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LT6600-20 Very Low Noise, Differential Amplifier and 20MHz Lowpass Filter
FEATURES
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DESCRIPTIO
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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, 800 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
The LT(R)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. 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.
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.
TYPICAL APPLICATIO
LT6600-20 5V 0.1F RIN 402 1 7 0.01F VIN RIN 402 2 8 3
An 8192 Point FFT Spectrum
A/D LTC1748 5V
AMPLITUDE (dB)
0 -10 -20 -30 -40 -50 -60 -70 -80 -90 -100 -110 -120
66002 TA01a
-
VMID VOCM
+ -
4
49.9 18pF
+
AIN
V+ DOUT VCM V- 1F
49.9 5
+
6
-
GAIN = 402/RIN
0
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INPUT 5.9MHz 2VP-P fSAMPLE = 80MHz 10 20 FREQUENCY (MHz)
66002 TA01b
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30
40
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LT6600-20
ABSOLUTE
(Note 1)
AXI U RATI GS
PACKAGE/ORDER I FOR ATIO
TOP VIEW IN - 1 VOCM 2 V+ 3 OUT + 4 8 7 6 5 IN + VMID V- OUT -
Total Supply Voltage ................................................ 11V Operating Temperature Range (Note 6) ...-40C to 85C Specified Temperature Range (Note 7) ....-40C to 85C Junction Temperature ........................................... 150C Storage Temperature Range ................. - 65C to 150C Lead Temperature (Soldering, 10 sec).................. 300C
ORDER PART NUMBER LT6600CS8-20 LT6600IS8-20 S8 PART MARKING 660020 600I20
S8 PACKAGE 8-LEAD PLASTIC SO
TJMAX = 150C, JA = 100C/W
Consult LTC Marketing for parts specified with wider operating temperature ranges.
The q denotes specifications that apply over the full operating temperature range, otherwise specifications are at TA = 25C. Unless otherwise specified VS = 5V (V+ = 5V, V - = 0V), RIN = 402, and RLOAD = 1k.
PARAMETER Filter Gain, VS = 3V 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 = 800 2nd Harmonic 3rd Harmonic 20MHz, 2VP-P, RL = 800 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 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 VRMS dBc dBc dBc dBc VP-P DIFF VP-P DIFF A
ELECTRICAL CHARACTERISTICS
q q q q q q q q q q q q
Filter Gain, VS = 5V
- 0.5 - 0.1 - 0.2 - 0.3 - 1.3
Filter Gain, VS = 5V Filter Gain, RIN = 100
- 0.6 11.6 11.5 11.4
Filter Gain Temperature Coefficient (Note 2) Noise Distortion (Note 4)
Differential Output Swing Input Bias Current
q q q
3.80 3.75 - 95
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LT6600-20
The q denotes specifications that apply over the full operating temperature range, otherwise specifications are at TA = 25C. Unless otherwise specified VS = 5V (V+ = 5V, V - = 0V), RIN = 402, and RLOAD = 1k.
PARAMETER Input Referred Differential Offset CONDITIONS RIN = 402 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 VS = 5 VS = 3 VOCM = VMID= VS/2 VS = 5V VS = 3V VS = 3V, VS = 5 VS = 3V, VS = 5 VS = 5V
q q q q q q q q q q q q q q q q q q q q q
ELECTRICAL CHARACTERISTICS
RIN = 100
Differential Offset Drift Input Common Mode Voltage (Note 3)
TYP 5 10 10 5 5 5 10
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
Differential Input = 500mVP-P, RIN = 100 Differential Input = 2VP-P, Pin 7 at Mid-Supply Common Mode Voltage at Pin 2
Output Common Mode Voltage (Note 5)
Output Common Mode Offset (with Respect to Pin 2) Common Mode Rejection Ratio Voltage at VMID (Pin 7) VMID Input Resistance VOCM Bias Current Power Supply Current
0.0 0.0 -2.5 1.0 1.5 -1.0 -35 -40 -55 2.46 4.35 -15 -10
5 0 -5 66 2.51 1.5 5.7 -3 -3 42 46
46 53 56
UNITS mV mV mV mV mV mV V/C V V V V V V mV mV mV dB V V k A A mA mA mA
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 100. 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.
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 -40C to 85C. Note 7: The LT6600C-20 is guaranteed to meet 0C to 70C specifications and is designed, characterized and expected to meet the extended temperature limits, but is not tested at -40C and 85C. The LT6600I-20 is guaranteed to meet specified performance from -40C to 85C.
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LT6600-20 TYPICAL PERFOR A CE CHARACTERISTICS
Amplitude Response
10 0 -10 -20
GAIN (dB)
-40 -50 -60 -70 -80 VS = 5V GAIN = 1 TA = 25C 1 10 FREQUENCY (MHz) 100
66002 G01
GAIN (dB)
GAIN (dB)
-30
-90 0.1
Passband Gain and Group Delay
14 12 10 8 GAIN VS = 5V GAIN = 4 TA = 25C 50 45
OUTPUT IMPEDANCE ()
GAIN (dB)
6 4 2 0 -2 -4 -6 0.5
30 25 20 15 10 5
CMRR (dB)
GROUP DELAY
6.5
18.5 24.5 12.5 FREQUENCY (MHz)
Power Supply Rejection Ratio
100 90 80 70 V + TO DIFFOUT VS = 3V TA = 25C
DISTORTION (dB)
-60 -70 -80 -90
DISTORTION (dB)
PSRR (dB)
60 50 40 30 20 10 0 0.001
0.01
0.1 1 FREQUENCY (MHz)
4
UW
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Passband Gain and Phase
2 0 -2 -4 -6 -8 -10 -12 -14 -16 -18 0.5 6.5 18.5 24.5 12.5 FREQUENCY (MHz) PHASE GAIN VS = 5V GAIN = 1 TA = 25C 95 50 5 -40 2 0 -2 -4
Passband Gain and Group Delay
GAIN VS = 5V GAIN = 1 TA = 25C 50 45 40 35 GROUP DELAY 30 25 20 15 10 5 6.5 18.5 24.5 12.5 FREQUENCY (MHz) 0 30.5
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GROUP DELAY (ns)
PHASE (DEG)
-85 -130 -175
-6 -8 -10 -12 -14 -16 -18 0.5
-220 -265 -310 -355 30.5
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Output Impedance
100 VS = 5V GAIN = 1 TA = 25C
80 75 70 65 60 55 50 45 40 35
Common Mode Rejection Ratio
INPUT = 1VP-P VS = 5V GAIN = 1 TA = 25C
40 35
GROUP DELAY (ns)
10
1
0 30.5
0.1 0.1
30
1 10 FREQUENCY (MHz)
100
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0.1
1 10 FREQUENCY (MHz)
100
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Distortion vs Frequency
-40 -50 DIFFERENTIAL INPUT, 2ND HARMONIC DIFFERENTIAL INPUT, 3RD HARMONIC SINGLE-ENDED INPUT, 2ND HARMONIC SINGLE-ENDED INPUT, 3RD HARMONIC -40 -50 -60 -70 -80 -90 -100
Distortion vs Signal Level, VS = 3V
3RD HARMONIC VS = 3V 10MHz INPUT RL = 800 AT EACH OUTPUT GAIN = 1 2ND TA = 25C HARMONIC 10MHz INPUT 3RD HARMONIC 1MHz INPUT 2ND HARMONIC 1MHz INPUT
-100
10 100
66002 G07
VIN = 2VP-P VS = 3V RL = 800 AT EACH OUTPUT GAIN = 1 TA = 25C 0.1 1 10 FREQUENCY (MHz) 100
66002 G08
0
1
2 3 INPUT LEVEL (VP-P)
4
5
66002 G09
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LT6600-20 TYPICAL PERFOR A CE CHARACTERISTICS
Distortion vs Signal Level, VS = 5V
-40 -50 DISTORTION (dB) -60 -70 -80 -90 -100 0 1 2ND HARMONIC, 10MHz INPUT 3RD HARMONIC, 10MHz INPUT 2ND HARMONIC, 1MHz INPUT 3RD HARMONIC, 1MHz INPUT -40
DISTORTION COMPONENT (dB)
-60 -70 -80 -90
DISTORTION COMPONENT (dB)
VS = 5V RL = 800 AT EACH OUTPUT GAIN = 1 TA = 25C 2 3 INPUT LEVEL (VP-P) 4 5
66002 G10
DISTORTION COMPONENT (dB)
Total Supply Current vs Total Supply Voltage
60 TA = 85C TA = 25C TA = -40C
TOTAL SUPPLY CURRENT (mA)
50 40 30 20 10 0 0 1
2345678 TOTAL SUPPLY VOLTAGE (V)
UW
Distortion vs Input Common Mode Level
2ND HARMONIC, VS = 3V 3RD HARMONIC, VS = 3V 2ND HARMONIC, VS = 5V 3RD HARMONIC, VS = 5V 2VP-P 1MHz INPUT RL = 800 AT EACH OUTPUT GAIN = 1 TA = 25C -40 -50 -60 -70 -80 -90
Distortion vs Input Common Mode Level
2ND HARMONIC, VS = 3V 3RD HARMONIC, VS = 3V 2ND HARMONIC, VS = 5V 3RD HARMONIC, VS = 5V
-50
-100 -3 -2 -1 0 1 2 3 INPUT COMMON MODE VOTLAGE RELATIVE TO PIN 7 (V)
66002 G11
-100
500mVP-P 1MHz INPUT, GAIN = 4, RL = 800 AT EACH OUTPUT
-3 -2 -1 0 1 2 3 INPUT COMMON MODE VOTLAGE RELATIVE TO PIN 7 (V)
66002 G12
Distortion vs Output Common Mode
-40 -50 -60 -70 -80 -90 -100 -110 2VP-P 1MHz INPUT, GAIN = 1, RL = 800 AT EACH OUTPUT -2 -1.5 -1 -0.5 0 0.5 1 1.5 VOLTAGE PIN 2 TO PIN 7 (V) 2 2ND HARMONIC, VS = 3V 3RD HARMONIC, VS = 3V 2ND HARMONIC, VS = 5V 3RD HARMONIC, VS = 5V 2ND HARMONIC, VS = 5V 3RD HARMONIC, VS = 5V
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Transient Response, Gain = 1
VOUT+ 50mV/DIV
DIFFERENTIAL INPUT 200mV/DIV
100ns/DIV
66002 G15
9
10
66002 G14
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LT6600-20
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 402/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 0.01F 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 0.1F 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 0.1F ceramic capacitor. OUT+ and OUT - (Pins 4, 5): Output Pins. Pins 4 and 5 are the filter differential outputs. Each pin can drive a 100 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 0.01F 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 5.5k impedance, and it can be overridden with an external low impedance voltage source.
BLOCK DIAGRA
VIN+
VIN-
6
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RIN 8
IN +
VMID 7 V+ 11k 402 11k
V- 6
OUT - 5
PROPRIETARY LOWPASS FILTER STAGE 200
V- OP AMP
+
VOCM
200
- +
200
+-
VOCM
-
-+
200 402 1 RIN IN - 2 VOCM 3 V+ 4
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OUT +
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APPLICATIO S I FOR ATIO
Interfacing to the LT6600-20 The LT6600-20 requires two equal external resistors, RIN, to set the differential gain to 402/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 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
V 3 2 1 0 VIN+
+ VIN
VIN
-
402
VIN-
t
402
V 0.1F 2 1 0 -1 VIN+ 0.1F t VIN
+
V 3 2 1 0 500mVP-P (DIFF) VIN+ VIN- t 62pF 0.01F
+ VIN
VIN
-
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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). 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 0.1F coupling capacitor and the 402 gain setting resistor form a high pass filter, attenuating signals below 4kHz. Larger values of coupling capacitors will proportionally reduce this highpass 3dB frequency.
3.3V 0.1F 1 7 0.01F 2 8 3 V 3
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-
LT6600-20
+
4
VOUT+ VOUT-
2 1 0
VOUT+ VOUT- t
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-5
Figure 1
3.3V 0.1F 402 1 7 0.01F 402 2 8 3 V 3 VOUT+ VOUT- 2 1 0 VOUT+ VOUT-
-
LT6600-20
+
4
+
6
-
5
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Figure 2
62pF 5V 0.1F 100 1 7 2 8 3 V 3 VOUT+ 2 VOUT- 1 0 VOUT- VOUT+
-
LT6600-20
+
4
+
6
-
5
100
+ -
2V
t
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Figure 3
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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 ( ) (R1+ R2)
By setting R1 + R2 = 402, the gain equation reduces to A = R1(). 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 = 49.9 and R2 = 348. 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 * 48.3
IIN is IIN+ or IIN-. The transimpedance in this example is 50.4. 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 (0) source impedance and a large (1k) load resistance. Among practical
CURRENT OUTPUT DAC IIN- R1 IIN+ R1 R2 0.01F R2 1 7 8 3.3V 0.1F 3
-+ +
6
4
VOUT+ VOUT-
2 LT6600-20
-
5
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Figure 4
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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 50 source impedance and 50 input impedance. For a unity gain configuration the LT6600-20 requires a 402 source resistance yet the network analyzer output is calibrated for a 50 load resistance. The 1:1 transformer, 53.6 and 388 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 50. The 4:1 turns (16:1 impedance) transformer and the two 402 resistors of Figure 5, present the output of the LT6600-20 with a 1600 differential load, or the equivalent of 800 to ground at each output. The impedance seen by the network analyzer input is still 50, reducing reflections in the cabling between the transformer and analyzer input. Differential and Common Mode Voltage Ranges 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
2.5V 0.1F NETWORK ANALYZER SOURCE 50 53.6 COILCRAFT TTWB-1010 1:1 388 1 7 2 8 388 COILCRAFT TTWB-16A 4:1 402 NETWORK ANALYZER INPUT 3
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+ -
6
4
LT6600-20
50 402
66002 F05
+
5
0.1F
- 2.5V
Figure 5
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APPLICATIO S I FOR ATIO
20 0
OUTPUT LEVEL (dBV)
1dB PASSBAND GAIN COMPRESSION POINTS
1MHz 25C 1MHz 85C
-20 -40 -60 -80 -100
3RD HARMONIC 85C 3RD HARMONIC 25C 2ND HARMONIC 25C 2ND HARMONIC 85C 0 1 4 3 5 2 1MHz INPUT LEVEL (VP-P) 6 7
-120
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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 0.01F 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.
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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). 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 402 feedback resistor and the external 100 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.
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APPLICATIO S I FOR ATIO
Noise
The noise performance of the LT6600-20 can be evaluated with the circuit of Figure 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. Example: With the IC removed and the 25 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:
NOISE SPECTRAL DENSITY (nVRMS/Hz)
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 LT6600-20 with RIN = 402 using the fixture of Figure 7 (the instrument noise has been subtracted from the results).
Table 1. Noise Performance
PASSBAND GAIN (V/V) 4 2 1 INPUT REFERRED INTEGRATED NOISE 10kHz TO 20MHz 42VRMS 67VRMS 118VRMS INPUT REFERRED NOISE dBm/Hz -148 -143 -139
RIN 100 200 402
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
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2.5V 0.1F VIN RIN 3 COILCRAFT TTWB-1010 25 1:1 SPECTRUM ANALYZER INPUT 1 7 2 8 RIN
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LT6600-20
50 25
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5 0.1F
- 2.5V
Figure 7
50 VS = 5V 250
40
200
INTEGRATED NOISE (VRMS)
30 SPECTRAL DENSITY 20
150
100
10
INTEGRATED
50
0 0.1 1 10 FREQUENCY (MHz)
0 100
66002 F08
Figure 8. Input Referred Noise, Gain = 1
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 150C. 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, JA, of about 85C/W. Without the extra metal trace connected to the V - pin to provide a heat sink, the thermal resistance will be around 105C/W. Table 2 can be used as a guide when considering thermal resistance.
66002f
LT6600-20
APPLICATIO S I FOR ATIO
COPPER AREA TOPSIDE (mm2) 1100 330 35 35 0 BACKSIDE (mm2) 1100 330 35 0 0 BOARD AREA (mm2) 2500 2500 2500 2500 2500
Table 2. LT6600-20 SO-8 Package Thermal Resistance
THERMAL RESISTANCE (JUNCTION-TO-AMBIENT) 65C/W 85C/W 95C/W 100C/W 105C/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 * JA) = TA + (VS * IS * JA) where the supply current, IS, is a function of signal level, load impedance, temperature and common mode voltages.
PACKAGE DESCRIPTIO
S8 Package 8-Lead Plastic Small Outline (Narrow .150 Inch)
(Reference LTC DWG # 05-08-1610)
.045 .005 .050 BSC 8 .189 - .197 (4.801 - 5.004) NOTE 3 7 6 5
.245 MIN
.030 .005 TYP RECOMMENDED SOLDER PAD LAYOUT .010 - .020 x 45 (0.254 - 0.508) .008 - .010 (0.203 - 0.254) 0- 8 TYP
.016 - .050 (0.406 - 1.270) NOTE: 1. DIMENSIONS IN
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)
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.
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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 85C, the supply current (current into Pin 3) measures 55.5mA. Assuming a PC board layout with a 35mm2 copper trace, the JA is 100C/W. The resulting junction temperature is: TJ = TA + (PD * JA) = 85 + (5 * 0.0555 * 100) = 113C When using higher supply voltages or when driving small impedances, more copper may be necessary to keep TJ below 150C.
.160 .005 .228 - .244 (5.791 - 6.197) .150 - .157 (3.810 - 3.988) NOTE 3 1 2 3 4 .053 - .069 (1.346 - 1.752) .004 - .010 (0.101 - 0.254) .014 - .019 (0.355 - 0.483) TYP .050 (1.270) BSC
SO8 0303
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66002f
11
LT6600-20
TYPICAL APPLICATIO
Amplitude Response
10 0 -10 -20
GAIN (dB)
-30 -40 -50 -60 -70 -80 VS = 2.5V GAIN = 1 C = 39pF R = 200 TA = 25C 10 1 FREQUENCY (MHz) 100
66002 TA04
-90 0.1
RELATED PARTS
PART NUMBER LTC 1565-31 LTC1566-1 LT1567 LT1568 LT6600-2.5 LT6600-10
(R)
DESCRIPTION 650kHz Linear Phase Lowpass Filter Low Noise, 2.3MHz Lowpass Filter Very Low Noise, High Frequency Filter Building Block Very Low Noise, 4th Order Building Block Very Low Noise Differential Amplifier and 2.5MHz Lowpass Filter Very Low Noise Differential Amplifier and 10MHz Lowpass Filter
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Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900
q
FAX: (408) 434-0507 q www.linear.com
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A 5th Order, 20MHz Lowpass Filter
V+ 0.1F VIN- R R C VIN+ C= R R 1 7 2 8 3
-
+ -
6
4
VOUT+ VOUT-
LT6600-20
+
5 0.1F
1 2 * R * 20MHz
GAIN = 402 , MAXIMUM GAIN = 4 V - 2R
66002 TA02a
Transient Response, Gain = 1
VOUT+ 50mV/DIV
DIFFERENTIAL INPUT 200mV/DIV
100ns/DIV
66002 TA03
COMMENTS Continuous Time, SO8 Package, Fully Differential Continuous Time, SO8 Package 1.4nV/Hz Op Amp, MSOP Package, Fully Differential Lowpass and Bandpass Filter Designs Up to 10MHz, Differential Outputs 86dB S/N with 3V Supply, SO-8 82dB S/N with 3V Supply, SO-8
66002f LT/TP 0503 1K * PRINTED IN USA
(c) LINEAR TECHNOLOGY CORPORATION 2003

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