40°C to 85°C
LT6600IS8-20#PBF
LT6600IS8-20#TRPBF
600I20
8-Lead Plastic SO
–40°C to 85°C
LEAD BASED FINISH
TAPE AND REEL
PART MARKING
PACKAGE DESCRIPTION
TEMPERATURE RANGE
LT6600CS8-20
LT6600CS8-20#TR
660020
8-Lead Plastic SO
–40°C to 85°C
LT6600IS8-20
LT6600IS8-20#TR
600I20
8-Lead Plastic SO
–40°C to 85°C
Consult LTC Marketing for parts specified with wider operating temperature ranges.
For more information on lead free part marking, go to: http://www.linear.com/leadfree/
For more information on tape and reel specifications, go to: http://www.linear.com/tapeandreel/
ELECTRICAL CHARACTERISTICS
The l denotes the specifications which 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
Filter Gain, VS = 3V
VIN = 2VP-P, fIN = DC to 260kHz
–0.4
0.1
0.5
dB
Filter Gain, VS = 5V
Filter Gain, VS = ±5V
UNITS
VIN = 2VP-P, fIN = 2MHz (Gain Relative to 260kHz)
l
–0.1
0
0.1
dB
VIN = 2VP-P, fIN = 10MHz (Gain Relative to 260kHz)
l
–0.2
0.1
0.5
dB
VIN = 2VP-P, fIN = 16MHz (Gain Relative to 260kHz)
l
–0.1
0.5
1.9
dB
VIN = 2VP-P, fIN = 20MHz (Gain Relative to 260kHz)
l
–0.8
0
1
dB
VIN = 2VP-P, fIN = 60MHz (Gain Relative to 260kHz)
l
–33
–28
dB
VIN = 2VP-P, fIN = 100MHz (Gain Relative to 260kHz)
l
–50
VIN = 2VP-P, fIN = DC to 260kHz
dB
–0.5
0
0.5
dB
VIN = 2VP-P, fIN = 2MHz (Gain Relative to 260kHz)
l
–0.1
0
0.1
dB
VIN = 2VP-P, fIN = 10MHz (Gain Relative to 260kHz)
l
–0.2
0.1
0.4
dB
VIN = 2VP-P, fIN = 16MHz (Gain Relative to 260kHz)
l
–0.3
0.4
1.6
dB
VIN = 2VP-P, fIN = 20MHz (Gain Relative to 260kHz)
l
–1.3
–0.4
0.6
dB
VIN = 2VP-P, fIN = 60MHz (Gain Relative to 260kHz)
l
–33
–28
VIN = 2VP-P, fIN = 100MHz (Gain Relative to 260kHz)
l
–50
VIN = 2VP-P, fIN = DC to 260kHz
–0.6
–0.1
dB
dB
0.4
dB
66002fb
2
LT6600-20
ELECTRICAL CHARACTERISTICS
The l denotes the specifications which 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
Filter Gain, RIN = 100
VIN = 0.5VP-P, fIN = DC to 260kHz, VS = 3V
VIN = 0.5VP-P, fIN = DC to 260kHz, VS = 5V
VIN = 0.5VP-P, fIN = DC to 260kHz, VS = 5V
11.6
11.5
11.4
12.1
12.0
11.9
12.6
12.5
12.4
dB
dB
dB
Filter Gain Temperature Coefficient (Note 2)
fIN = 250kHz, VIN = 2VP-P
780
ppm/C
Noise
Noise BW = 10kHz to 20MHz
118
μVRMS
Distortion (Note 4)
2.5MHz, 2VP-P, RL = 800Ω
2nd Harmonic
3rd Harmonic
83
88
dBc
dBc
20MHz, 2VP-P, RL = 800Ω
2nd Harmonic
3rd Harmonic
63
64
dBc
dBc
Differential Output Swing
Measured Between Pins 4 and 5
VS = 5V
VS = 3V
Input Bias Current
Average of Pin 1 and Pin 8
Input Referred Differential Offset
RIN = 402Ω
VS = 3V
VS = 5V
VS = ±5V
l
l
l
5
10
10
25
30
35
mV
mV
mV
RIN = 100Ω
VS = 3V
VS = 5V
VS = ±5V
l
l
l
5
5
5
15
17
20
mV
mV
mV
l
l
3.80
3.75
4.75
4.50
VP-P DIFF
VP-P DIFF
l
–95
–50
μA
Differential Offset Drift
10
μV/°C
Input Common Mode Voltage (Note 3)
Differential Input = 500mVP-P,
RIN = 100Ω
VS = 3V
VS = 5V
VS = ±5V
l
l
l
0.0
0.0
–2.5
1.5
3.0
1.0
V
V
V
Output Common Mode Voltage (Note 5)
Differential Input = 2VP-P,
Pin 7 = OPEN
Common Mode Voltage at Pin 2
VS = 3V
VS = 5V
VS = ±5V
l
l
l
1.0
1.5
–1.0
1.5
3.0
2.0
V
V
V
VS = 3V
VS = 5V
VS = ±5V
l
l
l
–35
–40
–55
40
40
35
mV
mV
mV
VS = 5
VS = 3
l
2.46
2.51
1.5
2.55
V
V
7.65
kΩ
Output Common Mode Offset
(with Respect to Pin 2)
Common Mode Rejection Ratio
66
Voltage at VMID (Pin 7)
l
4.35
5.7
VS = 5V
VS = 3V
l
l
–15
–10
–3
–3
VS = 3V, VS = 5
VS = 3V, VS = 5
VS = ±5V
l
l
VMID Input Resistance
VOCM Bias Current
5
0
–5
VOCM = VMID = VS/2
Power Supply Current
Note 1: Stresses beyond those listed under Absolute Maximum Ratings
may cause permanent damage to the device. Exposure to any Absolute
Maximum Rating condition for extended periods may affect device
reliability and lifetime.
Note 2: This is the temperature coefficient of the internal feedback
resistors assuming a temperature independent external resistor (RIN).
Note 3: The input common mode voltage is the average of the voltages
applied to the external resistors (RIN). Specification guaranteed for RIN ≥
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.
42
46
dB
μA
μA
46
53
56
mA
mA
mA
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.
Note 6: The LT6600C-20 is guaranteed functional over the operating
temperature range –40°C to 85°C.
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.
Note 8: The inputs are protected by back-to-back diodes. If the differential
input voltage exceeds 1.4V, the input current should be limited to less than
10mA.
66002fb
3
LT6600-20
TYPICAL PERFORMANCE CHARACTERISTICS
Passband Gain and Phase
0
0
–10
–2
–20
–4
–30
–6
–85
–8
–130
GAIN (dB)
2
–40
–50
VS = 5V
GAIN = 1
TA = 25°C
GAIN
–40
PHASE
–175
–12
–220
–70
–14
–265
–16
–310
VS = 5V
GAIN = 1
TA = 25°C
1
10
FREQUENCY (MHz)
–18
0.5
100
6.5
18.5
24.5
12.5
FREQUENCY (MHz)
66002 G01
GAIN (dB)
40
10
35
8
30
25
35
GROUP
DELAY
6
4
30
25
0
15
10
–2
10
5
–4
5
–12
15
–14
–16
–6
0.5
0
30.5
18.5
24.5
12.5
FREQUENCY (MHz)
6.5
INPUT = 1VP-P
VS = 5V
GAIN = 1
TA = 25°C
60
60
50
50
40
45
30
40
20
35
10
30
1
10
FREQUENCY (MHz)
100
66002 G06
0
0.001
DIFFERENTIAL INPUT,
2ND HARMONIC
DIFFERENTIAL INPUT,
3RD HARMONIC
SINGLE-ENDED INPUT,
2ND HARMONIC
SINGLE-ENDED INPUT,
3RD HARMONIC
–50
DISTORTION (dB)
70
–60
–70
VIN = 2VP-P
VS = 3V
RL = 800Ω AT
EACH OUTPUT
GAIN = 1
TA = 25°C
–80
–90
–100
0.01
0.1
1
FREQUENCY (MHz)
10
100
Distortion vs Frequency
–40
VS = 3V
TA = 25°C
80
55
1
10
FREQUENCY (MHz)
66002 G05
V+ TO DIFFOUT
90
65
0.1
1
Power Supply Rejection Ratio
100
PSRR (dB)
CMRR (dB)
70
10
66002 G04
Common Mode Rejection Ratio
75
VS = 5V
GAIN = 1
TA = 25°C
0.1
0.1
0
30.5
18.5
24.5
12.5
FREQUENCY (MHz)
66002 G03
80
40
20
20
6.5
45
2
–10
–18
0.5
VS = 5V
GAIN = 4
TA = 25°C
GAIN
GROUP DELAY (ns)
GROUP
DELAY
12
GROUP DELAY (ns)
–4
45
Output Impedance
100
50
OUTPUT IMPEDANCE (Ω)
–2
14
GAIN (dB)
VS = 5V
GAIN = 1
TA = 25°C
GAIN
0
50
–355
30.5
66002 G02
Passband Gain and Group Delay
Passband Gain and Group Delay
–8
5
–10
–90
0.1
–6
50
–60
–80
2
95
PHASE (DEG)
GAIN (dB)
Amplitude Response
10
100
66002 G07
0.1
1
10
FREQUENCY (MHz)
100
66002 G08
66002fb
4
LT6600-20
TYPICAL PERFORMANCE CHARACTERISTICS
Distortion
vs Signal Level, VS = 3V
–60
–70
3RD
HARMONIC
1MHz INPUT
–80
2ND HARMONIC
1MHz INPUT
–90
2
3
INPUT LEVEL (VP-P)
5
4
–70
–80
VS = ±5V
RL = 800Ω AT
EACH OUTPUT
GAIN = 1
TA = 25°C
–100
0
1
2
3
INPUT LEVEL (VP-P)
4
66002 G09
–60
–70
–80
–90
500mVP-P 1MHz INPUT, GAIN = 4,
RL = 800Ω AT EACH OUTPUT
–100
–70
–80
–90
–100
5
–3
3
–2
–1
0
1
2
INPUT COMMON MODE VOLTAGE
RELATIVE TO PIN 7 (V)
66002 G11
2ND HARMONIC,
VS = 3V
3RD HARMONIC,
VS = 3V
2ND HARMONIC,
VS = 5V
3RD HARMONIC,
VS = 5V
2ND HARMONIC,
VS = ±5V
3RD HARMONIC,
VS = ±5V
–50
–60
–70
–80
–90
–100
2VP-P 1MHz INPUT, GAIN = 1,
RL = 800Ω AT EACH OUTPUT
–110
–2 –1.5 –1 –0.5 0 0.5 1 1.5
VOLTAGE PIN 2 TO PIN 7 (V)
3
–2
–1
0
1
2
INPUT COMMON MODE VOLTAGE
RELATIVE TO PIN 7 (V)
66002 G12
–3
–60
–40
2ND HARMONIC,
VS = 3V
3RD HARMONIC,
VS = 3V
2ND HARMONIC,
VS = 5V
3RD HARMONIC,
VS = 5V
–50
–50
2VP-P 1MHz INPUT
RL = 800Ω AT
EACH OUTPUT
GAIN = 1
TA = 25°C
Distortion
vs Output Common Mode
DISTORTION COMPONENT (dB)
DISTORTION COMPONENT (dB)
–40
2ND HARMONIC,
VS = 3V
3RD HARMONIC,
VS = 3V
2ND HARMONIC,
VS = 5V
3RD HARMONIC,
VS = 5V
66002 G10
Distortion
vs Input Common Mode Level
2
66002 G13
Total Supply Current
vs Total Supply Voltage
Transient Response, Gain = 1
60
TOTAL SUPPLY CURRENT (mA)
1
–60
–90
–100
0
–40
2ND HARMONIC,
10MHz INPUT
3RD HARMONIC,
10MHz INPUT
2ND HARMONIC,
1MHz INPUT
3RD HARMONIC,
1MHz INPUT
–50
DISTORTION (dB)
–50
DISTORTION (dB)
–40
3RD HARMONIC
VS = 3V
10MHz INPUT
RL = 800Ω AT
EACH OUTPUT
GAIN = 1
2ND
TA = 25°C
HARMONIC
10MHz INPUT
Distortion
vs Input Common Mode Level
DISTORTION COMPONENT (dB)
–40
Distortion
vs Signal Level, VS = ±5V
50
TA = 85°C
40
TA = 25°C
30
TA = –40°C
VOUT+
50mV/DIV
DIFFERENTIAL
INPUT
200mV/DIV
20
10
100ns/DIV
66002 G15
0
0
1
2 3 4 5 6 7 8
TOTAL SUPPLY VOLTAGE (V)
9
10
66002 G14
66002fb
5
LT6600-20
PIN FUNCTIONS
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.
VOCM (Pin 2): Is the DC Common Mode Reference Voltagefor 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 ceramic
capacitor unless it is connected to a ground plane.
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 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.
VMID (Pin 7): The VMID pin is internally biased at midsupply, 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.
BLOCK DIAGRAM
VIN+
RIN
IN+
8
VMID
V–
OUT–
7
6
5
V+
11k
PROPRIETARY
LOWPASS
FILTER STAGE
402Ω
11k
200Ω
V–
OP AMP
+
200Ω
+ –
–
VOCM
–
VOCM
+
– +
200Ω
200Ω
402Ω
1
VIN–
RIN
IN–
2
3
4
VOCM
V+
OUT+
66002 BD
66002fb
6
LT6600-20
APPLICATIONS INFORMATION
Interfacing to the LT6600-20
output voltage is 1.65V, and the differential output voltage
is 2VP-P for frequencies below 20MHz. The common mode
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 section).
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
difference between VOUT+ and VOUT– is the differential
output voltage. The average of VOUT+ and VOUT– is the
common mode output voltage.
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 proportionally reduce this highpass 3dB frequency.
Figure 1 illustrates the LT6600-20 operating with a single
3.3V supply and unity passband gain; the input signal is
DC coupled. The common mode input voltage is 0.5V, and
the differential input voltage is 2VP-P. The common mode
3.3V
0.1μF
V
3
–
402Ω
1
VIN
VIN+
1
2
0.01μF
VIN
0
VIN–
t
4
+
VOUT+
LT6600-20
8
+
3
–
7
2
V
3
+
402Ω
VOUT–
–5
6
2
VOUT+
1
VOUT–
t
0
66002 F01
Figure 1
3.3V
0.1μF
V
0.1μF
2
402Ω
1
–
7
1
VIN+
0
0.1μF
t
+
VIN
–1
+
3
4
VOUT+
LT6600-20
2
0.01μF
V
3
8
–
+
402Ω
VOUT–
5
6
2
1
VOUT+
VOUT–
0
66002 F02
Figure 2
62pF
5V
0.1μF
V
–
3
100Ω
1
VIN
7
2
1
0
VIN+
VIN–
2
0.01μF
500mVP-P (DIFF)
8
+
VIN
–
+
–
+
4
LT6600-20
–
+
100Ω
t
V
3
6
2V
5
3
VOUT+
VOUT+
2
VOUT–
1
0
VOUT–
66002 F03
t
62pF
Figure 3
66002fb
7
LT6600-20
APPLICATIONS INFORMATION
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.
Use Figure 4 to determine the interface between the
LT6600-20 and a current output DAC. The gain, or “transimpedance,” 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:
R1
R1• R2
+IIN •
R1+ R2 + 402
R1+ R2
= 26mV +IIN • 48.3Ω
VDAC = VPIN7 •
amples 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 characterize
the LT6600-20 using single-ended instruments with 50
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.
Differential and Common Mode Voltage Ranges
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
ofthe LT6600-20 require some attention to the matching
networks between the LT6600-20 and other devices. The
previous examples assume an ideal (0Ω) source impedance
and a large (1kΩ) load resistance. Among practical ex-
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
2.5V
CURRENT
OUTPUT
DAC
0.1μF
3.3V
NETWORK
ANALYZER
SOURCE
0.1μF
IIN–
R1
R2
0.01μF
IIN+
R1
1
3
7
– +
2
LT6600-20
8
+
R2
–
4
5
VOUT
+
50Ω
COILCRAFT
TTWB-1010
1:1 388Ω 1
7
53.6Ω
2
8
VOUT–
388Ω
3
–
+
4
COILCRAFT
TTWB-16A
4:1
402Ω
LT6600-20
–
+
6
5
0.1μF
6
NETWORK
ANALYZER
INPUT
50Ω
402Ω
66002 F05
66002 F04
–2.5V
Figure 4
Figure 5
66002fb
8
LT6600-20
APPLICATIONS INFORMATION
20
OUTPUT LEVEL (dBV)
0
1dB PASSBAND GAIN
COMPRESSION POINTS
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 section).
1MHz 25°C
1MHz 85°C
–20
3RD HARMONIC
85°C
–40
3RD HARMONIC
25°C
–60
2ND HARMONIC
25°C
–80
2ND HARMONIC
85°C
–100
–120
0
1
4
3
5
2
1MHz INPUT LEVEL (VP-P)
6
7
66002 F06
Figure 6. Output Level vs Input Level,
Differential 1MHz Input, Gain = 1
place well before the output stage of the filter reaches the
supply rails, the input/output behavior of the IC shown
in Figure 6 is relatively independent of the power supply
voltage.
The two amplifiers inside the LT6600-20 have independent control of their output common mode voltage (see
the Block Diagram section). The following guidelines will
optimize the performance of the filter.
Pin 7 must be bypassed to an AC ground with a 0.01μF or
larger capacitor. Pin 7 can be driven from a low impedance
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.
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.
Common Mode DC Currents
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 between
input and output terminals. Minimize these currents to
decrease power dissipation and distortion.
Consider the application in Figure 3. Pin 7 sets the output
common mode voltage of the 1st differential amplifier inside
the LT6600-20 (see the Block Diagram 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 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 additional 2.5mA (1.25mA
per side) of DC current will flow 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.
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.
66002fb
9
LT6600-20
APPLICATIONS INFORMATION
Noise
2.5V
0.1μF
The noise performance of the LT6600-20 can be evaluated
with the circuit of Figure 7.
VIN
RIN
1
7
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.
2
8
RIN
COILCRAFT
TTWB-1010
25Ω
1:1
3
– +
4
SPECTRUM
ANALYZER
INPUT
LT6600-20
50Ω
–
+
6
5
0.1μF
25Ω
66002 F07
–2.5V
eIN =
2
(eO ) – (eS )
A
Table 1 lists the typical input referred integrated noise forvarious values of RIN. 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 subtracted from the results).
Table 1. Noise Performance
RIN
INPUT REFERRED
INTEGRATED NOISE
10kHz TO 20MHz
INPUT REFERRED
NOISE dBm/Hz
4
100Ω
42μVRMS
–148
2
200Ω
67μVRMS
–143
1
402Ω
118μVRMS
–139
PASSBAND
GAIN (V/V)
The noise at each output is comprised of a differential
component and a common mode component. Using a
transformer or combiner to convert the differential outputs
to single-ended signal rejects the common mode noise and
gives a true measure of the S/N achievable in the system.
Conversely, if each output is measured individually and the
noise power added together, the resulting calculated noise
level will be higher than the true differential noise.
50
NOISE SPECTRAL DENSITY (nVRMS/√Hz)
2
Figure 7
250
VS = 5V
200
40
150
30
SPECTRAL DENSITY
20
100
10
50
INTEGRATED
0
0.1
1
10
INTEGRATED NOISE (μVRMS)
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 connected,
set the frequency to 1MHz and adjust the amplitude until
VIN measures 100mVP-P. Measure the output amplitude,
VOUT, and compute the passband gain A = VOUT/VIN. Now
compute the input referred integratednoise (eIN) as:
0
100
FREQUENCY (MHz)
66002 F08
Figure 8. Input Referred Noise, Gain = 1
Power Dissipation
The LT6600-20 amplifiers combine high speed with largesignal currents in a small package. There is a need to
ensure that the die junction temperature does not exceed
150°C. The LT6600-20 package has Pin 6 fused to the lead
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. For example,
on a 3/32" FR-4 board with 2oz copper, a total of 660
square millimeters connected to Pin 6 of the LT6600-20
(330 square millimeters on each side of the PC board)
will result in a thermal resistance, θJA, of about 85°C/W.
Without the extra metal trace connected to the V– pin to
provide a heat sink, the thermal resistance will be around
105°C/W. Table 2 can be used as a guide when considering
thermal resistance.
66002fb
10
LT6600-20
APPLICATIONS INFORMATION
For a given supply voltage, the worst-case power dissipation
occurs when the differential input signal is maximum, the
common mode currents are maximum (see the Applications
Information section regarding common mode DC currents),
the load impedance is small and the ambient temperature is
maximum. To compute the junction temperature, measure
the supply current under these worst-case conditions, estimate the thermal resistance from Table 2, then apply the
equation for TJ. For example, 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, the θJA is 100°C/W. The resulting
junction temperature is:
Table 2. LT6600-20 SO-8 Package Thermal Resistance
COPPER AREA
TOPSIDE
(mm2)
BACKSIDE
(mm2)
BOARD AREA
(mm2)
THERMAL RESISTANCE
(JUNCTION-TO-AMBIENT)
1100
1100
2500
65°C/W
330
330
2500
85°C/W
35
35
2500
95°C/W
35
0
2500
100°C/W
0
0
2500
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.
PACKAGE DESCRIPTION
S8 Package
8-Lead Plastic Small Outline (Narrow .150 Inch)
(Reference LTC DWG # 05-08-1610)
.050 BSC
.189 – .197
(4.801 – 5.004)
NOTE 3
.045 ±.005
8
.245
MIN
.160 ±.005
.010 – .020
× 45°
(0.254 – 0.508)
NOTE:
1. DIMENSIONS IN
5
.150 – .157
(3.810 – 3.988)
NOTE 3
1
RECOMMENDED SOLDER PAD LAYOUT
.053 – .069
(1.346 – 1.752)
0°– 8° TYP
.016 – .050
(0.406 – 1.270)
6
.228 – .244
(5.791 – 6.197)
.030 ±.005
TYP
.008 – .010
(0.203 – 0.254)
7
.014 – .019
(0.355 – 0.483)
TYP
INCHES
(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)
2
3
4
.004 – .010
(0.101 – 0.254)
.050
(1.270)
BSC
SO8 0303
66002fb
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 representation that the interconnection of its circuits as described herein will not infringe on existing patent rights.
11
LT6600-20
TYPICAL APPLICATION
A 5th Order, 20MHz Lowpass Filter
V+
0.1μF
VIN–
VIN+
C=
R
R
1
7
C
2
R
8
R
1
2π • R • 20MHz
3
–
+
4
VOUT+
LT6600-20
–
+
5
0.1μF
6
GAIN = 402Ω , MAXIMUM GAIN = 4 V–
2R
Amplitude Response
VOUT–
66002 TA02a
Transient Response, Gain = 1
10
0
–10
GAIN (dB)
–20
VOUT+
50mV/DIV
–30
–40
DIFFERENTIAL
INPUT
200mV/DIV
–50
–60
–70
–80
VS = ±2.5V
GAIN = 1
C = 39pF
R = 200Ω
TA = 25°C
–90
0.1
66002 TA03
100ns/DIV
10
1
FREQUENCY (MHz)
100
66002 TA04
RELATED PARTS
PART NUMBER
DESCRIPTION
COMMENTS
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66002fb
12 Linear Technology Corporation
LT 0409 REV B • PRINTED IN USA
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