LINER LT6600IS8-20 Very low noise, differential amplifier and 20mhz lowpass filter Datasheet

LT6600-20
Very Low Noise, Differential
Amplifier and 20MHz Lowpass Filter
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FEATURES
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DESCRIPTIO
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.
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
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APPLICATIO S
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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
, LTC and LT are registered trademarks of Linear Technology Corporation.
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 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.
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TYPICAL APPLICATIO
An 8192 Point FFT Spectrum
A/D
LTC1748
LT6600-20
5V
0.1µF
7
0.01µF
VIN
2
8
RIN
402Ω
3
–
VMID
VOCM
+
+
–
4
49.9Ω
49.9Ω
5
6
V+
+
18pF
DOUT
AIN
–
VCM
INPUT 5.9MHz
2VP-P
fSAMPLE = 80MHz
–20
5V
AMPLITUDE (dB)
RIN
402Ω 1
0
–10
–30
–40
–50
–60
–70
–80
–90
V–
–100
–110
1µF
–120
GAIN = 402Ω/RIN
66002 TA01a
0
10
20
30
40
FREQUENCY (MHz)
66002 TA01b
66002f
1
LT6600-20
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ABSOLUTE
AXI U RATI GS
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PACKAGE/ORDER I FOR ATIO
(Note 1)
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
NUMBER
TOP VIEW
IN – 1
8
IN +
VOCM 2
7
VMID
V+ 3
6
V–
OUT + 4
5
OUT –
LT6600CS8-20
LT6600IS8-20
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, VS = 3V
Filter Gain, VS = 5V
Filter Gain, VS = ±5V
Filter Gain, RIN = 100Ω
Filter Gain Temperature Coefficient (Note 2)
Noise
Distortion (Note 4)
Differential Output Swing
Input Bias Current
CONDITIONS
VIN = 2VP-P, fIN = DC to 260kHz
VIN = 2VP-P, fIN = 2MHz (Gain Relative to 260kHz)
VIN = 2VP-P, fIN = 10MHz (Gain Relative to 260kHz)
VIN = 2VP-P, fIN = 16MHz (Gain Relative to 260kHz)
VIN = 2VP-P, fIN = 20MHz (Gain Relative to 260kHz)
VIN = 2VP-P, fIN = 60MHz (Gain Relative to 260kHz)
VIN = 2VP-P, fIN = 100MHz (Gain Relative to 260kHz)
VIN = 2VP-P, fIN = DC to 260kHz
VIN = 2VP-P, fIN = 2MHz (Gain Relative to 260kHz)
VIN = 2VP-P, fIN = 10MHz (Gain Relative to 260kHz)
VIN = 2VP-P, fIN = 16MHz (Gain Relative to 260kHz)
VIN = 2VP-P, fIN = 20MHz (Gain Relative to 260kHz)
VIN = 2VP-P, fIN = 60MHz (Gain Relative to 260kHz)
VIN = 2VP-P, fIN = 100MHz (Gain Relative to 260kHz)
VIN = 2VP-P, fIN = DC to 260kHz
VIN = 2VP-P, fIN = DC to 260kHz, VS = 3V
VIN = 2VP-P, fIN = DC to 260kHz, VS = 5V
VIN = 2VP-P, fIN = DC to 260kHz, VS = ±5V
fIN = 250kHz, VIN = 2VP-P
Noise BW = 10kHz to 20MHz
2.5MHz, 2VP-P, RL = 800Ω
2nd Harmonic
3rd Harmonic
20MHz, 2VP-P, RL = 800Ω
2nd Harmonic
3rd Harmonic
Measured Between Pins 4 and 5
VS = 5V
VS = 3V
Average of Pin 1 and Pin 8
●
●
●
●
MIN
– 0.4
– 0.1
– 0.2
– 0.1
– 0.8
●
●
●
●
●
●
– 0.5
– 0.1
– 0.2
– 0.3
– 1.3
●
●
– 0.6
11.6
11.5
11.4
●
●
●
3.80
3.75
– 95
TYP
0.1
0
0.1
0.5
0
– 33
– 50
0
0
0.1
0.4
–0.4
– 33
– 50
– 0.1
12.1
12.0
11.9
780
118
83
88
63
64
4.75
4.50
– 50
MAX
0.5
0.1
0.5
1.9
1
– 28
0.5
0.1
0.4
1.6
0.6
–28
0.4
12.6
12.5
12.4
UNITS
dB
dB
dB
dB
dB
dB
dB
dB
dB
dB
dB
dB
dB
dB
dB
dB
dB
dB
ppm/C
µVRMS
dBc
dBc
dBc
dBc
VP-P DIFF
VP-P DIFF
µA
66002f
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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
Input Referred Differential Offset
CONDITIONS
RIN = 402Ω
RIN = 100Ω
Differential Offset Drift
Input Common Mode Voltage (Note 3)
Output Common Mode Voltage (Note 5)
Differential Input = 500mVP-P,
RIN = 100Ω
Differential Input = 2VP-P,
Pin 7 at Mid-Supply
Common Mode Voltage at Pin 2
Output Common Mode Offset
(with Respect to Pin 2)
Common Mode Rejection Ratio
Voltage at VMID (Pin 7)
VMID Input Resistance
VOCM Bias Current
VOCM = VMID= VS/2
Power Supply Current
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
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.
MIN
VS = 3V
VS = 5V
VS = ±5V
VS = 3V
VS = 5V
VS = ±5V
●
●
●
VS = 3V
VS = 5V
VS = ±5V
VS = 3V
VS = 5V
VS = ±5V
VS = 3V
VS = 5V
VS = ±5V
●
●
●
●
●
●
0.0
0.0
–2.5
1.0
1.5
–1.0
–35
–40
–55
VS = 5
VS = 3
●
2.46
●
4.35
–15
–10
VS = 5V
VS = 3V
VS = 3V, VS = 5
VS = 3V, VS = 5
VS = ±5V
●
●
●
●
●
●
●
●
●
●
TYP
5
10
10
5
5
5
10
5
0
–5
66
2.51
1.5
5.7
–3
–3
42
46
MAX
25
30
35
15
17
20
1.5
3.0
1.0
1.5
3.0
2.0
40
40
35
2.55
7.65
46
53
56
UNITS
mV
mV
mV
mV
mV
mV
µV/°C
V
V
V
V
V
V
mV
mV
mV
dB
V
V
kΩ
µA
µA
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.
66002f
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LT6600-20
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TYPICAL PERFOR A CE CHARACTERISTICS
Passband Gain and Phase
0
–10
–2
–20
–4
GAIN (dB)
–30
–40
–50
–60
–70
–80
VS = 5V
GAIN = 1
TA = 25°C
–90
0.1
1
10
FREQUENCY (MHz)
PHASE
–130
–175
–12
GAIN (dB)
4
–8
20
15
–14
–265
–14
10
–16
–310
–16
5
6.5
100
50
45
40
30
25
–18
0.5
–355
30.5
18.5
24.5
12.5
FREQUENCY (MHz)
6.5
Common Mode Rejection Ratio
80
VS = 5V
GAIN = 1
TA = 25°C
70
65
10
60
55
50
–2
10
40
–4
5
35
1
0.1
0.1
45
30
1
10
FREQUENCY (MHz)
–40
60
50
40
30
20
–60
–70
10
–100
0.1
1
FREQUENCY (MHz)
10
100
66002 G07
0.1
3RD HARMONIC
VS = 3V
10MHz INPUT
RL = 800Ω AT
EACH OUTPUT
GAIN = 1
2ND
TA = 25°C
HARMONIC
10MHz INPUT
–50
VIN = 2VP-P
VS = 3V
RL = 800Ω AT
EACH OUTPUT
GAIN = 1
TA = 25°C
–80
–90
0.01
–40
DIFFERENTIAL INPUT,
2ND HARMONIC
DIFFERENTIAL INPUT,
3RD HARMONIC
SINGLE-ENDED INPUT,
2ND HARMONIC
SINGLE-ENDED INPUT,
3RD HARMONIC
–50
DISTORTION (dB)
70
0
0.001
Distortion vs Signal Level,
VS = 3V
Distortion vs Frequency
V + TO DIFFOUT
VS = 3V
TA = 25°C
80
100
66002 G06
1
10
FREQUENCY (MHz)
100
66002 G08
DISTORTION (dB)
Power Supply Rejection Ratio
90
1
10
FREQUENCY (MHz)
0.1
100
66002 G05
66002 G04
100
INPUT = 1VP-P
VS = 5V
GAIN = 1
TA = 25°C
75
15
0
30.5
0
30.5
18.5
24.5
12.5
FREQUENCY (MHz)
66002 G03
20
18.5
24.5
12.5
FREQUENCY (MHz)
25
–12
0
6.5
30
–220
2
–6
0.5
PSRR (dB)
35
CMRR (dB)
GROUP
DELAY
40
GROUP
DELAY
–6
66002 G02
35
45
–10
GROUP DELAY (ns)
8
6
–4
–8
OUTPUT IMPEDANCE (Ω)
10
–40
50
VS = 5V
GAIN = 1
TA = 25°C
GAIN
Output Impedance
VS = 5V
GAIN = 4
TA = 25°C
GAIN
–2
–10
Passband Gain and Group Delay
12
0
5
–85
66002 G01
14
50
–6
–18
0.5
100
VS = 5V
GAIN = 1
TA = 25°C
GAIN
GROUP DELAY (ns)
0
Passband Gain and Group Delay
2
95
PHASE (DEG)
GAIN (dB)
2
GAIN (dB)
Amplitude Response
10
–60
–70
3RD
HARMONIC
1MHz INPUT
–80
2ND HARMONIC
1MHz INPUT
–90
–100
0
1
2
3
INPUT LEVEL (VP-P)
4
5
66002 G09
66002f
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LT6600-20
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TYPICAL PERFOR A CE CHARACTERISTICS
–40
–40
–60
–70
–80
DISTORTION COMPONENT (dB)
2ND HARMONIC,
10MHz INPUT
3RD HARMONIC,
10MHz INPUT
2ND HARMONIC,
1MHz INPUT
3RD HARMONIC,
1MHz INPUT
–50
DISTORTION (dB)
Distortion
vs Input Common Mode Level
VS = ±5V
RL = 800Ω AT
EACH OUTPUT
GAIN = 1
TA = 25°C
–90
–100
2
3
INPUT LEVEL (VP-P)
4
–50
–60
–70
2VP-P 1MHz INPUT
RL = 800Ω AT
EACH OUTPUT
GAIN = 1
TA = 25°C
–80
–90
–100
–3
–2
–1
0
1
2
3
INPUT COMMON MODE VOTLAGE RELATIVE TO PIN 7 (V)
5
66002 G10
–40
–50
–60
–70
2ND HARMONIC,
VS = 3V
3RD HARMONIC,
VS = 3V
2ND HARMONIC,
VS = 5V
3RD HARMONIC,
VS = 5V
–80
–90
–100
500mVP-P 1MHz INPUT, GAIN = 4,
RL = 800Ω AT EACH OUTPUT
–3
–2
–1
0
1
2
3
INPUT COMMON MODE VOTLAGE RELATIVE TO PIN 7 (V)
66002 G11
66002 G12
Distortion
vs Output Common Mode
–40
DISTORTION COMPONENT (dB)
1
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)
2
66002 G13
Total Supply Current
vs Total Supply Voltage
Transient Response, Gain = 1
60
TOTAL SUPPLY CURRENT (mA)
0
2ND HARMONIC,
VS = 3V
3RD HARMONIC,
VS = 3V
2ND HARMONIC,
VS = 5V
3RD HARMONIC,
VS = 5V
Distortion
vs Input Common Mode Level
DISTORTION COMPONENT (dB)
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
66002f
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LT6600-20
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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.
VOCM (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
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.
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BLOCK DIAGRA
VIN+
RIN
IN +
VMID
8
7
V+
V–
OUT –
6
5
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
66002f
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LT6600-20
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APPLICATIO S I FOR ATIO
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).
Interfacing to the LT6600-20
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
0
VIN–
2
0.01µF
+
VIN
t
3
–
7
2
V
3
+
4
VOUT+
LT6600-20
8
–5
+
402Ω
VOUT–
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
2
0.01µF
8
+
V
3
–
+
4
LT6600-20
–
+
402Ω
5
3
VOUT+
2
VOUT–
1
6
VOUT+
VOUT–
0
66002 F02
Figure 2
62pF
5V
0.1µF
V
–
3
100Ω
1
VIN
7
2
1
0
0.01µF
500mVP-P (DIFF)
VIN+
VIN–
+
VIN
2
8
–
+
–
+
4
LT6600-20
–
+
100Ω
t
V
3
6
2V
5
3
VOUT+
VOUT+
2
VOUT–
1
0
VOUT–
t
66002 F03
62pF
Figure 3
66002f
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LT6600-20
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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.
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:
A=
402 • R1
(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
VDAC = VPIN7 •
+ IIN •
R1 + R2 + 402
R1 + R2
= 26mV + IIN • 48.3Ω
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 impedance and a large (1kΩ) load resistance. Among practical
CURRENT
OUTPUT
DAC
3.3V
R1
R2
7
0.01µF
3
– +
8
R2
–
+
4
2.5V
0.1µF
COILCRAFT
TTWB-1010
1:1 388Ω 1
7
VOUT+
5
VOUT–
53.6Ω
2
8
388Ω
3
–
+
4
COILCRAFT
TTWB-16A
4:1
402Ω
LT6600-20
–
+
6
5
NETWORK
ANALYZER
INPUT
50Ω
402Ω
0.1µF
66002 F05
6
66002 F04
Figure 4
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
50Ω
2 LT6600-20
IIN+
R1
1
Differential and Common Mode Voltage Ranges
NETWORK
ANALYZER
SOURCE
0.1µF
IIN–
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 characterize the LT6600-20 using single-ended instruments with
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.
– 2.5V
Figure 5
66002f
8
LT6600-20
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APPLICATIO S I FOR ATIO
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).
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
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.
66002f
9
LT6600-20
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APPLICATIO S I FOR ATIO
2.5V
Noise
0.1µF
The noise performance of the LT6600-20 can be evaluated
with the circuit of Figure 7.
RIN
VIN
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
– +
4
LT6600-20
50Ω
–
+
5
0.1µF
6
25Ω
66002 F07
– 2.5V
Figure 7
NOISE SPECTRAL DENSITY (nVRMS/√Hz)
50
250
VS = 5V
40
200
30
150
SPECTRAL DENSITY
20
100
10
0
0.1
eIN =
(eO )2 – (eS )2
A
10
0
100
66002 F08
Figure 8. Input Referred Noise, Gain = 1
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
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
10
1
FREQUENCY (MHz)
Table 1 lists the typical input referred integrated noise for
various values of RIN.
PASSBAND
GAIN (V/V)
50
INTEGRATED
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 integrated
noise (eIN) as:
1
SPECTRUM
ANALYZER
INPUT
COILCRAFT
TTWB-1010
25Ω
1:1
3
noise power added together, the resulting calculated noise
level will be higher than the true differential noise.
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.
66002f
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 dissipation occurs when the differential input signal is maximum, 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 temperature, measure the supply current under these worstcase 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:
COPPER AREA
TOPSIDE
(mm2)
BACKSIDE
(mm2)
BOARD AREA
(mm2)
THERMAL RESISTANCE
(JUNCTION-TO-AMBIENT)
1100
330
1100
2500
65°C/W
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.
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PACKAGE DESCRIPTIO
S8 Package
8-Lead Plastic Small Outline (Narrow .150 Inch)
(Reference LTC DWG # 05-08-1610)
.189 – .197
(4.801 – 5.004)
NOTE 3
.045 ±.005
.050 BSC
8
.245
MIN
7
6
5
.160 ±.005
.150 – .157
(3.810 – 3.988)
NOTE 3
.228 – .244
(5.791 – 6.197)
.030 ±.005
TYP
1
RECOMMENDED SOLDER PAD LAYOUT
.010 – .020
× 45°
(0.254 – 0.508)
.008 – .010
(0.203 – 0.254)
0°– 8° TYP
.016 – .050
(0.406 – 1.270)
NOTE:
1. DIMENSIONS IN
.053 – .069
(1.346 – 1.752)
.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
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 representation that the interconnection of its circuits as described herein will not infringe on existing patent rights.
11
LT6600-20
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TYPICAL APPLICATIO
A 5th Order, 20MHz Lowpass Filter
V+
0.1µF
VIN–
VIN+
C=
R
R
1
7
R
C
2
R
8
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
–50
–60
–70
–80
DIFFERENTIAL
INPUT
200mV/DIV
VS = ±2.5V
GAIN = 1
C = 39pF
R = 200Ω
TA = 25°C
–90
0.1
10
1
FREQUENCY (MHz)
100ns/DIV
100
66002 TA03
66002 TA04
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82dB S/N with 3V Supply, SO-8
66002f
12
Linear Technology Corporation
LT/TP 0503 1K • PRINTED IN USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900
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