LINER LT6600CS8-10

LT6600-10
Very Low Noise, Differential
Amplifier and 10MHz Lowpass Filter
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FEATURES
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DESCRIPTIO
The LT®6600-10 combines a fully differential amplifier
with a 4th order 10MHz lowpass filter approximating a
Chebyshev frequency response. Most differential amplifiers require many precision external components to tailor
gain and bandwidth. In contrast, with the LT6600-10, two
external resistors program differential gain, and the filter’s
10MHz cutoff frequency and passband ripple are internally
set. The LT6600-10 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 10MHz
Cutoff
82dB S/N with 3V Supply and 2VP-P Output
Low Distortion, 2VP-P, 800Ω Load
1MHz: 88dBc 2nd, 97dBc 3rd
5MHz: 74dBc 2nd, 77dBc 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 LT6600-10
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 82dB. 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 LT6600-10 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.
For similar devices with other cutoff frequencies, refer to
the LT6600-20 and LT6600-2.5.
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TYPICAL APPLICATIO
An 8192 Point FFT Spectrum
0
LT6600-10
–20
0.1µF
7
0.01µF
VIN
2
8
RIN
402Ω
3
–
VMID
VOCM
+
+
–
4
5V
49.9Ω
49.9Ω
5
6
–30
V+
+
18pF
AIN
LTC1748
–
VCM
DOUT
FREQUENCY (dB)
RIN
402Ω 1
INPUT IS A 4.7MHz SINEWAVE
2VP-P
fSAMPLE = 66MHz
–10
5V
–40
–50
–60
–70
–80
V–
–90
–100
1µF
GAIN = 402Ω/RIN
–110
6600 TA01a
0
4
8
12 16 20 24
FREQUENCY (MHz)
28
32
6600 TA01b
6600f
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LT6600-10
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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 –
LT6600CS8-10
LT6600IS8-10
S8 PART MARKING
S8 PACKAGE
8-LEAD PLASTIC SO
660010
600I10
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
CONDITIONS
MIN
VIN = 2VP-P, fIN = DC to 260kHz
TYP
MAX
UNITS
– 0.4
0
0.5
dB
VIN = 2VP-P, fIN = 1MHz (Gain Relative to 260kHz)
●
– 0.1
0
0.1
dB
VIN = 2VP-P, fIN = 5MHz (Gain Relative to 260kHz)
●
– 0.4
– 0.1
0.3
dB
VIN = 2VP-P, fIN = 8MHz (Gain Relative to 260kHz)
●
– 0.3
0.1
1
dB
VIN = 2VP-P, fIN = 10MHz (Gain Relative to 260kHz)
●
–0.2
0.3
1.7
dB
VIN = 2VP-P, fIN = 30MHz (Gain Relative to 260kHz)
●
– 28
– 25
dB
VIN = 2VP-P, fIN = 50MHz (Gain Relative to 260kHz)
●
– 44
VIN = 2VP-P, fIN = DC to 260kHz
– 0.5
dB
0
0.5
dB
VIN = 2VP-P, fIN = 1MHz (Gain Relative to 260kHz)
●
– 0.1
0
0.1
dB
VIN = 2VP-P, fIN = 5MHz (Gain Relative to 260kHz)
●
– 0.4
– 0.1
0.3
dB
VIN = 2VP-P, fIN = 8MHz (Gain Relative to 260kHz)
●
– 0.4
0.1
0.9
dB
VIN = 2VP-P, fIN = 10MHz (Gain Relative to 260kHz)
●
– 0.3
0.2
1.4
dB
VIN = 2VP-P, fIN = 30MHz (Gain Relative to 260kHz)
●
– 28
–25
dB
VIN = 2VP-P, fIN = 50MHz (Gain Relative to 260kHz)
●
– 44
dB
Filter Gain, VS = ±5V
VIN = 2VP-P, fIN = DC to 260kHz
– 0.6
– 0.1
0.4
dB
Filter Gain, RIN = 100Ω, VS = 3V, 5V, ±5V
VIN = 2VP-P, fIN = DC to 260kHz
11.4
12
12.6
dB
Filter Gain Temperature Coefficient (Note 2)
fIN = 260kHz, VIN = 2VP-P
Noise
Noise BW = 10kHz to 10MHz, RIN = 402Ω
Distortion (Note 4)
1MHz, 2VP-P, RL = 800Ω
2nd Harmonic
3rd Harmonic
5MHz, 2VP-P, RL = 800Ω
2nd Harmonic
3rd Harmonic
74
77
dBc
dBc
Differential Output Swing
Measured Between Pins 4 and 5
Pin 7 Shorted to Pin 2
VS = 5V
VS = 3V
Input Bias Current
Average of Pin 1 and Pin 8
780
ppm/C
56
µVRMS
88
97
dBc
dBc
●
●
3.85
3.85
5.0
4.9
VP-P DIFF
VP-P DIFF
●
– 85
– 40
µA
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LT6600-10
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
Input Referred Differential Offset
RIN = 402Ω
VS = 3V
VS = 5V
VS = ±5V
RIN = 100Ω
VS = 3V
VS = 5V
VS = ±5V
TYP
MAX
●
●
●
5
10
8
20
30
35
mV
mV
mV
●
●
●
5
5
5
13
22
30
mV
mV
mV
Differential Offset Drift
UNITS
µV/°C
10
Input Common Mode Voltage (Note 3)
Differential Input = 500mVP-P,
RIN = 100Ω
VS = 3V
VS = 5V
VS = ±5V
●
●
●
0.0
0.0
–2.5
1.5
3.0
1.0
V
V
V
Output Common Mode Voltage (Note 5)
Differential Output = 2VP-P,
Pin 7 at Midsupply
VS = 3V
VS = 5V
VS = ±5V
●
●
●
1.0
1.5
–1.0
1.5
3.0
2.0
V
V
V
VS = 3V
VS = 5V
VS = ±5V
●
●
●
–35
–40
–55
40
40
35
mV
mV
mV
Output Common Mode Offset
(with respect to Pin 2)
Common Mode Rejection Ratio
61
Voltage at VMID (Pin 7)
VS = 5
VS = 3
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.
dB
●
2.46
2.51
1.5
2.55
●
4.3
5.5
7.7
VS = 5V
VS = 3V
●
●
–15
–10
–3
–3
VS = 3V, VS = 5
VS = 3V, VS = 5
VS = ±5V
●
●
VMID Input Resistance
VOCM Bias Current
5
0
–5
35
36
V
V
kΩ
µA
µA
39
43
46
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 is guaranteed functional over the operating
temperature range –40°C to 85°C.
Note 7: The LT6600C 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 is guaranteed to
meet specified performance from –40°C to 85°C.
6600f
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LT6600-10
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TYPICAL PERFOR A CE CHARACTERISTICS
Amplitude Response
Passband Gain and Group Delay
10
VS = 5V
GAIN = 1
0
60
0
55
–1
50
–2
45
–3
40
–4
35
–5
30
–6
25
–7 V = 5V
S
–8 GAIN = 1
TA = 25°C
–9
0.5
20
GAIN (dB)
–30
–40
(
GAIN 20LOG
–20
–50
–60
–70
–80
100k
1M
10M
FREQUENCY (Hz)
100M
15
10
14.9
5.3
10.1
FREQUENCY (MHz)
6600 G01
60
11
55
10
50
9
45
8
40
7
35
6
30
5
25
4 V = 5V
S
3 GAIN = 4
TA = 25°C
2
0.5
20
45
15
40
80
10
65
10
14.9
5.3
10.1
FREQUENCY (MHz)
VS = 5V
75 GAIN = 1
VIN = 1VP-P
70 TA = 25°C
CMRR (dB)
OUTPUT IMPEDANCE (Ω)
100
0.1
100k
50
1M
10M
FREQUENCY (Hz)
–40
DISTORTION (dB)
50
40
30
VS = 3V
VIN = 200mVP-P
TA = 25°C
V + TO DIFFOUT
1k
10k
100k
1M
FREQUENCY (Hz)
–60
–70
–40
–80
6600 G06
–60
–70
–80
–90
–100
–100
100M
DIFFERENTIAL INPUT,
2ND HARMONIC
DIFFERENTIAL INPUT,
3RD HARMONIC
SINGLE-ENDED INPUT,
2ND HARMONIC
SINGLE-ENDED INPUT,
3RD HARMONIC
–50
–90
10M
10M
6600 G05
DISTORTION (dB)
–50
70
60
100k
1M
FREQUENCY (Hz)
Distortion vs Frequency
VIN = 2VP-P, VS = ±5V, RL = 800Ω
at Each Output, TA = 25°C
DIFFERENTIAL INPUT,
2ND HARMONIC
DIFFERENTIAL INPUT,
3RD HARMONIC
SINGLE-ENDED INPUT,
2ND HARMONIC
SINGLE-ENDED INPUT,
3RD HARMONIC
80
0
35
10k
100M
Distortion vs Frequency
VIN = 2VP-P, VS = 3V, RL = 800Ω
at Each Output, TA = 25°C
Power Supply Rejection Ratio
20
55
6600 G04
90
10
60
1
6600 G03
PSRR (dB)
Common Mode Rejection Ratio
12
GROUP DELAY (ns)
GAIN (dB)
6600 G02
Output Impedance vs Frequency
(OUT + or OUT –)
Passband Gain and Group Delay
GROUP DELAY (ns)
DIFFOUT
DIFFIN
)
–10
1
0.1
1
FREQUENCY (MHz)
10
6600 G07
0.1
1
FREQUENCY (MHz)
10
6600 G08
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LT6600-10
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TYPICAL PERFOR A CE CHARACTERISTICS
–40
–60
–70
–60
–80
–70
–80
–90
–90
–100
–110
–100
0
1
2
3
INPUT LEVEL (VP-P)
4
1
0
5
2
3
–70
–80
–90
–100
–3
–1
0
1
2
–2
INPUT COMMON MODE VOLTAGE
RELATIVE TO PIN 7 (V)
3
6600 G11
Transient Response,
Differential Gain = 1
40
POWER SUPPLY CURRENT (mA)
DISTORTION COMPONENT (dB)
–60
Power Supply Current
vs Power Supply Voltage
2ND HARMONIC,
VS = 3V
3RD HARMONIC,
VS = 3V
2ND HARMONIC,
VS = 5V
3RD HARMONIC,
VS = 5V
–60
–50
6600 G10
Distortion vs Input Common Mode
Level, 0.5VP-P, 1MHz Input, 4x
Gain, RL = 800Ω at Each Output,
TA = 25°C
–50
5
4
2ND HARMONIC,
VS = 3V
3RD HARMONIC,
VS = 3V
2ND HARMONIC,
VS = 5V
3RD HARMONIC,
VS = 5V
INPUT LEVEL (VP-P)
6600 G09
–40
Distortion vs Input Common Mode
Level, 2VP-P, 1MHz Input, 1x Gain,
RL = 800Ω at Each Output, TA = 25°C
–40
2ND HARMONIC,
5MHz INPUT
3RD HARMONIC,
5MHz INPUT
2ND HARMONIC,
1MHz INPUT
3RD HARMONIC,
1MHZ INPUT
–50
DISTORTION (dB)
DISTORTION (dB)
–40
2ND HARMONIC,
5MHz INPUT
3RD HARMONIC,
5MHz INPUT
2ND HARMONIC,
1MHz INPUT
3RD HARMONIC,
1MHZ INPUT
–50
Distortion vs Signal Level
VS = ±5V, RL = 800Ω at Each Output,
TA = 25°C
DISTORTION COMPONENT (dB)
Distortion vs Signal Level
VS = 3V, RL = 800Ω at Each Output,
TA = 25°C
–70
–80
–90
38
TA = 85°C
VOUT+
50mV/DIV
36
34
TA = 25°C
DIFFERENTIAL
INPUT
200mV/DIV
32
30
TA = –40°C
28
100ns/DIV
6600 G13
26
–3
2
–1
0
1
–2
INPUT COMMON MODE VOLTAGE
RELATIVE TO PIN 7 (V)
24
3
2
6600 G12
3
6
7
4
5
8
9
TOTAL SUPPLY VOLTAGE (V)
10
6600 G14
Distortion vs Output Common Mode,
2VP-P 1MHz Input, 1x Gain, TA = 25°C
–40
DISTORTION COMPONENT (dB)
–100
–50
–60
2ND HARMONIC, VS = 3V
3RD HARMONIC, VS = 3V
2ND HARMONIC, VS = 5V
3RD HARMONIC, VS = 5V
2ND HARMONIC, VS = ±5V
3RD HARMONIC, VS = ±5V
–70
–80
–90
–100
–1
0
0.5
1
1.5
–0.5
OUTPUT COMMON MODE VOLTAGE (V)
2
6600 G15
6600f
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LT6600-10
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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.
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 to AC ground.
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.
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.
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
W
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 +
6600 BD
6600f
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LT6600-10
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APPLICATIO S I FOR ATIO
Interfacing to the LT6600-10
is 2VP-P for frequencies below 10MHz. 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
(refer to the Distortion vs Input Common Mode Level
graphs in the Typical Performance Characteristics).
The LT6600-10 requires 2 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 LT6600-10 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
LT6600-10. 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 LT6600-10 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
output voltage is 1.65V and the differential output voltage
In Figure 3 the LT6600-10 is providing 12dB of gain. The
gain resistor has an optional 62pF in parallel to improve
3.3V
0.1µF
V
3
–
402Ω
1
VIN
7
2
VIN+
1
2
0.01µF
8
+
VIN
0
VIN–
3
–
+
4
VOUT+
LT6600-10
–5
+
402Ω
t
V
3
VOUT–
6
2
VOUT+
1
VOUT–
t
0
6600 F01
Figure 1
3.3V
0.1µF
V
0.1µF
2
402Ω
1
7
1
VIN+
0
0.1µF
t
VIN
–1
0.01µF
2
8
+
V
3
–
3
4
+
VOUT+
LT6600-10
–
+
402Ω
2
VOUT–
5
1
6
VOUT+
VOUT–
0
6600 F02
Figure 2
62pF
5V
0.1µF
V
–
3
100Ω
1
VIN
7
2
1
0
VIN+
VIN–
2
0.01µF
500mVP-P (DIFF)
+
VIN
100Ω
t
8
+
–
V
3
–
+
4
LT6600-10
–
+
6
2V
5
3
VOUT+
VOUT+
2
VOUT–
1
0
VOUT–
t
6600 F03
0.01µF
62pF
Figure 3
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LT6600-10
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APPLICATIO S I FOR ATIO
the passband flatness near 10MHz. The common mode
output voltage is set to 2V.
Use Figure 4 to determine the interface between the
LT6600-10 and a current output DAC. The gain, or “transimpedance”, is defined as A = VOUT/IIN Ω. 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 = R1Ω.
The voltage at the pins of the DAC is determined by R1,
R2, the voltage on Pin 7 and the DAC output current (IIN+
or IIN–). 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:
Figure 5 is a laboratory setup that can be used to characterize the LT6600-10 using single-ended instruments with
50Ω source impedance and 50Ω input impedance. For a
unity gain configuration the LT6600-10 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 the
LT6600-10 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 LT6600-10
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
0.1µF
R1
R1 • R2
VDAC = VPIN7 •
+ IIN
R1 + R2 + 402
R1 + R2
= 103mV + IIN 43.6Ω
NETWORK
ANALYZER
SOURCE
IIN is IIN– or IIN+.The transimpedance in this example is
50.4Ω.
COILCRAFT
TTWB-1010
1:1 388Ω 1
7
50Ω
53.6Ω
2
8
388Ω
3
–
+
4
COILCRAFT
TTWB-16A
4:1
402Ω
NETWORK
ANALYZER
INPUT
LT6600-10
–
+
6
5
50Ω
402Ω
0.1µF
6600 F05
CURRENT
OUTPUT
DAC
3.3V
0.1µF
IIN–
R2
R1
IIN+
1
7
0.01µF
R2
– 2.5V
Figure 5
3
–
+
4
VOUT+
2 LT6600-10
8
–
+
5
VOUT–
6
R1
6600 F04
Figure 4
Evaluating the LT6600-10
The low impedance levels and high frequency operation of
the LT6600-10 require some attention to the matching
networks between the LT6600-10 and other devices. The
previous examples assume an ideal (0Ω) source impedance and a large (1kΩ) load resistance. Among practical
examples where impedance must be considered is the
evaluation of the LT6600-10 with a network analyzer.
Differential and Common Mode Voltage Ranges
The differential amplifiers inside the LT6600-10 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 LTC6600-10 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 supply rails, the input/output behavior of the
IC shown in Figure 6 is relatively independent of the power
supply voltage.
6600f
8
LT6600-10
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APPLICATIO S I FOR ATIO
20
OUTPUT LEVEL (dBV)
0
1dB PASSBAND GAIN
COMPRESSION POINTS
average of VIN+ and VIN– in Figure 1) is determined by
the power supply level and gain setting (see “Electrical
Characteristics”).
1MHz 25°C
1MHz 85°C
–20
3RD HARMONIC
85°C
–40
Common Mode DC Currents
3RD HARMONIC
25°C
2ND HARMONIC
85°C
–60
–80
2ND HARMONIC
25°C
–100
–120
0
1
4
3
5
2
1MHz INPUT LEVEL (VP-P)
6
6600 F06
Figure 6
The two amplifiers inside the LT6600-10 have independent control of their output common mode voltage (see
the “block diagram” section). The following guidelines will
optimize the performance of the filter for single supply
operation.
Pin 7 must be bypassed to an AC ground with a 0.01µF or
higher 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 can be set above mid supply.
The voltage on Pin 2 should not be more than 1V below the
voltage on Pin 7. The voltage on Pin 2 should not be more
than 2V above the voltage on PIn 7. Pin 2 is a high
impedance input.
The LT6600-10 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
In applications like Figure 1 and Figure 3 where the
LT6600-10 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 LT6600-10 (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 LT6600-10, and therefore sets the
common mode output voltage of the filter. Since in the
example, 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.
Noise
The noise performance of the LT6600-10 can be evaluated
with the circuit of Figure 7.
Given the low noise output of the LT6600-10 and the 6dB
attenuation of the transformer coupling network, it will be
necessary to measure the noise floor of the spectrum
analyzer and subtract the instrument noise from the filter
noise measurement.
6600f
9
LT6600-10
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APPLICATIO S I FOR ATIO
2.5V
RIN
1
7
2
8
RIN
3
– +
4
SPECTRUM
ANALYZER
INPUT
LT6600-10
50Ω
–
+
6
5
0.1µF
25Ω
6600 F07
35
140
30
120
25
100
80
20
15
SPECTRAL DENSITY
10
60
40
INTEGRATED
NOISE
5
INTEGRATED NOISE (µVRMS)
VIN
COILCRAFT
TTWB-1010
25Ω
1:1
SPECTRAL DENSITY (nVRMS/√Hz)
0.1µF
20
– 2.5V
0
Figure 7
0.1
Example: With the IC removed and the 25Ω resistors
grounded, measure the total integrated noise (eS) of the
spectrum analyzer from 10kHz to 10MHz. 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:
eIN =
(eO )2 – (eS )2
A
Table 1 lists the typical input referred integrated noise for
various values of RIN.
Figure 8 is plot of the noise spectral density as a function
of frequency for an LT6600-10 with RIN = 402Ω using the
fixture of Figure 7 (the instrument noise has been subtracted from the results).
Table 1. Noise Performance
PASSBAND
GAIN (V/V)
4
RIN
INPUT REFERRED
INTEGRATED NOISE
10kHz TO 10MHz
INPUT REFERRED
NOISE dBm/Hz
100Ω
24µVRMS
–149
2
200Ω
34µVRMS
–146
1
402Ω
56µVRMS
–142
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
1.0
10
FREQUENCY (MHz)
0
100
6600 F08
Figure 8
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.
Power Dissipation
The LT6600-10 amplifiers combine high speed with largesignal currents in a small package. There is a need to
ensure that the dies’s junction temperature does not
exceed 150°C. The LT6600-10 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 LT6600-10 (330 square millimeters on each side
of the PC board) will result in a thermal resistance, θJA, of
about 85°C/W. Without extra metal trace connected to the
Table 2. LT6600-10 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
6600f
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LT6600-10
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APPLICATIO S I FOR ATIO
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.
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 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 48.9mA. Assuming
a PC board layout with a 35mm2 copper trace, the θJA is
100°C/W. The resulting junction temperature is:
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)
where the supply current, IS, is a function of signal level,
load impedance, temperature and common mode voltages.
TJ = TA + (PD • θJA) = 85 + (5 • 0.0489 • 100) = 109°C
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
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)
3
4
.053 – .069
(1.346 – 1.752)
.004 – .010
(0.101 – 0.254)
0°– 8° TYP
.016 – .050
(0.406 – 1.270)
NOTE:
1. DIMENSIONS IN
2
.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)
.050
(1.270)
BSC
SO8 0303
6600f
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
LT6600-10
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TYPICAL APPLICATIO S
5th Order, 10MHz Lowpass Filter
Amplitude Response
0.1µF
R
R
VIN+
C=
1
–
7
C
R
0
3
–10
+
4
LT6600-10
2
8
–
+
5
0.1µF
6
1
2π • R • 10MHz
VOUT+
VOUT
–20
GAIN (dB)
R
VIN–
Transient Response
5th Order, 10MHz Lowpass Filter
Differential Gain = 1
10
V+
–
VOUT+
50mV/DIV
–30
–40
DIFFERENTIAL
INPUT
200mV/DIV
–50
–60
6600 TA02a
GAIN = 402Ω , MAXIMUM GAIN = 4 V –
2R
DIFFERENTIAL GAIN = 1
–70 R = 200Ω
C = 82pF
–80
1M
10M
100k
FREQUENCY (Hz)
100ns/DIV
6600 TA02c
100M
6600 TA02b
A WCDMA Transmit Filter
(10MHz Lowpass Filter with a 28MHz Notch)
Amplitude Response
22
33pF
V+
12
0.1µF
VIN–
33pF
1µH
VIN+
100Ω
1
RQ
301Ω
27pF 100Ω
7
2
8
GAIN = 12dB
INDUCTORS ARE COILCRAFT 1008PS-102M
2
3
–
–8
+
4
VOUT+
LT6600-10
VOUT–
–
+
6
5
0.1µF
GAIN (dB)
1µH
–18
–28
–38
–48
–58
V–
6600 TA03a
–68
–78
200k
1M
10M
FREQUENCY (Hz)
100M
6600 TA03b
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PART NUMBER
®
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Very Low Noise, Differential Amplifier
and 20MHz Lowpass Filter
Adjustable Output Common Mode Voltage
6600f
12
Linear Technology Corporation
LT/TP 0403 2K • PRINTED IN USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900
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 LINEAR TECHNOLOGY CORPORATION 2002