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LT6600-10 Very Low Noise, Differential Amplifier and 10MHz 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 10MHz Cutoff 82dB S/N with 3V Supply and 2VP-P Output Low Distortion, 2VP-P, 800 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
The LT(R)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. 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.
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
LT6600-10 5V 0.1F RIN 402 1 7 0.01F VIN RIN 402 2 8 3
An 8192 Point FFT Spectrum
0 -10 -20 5V -30 INPUT IS A 4.7MHz SINEWAVE 2VP-P fSAMPLE = 66MHz
FREQUENCY (dB)
-
VMID VOCM
+ -
4
49.9 18pF
+
AIN LTC1748 VCM
V+ DOUT V- 1F
-40 -50 -60 -70 -80 -90
49.9 5
+
6
-
-100 -110
6600 TA01a
GAIN = 402/RIN
0
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4 8 12 16 20 24 FREQUENCY (MHz) 28 32
6600 TA01b
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LT6600-10
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 LT6600CS8-10 LT6600IS8-10 S8 PART MARKING 660010 600I10
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 = 1MHz (Gain Relative to 260kHz) VIN = 2VP-P, fIN = 5MHz (Gain Relative to 260kHz) VIN = 2VP-P, fIN = 8MHz (Gain Relative to 260kHz) VIN = 2VP-P, fIN = 10MHz (Gain Relative to 260kHz) VIN = 2VP-P, fIN = 30MHz (Gain Relative to 260kHz) VIN = 2VP-P, fIN = 50MHz (Gain Relative to 260kHz) Filter Gain, VS = 5V VIN = 2VP-P, fIN = DC to 260kHz VIN = 2VP-P, fIN = 1MHz (Gain Relative to 260kHz) VIN = 2VP-P, fIN = 5MHz (Gain Relative to 260kHz) VIN = 2VP-P, fIN = 8MHz (Gain Relative to 260kHz) VIN = 2VP-P, fIN = 10MHz (Gain Relative to 260kHz) VIN = 2VP-P, fIN = 30MHz (Gain Relative to 260kHz) VIN = 2VP-P, fIN = 50MHz (Gain Relative to 260kHz) Filter Gain, VS = 5V Filter Gain, RIN = 100, VS = 3V, 5V, 5V Filter Gain Temperature Coefficient (Note 2) Noise Distortion (Note 4) VIN = 2VP-P, fIN = DC to 260kHz VIN = 2VP-P, fIN = DC to 260kHz fIN = 260kHz, VIN = 2VP-P Noise BW = 10kHz to 10MHz, RIN = 402 1MHz, 2VP-P, RL = 800 5MHz, 2VP-P, RL = 800 Differential Output Swing Input Bias Current Measured Between Pins 4 and 5 Pin 7 Shorted to Pin 2 Average of Pin 1 and Pin 8 2nd Harmonic 3rd Harmonic 2nd Harmonic 3rd Harmonic VS = 5V VS = 3V
q q q q q q q q q q q q q q q
ELECTRICAL CHARACTERISTICS
MIN - 0.4 - 0.1 - 0.4 - 0.3 -0.2
TYP 0 0 - 0.1 0.1 0.3 - 28 - 44
MAX 0.5 0.1 0.3 1 1.7 - 25 0.5 0.1 0.3 0.9 1.4 -25 0.4 12.6
UNITS 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
- 0.5 - 0.1 - 0.4 - 0.4 - 0.3
0 0 - 0.1 0.1 0.2 - 28 - 44
- 0.6 11.4
- 0.1 12 780 56 88 97 74 77
3.85 3.85 - 85
5.0 4.9 - 40
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LT6600-10
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 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
ELECTRICAL CHARACTERISTICS
MIN
TYP 5 10 8 5 5 5 10
MAX 20 30 35 13 22 30 1.5 3.0 1.0 1.5 3.0 2.0
UNITS mV mV mV mV mV mV V/C V V V V V V mV mV mV dB V V k A A
RIN = 100
Differential Offset Drift Input Common Mode Voltage (Note 3) Differential Input = 500mVP-P, RIN = 100 Differential Output = 2VP-P, Pin 7 at Midsupply
q q q q q q q q q
0.0 0.0 -2.5 1.0 1.5 -1.0 -35 -40 -55 2.46 4.3 -15 -10 5 0 -5 61 2.51 1.5 5.5 -3 -3 35
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
40 40 35 2.55 7.7
q q q q q q
36
39 43 46
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 is guaranteed functional over the operating temperature range -40C to 85C. Note 7: The LT6600C 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 is guaranteed to meet specified performance from -40C to 85C.
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LT6600-10 TYPICAL PERFOR A CE CHARACTERISTICS
Amplitude Response
10 0
DIFFOUT DIFFIN
)
-10
-20 -30 -40 -50 -60 -70 -80 100k 1M 10M FREQUENCY (Hz) 100M
6600 G01
GAIN 20LOG
GAIN (dB)
(
Passband Gain and Group Delay
12 11 10 9 60 55 50 45 40 35 30 25 20 15 5.3 10.1 FREQUENCY (MHz) 10 14.9
6600 G03
OUTPUT IMPEDANCE ()
CMRR (dB)
GAIN (dB)
8 7 6 5 4 V = 5V S 3 GAIN = 4 TA = 25C 2 0.5
Power Supply Rejection Ratio
90 80
70
DISTORTION (dB)
DISTORTION (dB)
60
PSRR (dB)
50 40 30 20 10 0 VS = 3V VIN = 200mVP-P TA = 25C V + TO DIFFOUT 1k 10k 100k 1M FREQUENCY (Hz) 10M 100M
6600 G06
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UW
Passband Gain and Group Delay
VS = 5V GAIN = 1 1 0 -1 -2 -3 -4 -5 -6 -7 V = 5V S -8 GAIN = 1 TA = 25C -9 0.5 60 55 50 45 40 35 30 25 20 15 5.3 10.1 FREQUENCY (MHz) 10 14.9
6600 G02
GROUP DELAY (ns)
Output Impedance vs Frequency (OUT + or OUT -)
100
80
Common Mode Rejection Ratio
VS = 5V 75 GAIN = 1 VIN = 1VP-P 70 TA = 25C 65 60 55 50 45 40
GROUP DELAY (ns)
10
1
0.1 100k
1M 10M FREQUENCY (Hz)
100M
6600 G04
35 10k
100k 1M FREQUENCY (Hz)
10M
6600 G05
Distortion vs Frequency VIN = 2VP-P, VS = 3V, RL = 800 at Each Output, TA = 25C
-40 -50 -60 -70 -80 -90 -100 0.1 1 FREQUENCY (MHz) 10
6600 G07
Distortion vs Frequency VIN = 2VP-P, VS = 5V, RL = 800 at Each Output, TA = 25C
-40 -50 -60 -70 -80 -90 -100 0.1 1 FREQUENCY (MHz) 10
6600 G08
DIFFERENTIAL INPUT, 2ND HARMONIC DIFFERENTIAL INPUT, 3RD HARMONIC SINGLE-ENDED INPUT, 2ND HARMONIC SINGLE-ENDED INPUT, 3RD HARMONIC
DIFFERENTIAL INPUT, 2ND HARMONIC DIFFERENTIAL INPUT, 3RD HARMONIC SINGLE-ENDED INPUT, 2ND HARMONIC SINGLE-ENDED INPUT, 3RD HARMONIC
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LT6600-10 TYPICAL PERFOR A CE CHARACTERISTICS
Distortion vs Signal Level VS = 3V, RL = 800 at Each Output, TA = 25C
-40 -50 2ND HARMONIC, 5MHz INPUT 3RD HARMONIC, 5MHz INPUT 2ND HARMONIC, 1MHz INPUT 3RD HARMONIC, 1MHZ INPUT
-40 -50 -60 -70 -80 -90 -100 -110
DISTORTION COMPONENT (dB)
DISTORTION (dB)
DISTORTION (dB)
-60 -70 -80 -90
-100 0 1 2 3 INPUT LEVEL (VP-P) 4 5
6600 G09
Distortion vs Input Common Mode Level, 0.5VP-P, 1MHz Input, 4x Gain, RL = 800 at Each Output, TA = 25C
-40
DISTORTION COMPONENT (dB)
POWER SUPPLY CURRENT (mA)
-50 -60 -70 -80 -90
2ND HARMONIC, VS = 3V 3RD HARMONIC, VS = 3V 2ND HARMONIC, VS = 5V 3RD HARMONIC, VS = 5V
-100
-3
2 -1 0 1 -2 INPUT COMMON MODE VOLTAGE RELATIVE TO PIN 7 (V)
DISTORTION COMPONENT (dB)
UW
3
6600 G12
Distortion vs Signal Level VS = 5V, RL = 800 at Each Output, TA = 25C
2ND HARMONIC, 5MHz INPUT 3RD HARMONIC, 5MHz INPUT 2ND HARMONIC, 1MHz INPUT 3RD HARMONIC, 1MHZ INPUT -40 -50 -60 -70 -80 -90
Distortion vs Input Common Mode Level, 2VP-P, 1MHz Input, 1x Gain, RL = 800 at Each Output, TA = 25C
2ND HARMONIC, VS = 3V 3RD HARMONIC, VS = 3V 2ND HARMONIC, VS = 5V 3RD HARMONIC, VS = 5V
0
1
2
3
4
5
6600 G10
-100
-3
INPUT LEVEL (VP-P)
-1 0 1 2 -2 INPUT COMMON MODE VOLTAGE RELATIVE TO PIN 7 (V)
3
6600 G11
Power Supply Current vs Power Supply Voltage
40 38 36 34 32 30 28 26 24 2 3 6 7 4 5 8 9 TOTAL SUPPLY VOLTAGE (V) 10 TA = -40C TA = 25C TA = 85C
Transient Response, Differential Gain = 1
VOUT+ 50mV/DIV DIFFERENTIAL INPUT 200mV/DIV 100ns/DIV
6600 G13
6600 G14
Distortion vs Output Common Mode, 2VP-P 1MHz Input, 1x Gain, TA = 25C
-40 -50 -60 -70 -80 -90 2ND HARMONIC, VS = 3V 3RD HARMONIC, VS = 3V 2ND HARMONIC, VS = 5V 3RD HARMONIC, VS = 5V 2ND HARMONIC, VS = 5V 3RD HARMONIC, VS = 5V
-100
-1
0 0.5 1 1.5 -0.5 OUTPUT COMMON MODE VOLTAGE (V)
2
6600 G15
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LT6600-10
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 to AC ground. 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 LT6600-10 The LT6600-10 requires 2 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 LT6600-10 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 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
V 3 2 1 0 VIN+ VIN VIN- t
+
VIN
-
402
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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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). 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 0.1F 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. In Figure 3 the LT6600-10 is providing 12dB of gain. The gain resistor has an optional 62pF in parallel to improve
3.3V 0.1F 1 7 0.01F 2 8 3 V 3
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-
LT6600-10
+
4
VOUT+ VOUT-
2 1 0
VOUT+ VOUT- t
6600 F01
+
6
-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-
-
LT6600-10
+
4
+
6
-
5
6600 F02
Figure 2
62pF 5V 0.1F 100 1 7 2 8 2V 0.01F 3 V 3 VOUT+ 2 VOUT- 1 0 VOUT- VOUT+
-
LT6600-10
+
4
+
6
-
5
100
+ -
t
6600 F03
Figure 3
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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 . 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 (IIN+ or IIN-). 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 = 103mV + IIN 43.6
IIN is IIN- or IIN+.The transimpedance in this example is 50.4.
CURRENT OUTPUT DAC IIN- R1 IIN+ R1 R2 1 7 0.01F R2 8 3.3V 0.1F 3
-
+ -
6
4
VOUT+ VOUT-
2 LT6600-10
+
5
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 (0) source impedance and a large (1k) load resistance. Among practical examples where impedance must be considered is the evaluation of the LT6600-10 with a network analyzer.
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Figure 5 is a laboratory setup that can be used to characterize the LT6600-10 using single-ended instruments with 50 source impedance and 50 input impedance. For a unity gain configuration the LT6600-10 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 the LT6600-10 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 LT6600-10 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.
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
LT6600-10
50 402
6600 F05
+
5
0.1F
- 2.5V
Figure 5
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.
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APPLICATIO S I FOR ATIO
20 0 1dB PASSBAND GAIN COMPRESSION POINTS 1MHz 25C 1MHz 85C 3RD HARMONIC 85C 3RD HARMONIC 25C 2ND HARMONIC 85C 2ND HARMONIC 25C
OUTPUT LEVEL (dBV)
-20 -40 -60 -80 -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 0.01F 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
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average of VIN+ and VIN- in Figure 1) is determined by the power supply level and gain setting (see "Electrical Characteristics"). Common Mode DC Currents 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 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 LT6600-10, 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.
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LT6600-10
APPLICATIO S I FOR ATIO
2.5V 0.1F VIN RIN 3 COILCRAFT TTWB-1010 25 1:1
1 7 2 8
-+ +
6
4
SPECTRUM ANALYZER INPUT
SPECTRAL DENSITY (nVRMS/Hz)
LT6600-10
-
RIN
5 0.1F
25
6600 F07
- 2.5V
Figure 7
Example: With the IC removed and the 25 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 = 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 10MHz 24VRMS 34VRMS 56VRMS INPUT REFERRED NOISE dBm/Hz -149 -146 -142
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
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35 30 25 20 15 10 5 0 0.1 1.0 10 FREQUENCY (MHz) SPECTRAL DENSITY 140 120
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INTEGRATED NOISE (VRMS)
100 80 60 40 20 0 100
6600 F08
50
INTEGRATED NOISE
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 150C. 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, JA, of about 85C/W. Without extra metal trace connected to the
Table 2. LT6600-10 SO-8 Package Thermal Resistance
COPPER AREA TOPSIDE (mm2) 1100 330 35 35 0 BACKSIDE (mm2) 1100 330 35 0 0 BOARD AREA (mm2) 2500 2500 2500 2500 2500 THERMAL RESISTANCE (JUNCTION-TO-AMBIENT) 65C/W 85C/W 95C/W 100C/W 105C/W
6600f
LT6600-10
APPLICATIO S I FOR ATIO
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. 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. 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
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 8 7 6 5
.045 .005 .050 BSC
.245 MIN
.160 .005 .228 - .244 (5.791 - 6.197)
.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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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 85C, the supply current (current into Pin 3) measures 48.9mA. 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 * 0.0489 * 100) = 109C When using higher supply voltages or when driving small impedances, more copper may be necessary to keep TJ below 150C.
.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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W
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6600f
11
LT6600-10
TYPICAL APPLICATIO S
5th Order, 10MHz Lowpass Filter
V+ 0.1F VIN- R R C VIN+ C= R R 1 7 2 8 3 10 0 -10
-
GAIN (dB)
LT6600-10
+
4
VOUT+ VOUT
-
+
6
-
5 0.1F
1 2 * R * 10MHz
GAIN = 402 , MAXIMUM GAIN = 4 V - 2R
6600 TA02a
A WCDMA Transmit Filter (10MHz Lowpass Filter with a 28MHz Notch)
22
33pF
VIN-
1H 33pF 1H
100 RQ 301 27pF 100
1 7 2 8
LT6600-10
GAIN (dB)
VIN+
GAIN = 12dB INDUCTORS ARE COILCRAFT 1008PS-102M
RELATED PARTS
PART NUMBER LTC 1565-31 LTC1566-1 LT1567 LT1568 LTC6600-2.5 LTC6600-20
(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 20MHz 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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Amplitude Response
Transient Response 5th Order, 10MHz Lowpass Filter Differential Gain = 1
-20 -30 -40 -50 -60 DIFFERENTIAL GAIN = 1 -70 R = 200 C = 82pF -80 1M 10M 100k FREQUENCY (Hz)
VOUT+ 50mV/DIV
DIFFERENTIAL INPUT 200mV/DIV 100ns/DIV
100M
6600 TA02b
6600 TA02c
Amplitude Response
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V+ 0.1F 3
2 -8
-
+
4
VOUT+ VOUT-
-18 -28 -38 -48 -58
+
6
-
5 0.1F
V-
6600 TA03a
-68 -78 200k 1M 10M FREQUENCY (Hz) 100M
6600 TA03b
COMMENTS Continuous Time, SO8 Package, Fully Differential Continuous Time, SO8 Package, Fully Differential 1.4nV/Hz Op Amp, MSOP Package, Differential Output Lowpass and Bandpass Filter Designs Up to 10MHz, Differential Outputs Adjustable Output Common Mode Voltage Adjustable Output Common Mode Voltage
6600f LT/TP 0403 2K * PRINTED IN USA
(c) LINEAR TECHNOLOGY CORPORATION 2002

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