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LT6600-2.5 Very Low Noise, Differential Amplifier and 2.5MHz Lowpass Filter FEATURES
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DESCRIPTION
The LT(R)6600-2.5 combines a fully differential amplifier with a 4th order 2.5MHz lowpass filter approximating a Chebyshev frequency response. Most differential amplifiers require many precision external components to tail or gain and bandwidth. In contrast, with the LT6600-2.5, two external resistors program differential gain, and the filter's 2.5MHz cutoff frequency and passband ripple are internally set. The LT6600-2.5 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 LT6600-2.5 integrates an antialiasing filter and a differential amplifier/driver without compromising distortion or low noise performance. At unity gain the measured in band signalto-noise ratio is an impressive 86dB. 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 LT6600-2.5 also features low voltage operation. The differential design provides outstanding performance for a 4VP-P signal level while the part operates with a single 3V supply. The LT6600-2.5 is available in SO-8 and DFN-12 packages. For similar devices with higher cutoff frequency, refer to the LT6600-5, LT6600-10, LT6600-15 and LT6600-20 data sheets.
0.6dB (Max) Ripple 4th Order Lowpass Filter with 2.5MHz Cutoff Programmable Differential Gain via Two External Resistors Adjustable Output Common Mode Voltage Operates and Specified with 3V, 5V, 5V Supplies 86dB S/N with 3V Supply and 1VRMS Output Low Distortion, 1VRMS, 800 Load 1MHz: 95dBc 2nd, 88dBc 3rd Fully Differential Inputs and Outputs Compatible with Popular Differential Amplifier Pinouts SO-8 and DFN-12 Packages
APPLICATIONS
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High Speed ADC Antialiasing and DAC Smoothing in Networking or Cellular Base Station Applications High Speed Test and Measurement Equipment Medical Imaging Drop-in Replacement for Differential Amplifiers
L, LT, LTC and LTM are registered trademarks of Linear Technology Corporation. All other trademarks are the property of their respective owners.
TYPICAL APPLICATION
DAC Output Filter
5V 52.3 16 BIT 4kHz to 2.5MHz DISCRETE MULTI-TONE SIGNAL AT 50MSPS LADCOM IOUT A LTC1668 IOUT B CLK -5V 50MHz 52.3
(S8 Pin Numbers Shown) DAC Output Spectrum
5V 0.1F 0 -10 -20 1 7 2 8 BASEBAND SIGNAL 0 -10 -20 -30 (dBm) -40 -50 -60 -70 -80 -90 0 12 24 36 48 60 72 84 96 108 120 FREQUENCY (MHz)
660025 TA01b
LT6600-2.5 Output Spectrum
1580
-
3
(dBm)
LT6600-2.5
+
4
VOUT+ VOUT-
-30 -40 -50 -60 -70 -80 DAC OUTPUT IMAGE
+
-
6
5
1580
0.1F
-5V
660025 TA01a
-90
0
12 24 36 48 60 72 84 96 108 120 FREQUENCY (MHz)
660025 TA01c
660025fb
1
LT6600-2.5 ABSOLUTE MAXIMUM RATINGS
(Note 1)
Total Supply Voltage ...................................................1V Input Voltage (Note 8)...............................................VS Input Current (Note 8)..........................................10mA 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
PIN CONFIGURATION
TOP VIEW IN- NC VOCM V+ NC OUT+ 1 2 3 4 5 6 12 12 IN+ 11 NC 10 VMID 9 V- 8 V- 7 OUT- IN- 1 TOP VIEW 8 7 6 5 IN+ VMID V- OUT-
VOCM 2 V+ 3 OUT+ 4
DF PACKAGE 12-LEAD (4mm x 4mm) PLASTIC DFN TJMAX = 150C, JA = 43C/W, JC = 4C/W EXPOSED PAD (PIN 13) IS V-, MUST BE SOLDERED TO PCB
S8 PACKAGE 8-LEAD PLASTIC SO TJMAX = 150C, JA = 100C/W
ORDER INFORMATION
LEAD FREE FINISH LT6600CS8-2.5#PBF LT6600IS8-2.5#PBF LT6600CDF-2.5#PBF LT6600IDF-2.5#PBF LEAD BASED FINISH LT6600CS8-2.5 LT6600IS8-2.5 TAPE AND REEL LT6600CS8-2.5#TRPBF LT6600IS8-2.5#TRPBF LT6600CDF-2.5#TRPBF LT6600IDF-2.5#TRPBF TAPE AND REEL LT6600CS8-2.5#TR LT6600IS8-2.5#TR PART MARKING 660025 6600I25 60025 60025 PART MARKING 660025 600I25 PACKAGE DESCRIPTION 8-Lead Plastic SO 8-Lead Plastic SO 12-Lead (4mm x 4mm) Plastic DFN 12-Lead (4mm x 4mm) Plastic DFN PACKAGE DESCRIPTION 8-Lead Plastic SO 8-Lead Plastic SO TEMPERATURE RANGE 0C to 70C -40C to 85C 0C to 70C -40C to 85C TEMPERATURE RANGE 0C to 70C -40C to 85C
Consult LTC Marketing for parts specified with wider operating temperature ranges. Consult LTC Marketing for information on nonstandard lead based finish parts. For more information on lead free part marking, go to: http://www.linear.com/leadfree/ For more information on tape and reel specifications, go to: http://www.linear.com/tapeandreel/
The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25C. Unless otherwise specified VS = 5V (V+ = 5V, V- = 0V), RIN = 1580, and RLOAD = 1k.
PARAMETER Filter Gain, VS = 3V RIN = 1580 CONDITIONS VIN = 2VP-P, fIN = DC to 260kHz VIN = 2VP-P, fIN = 700kHz (Gain Relative to 260kHz) VIN = 2VP-P, fIN = 1.9MHz (Gain Relative to 260kHz) VIN = 2VP-P, fIN = 2.2MHz (Gain Relative to 260kHz) VIN = 2VP-P, fIN = 2.5MHz (Gain Relative to 260kHz)
l l l l
ELECTRICAL CHARACTERISTICS
MIN -0.5 -0.15 -0.2 -0.6 -2.1
TYP 0.1 0 0.2 0.1 -0.9
MAX 0.4 0.1 0.6 0.5 0
UNITS dB dB dB dB dB
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LT6600-2.5 ELECTRICAL CHARACTERISTICS
PARAMETER CONDITIONS VIN = 2VP-P, fIN = 7.5MHz (Gain Relative to 260kHz) VIN = 2VP-P, fIN = 12.5MHz (Gain Relative to 260kHz) Filter Gain, VS = 5V RIN = 1580 VIN = 2VP-P, fIN = DC to 260kHz VIN = 2VP-P, fIN = 700kHz (Gain Relative to 260kHz) VIN = 2VP-P, fIN = 2.2MHz (Gain Relative to 260kHz) VIN = 2VP-P, fIN = 2.2MHz (Gain Relative to 260kHz) VIN = 2VP-P, fIN = 2.5MHz (Gain Relative to 260kHz) VIN = 2VP-P, fIN = 7.5MHz (Gain Relative to 260kHz) VIN = 2VP-P, fIN = 12.5MHz (Gain Relative to 260kHz) Filter Gain, VS = 5V Filter Gain, RIN = 402 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 Noise BW = 10kHz to 2.5MHz 1MHz, 1VRMS, RL = 800 Measured Between Pins 4 and 5 Average of Pin 1 and Pin 8 RIN = 1580, Differential Gain = 1V/V 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 = 5V (S8) VS = 5V (DFN) VS = 3V VOCM = VMID = VS /2 VS = 5V VS = 3V VS = 3V, VS = 5V VS = 3V, VS = 5V VS = 5V 2nd Harmonic 3rd Harmonic VS = 5V VS = 3V
l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l
The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25C. Unless otherwise specified VS = 5V (V+ = 5V, V- = 0V), RIN = 1580, and RLOAD = 1k.
MIN TYP -34 -51 -0.5 -0.15 -0.2 -0.6 -2.1 -0.1 0 0.2 0.1 -0.9 -34 -51 -0.6 11.3 11.3 11.2 -0.1 11.8 11.8 11.7 780 51 95 88 8.8 5.1 -35 9.3 5.5 -15 5 5 5 3 3 3 10 Differential Input = 500mVP-P, RIN 402 Differential Input = 2VP-P, Pin 7 at Mid-Supply 0.0 0.0 -2.5 1.0 1.5 -1.0 -25 -30 -55 2.46 2.45 4.3 -15 -10 10 5 -10 63 2.51 2.51 1.5 5.7 -3 -3 26 28 30 33 36 2.55 2.56 7.7 1.5 3.0 1.0 1.5 3.0 2.0 45 45 35 25 30 35 13 16 20 0.4 12.3 12.3 12.2 0.4 0.1 0.6 0.5 0 -31 MAX -31 UNITS dB dB dB dB dB dB dB dB dB dB dB dB dB ppm/C VRMS dBc dBc VP-P DIFF VP-P DIFF A mV mV mV mV mV mV V/C V V V V V V mV mV mV dB V V V k A A mA mA mA
660025fb
Filter Gain Temperature Coefficient (Note 2) fIN = 260kHz, VIN = 2VP-P Noise Distortion (Note 4) Differential Output Swing Input Bias Current Input Referred Differential Offset
RIN = 402, Differential Gain = 4V/V
Differential Offset Drift Input Common Mode Voltage (Note 3)
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
3
LT6600-2.5 ELECTRICAL CHARACTERISTICS
Note 1: Stresses beyond those listed under Absolute Maximum Ratings may cause permanent damage to the device. Exposure to any Absolute Maximum Rating condition for extended periods may affect device reliability and lifetime. Note 2: This is the temperature coefficient of the internal feedback resistors assuming a temperature independent external resistor (RIN). Note 3: The input common mode voltage is the average of the voltages applied to the external resistors (RIN). Specification guaranteed for RIN 402. For 5V supplies, the minimum input common mode voltage is guaranteed by design to reach -5V. Note 4: Distortion is measured differentially using a single-ended stimulus. The input common mode voltage, the voltage at VOCM, and the voltage at VMID 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-2.5 is guaranteed functional over the operating temperature range of -40C to 85C. Note 7: The LT6600C-2.5 is guaranteed to meet specified performance from 0C to 70C and is designed, characterized and expected to meet specified performance from -40C and 85C, but is not tested or QA sampled at these temperatures. The LT6600I-2.5 is guaranteed to meet specified performance from -40C to 85C. Note 8: The inputs are protected by back-to-back diodes. If the differential input voltage exceeds 1.4V, the input current should be limited to less than 10mA.
TYPICAL PERFORMANCE CHARACTERISTICS
Amplitude Response
12 0 -12 -24 GAIN (dB) -36 -48 -60 -72 -84 -96 100k 1M 10M FREQUENCY (Hz) 50M
660025 G01
Passband Gain and Group Delay
VS = 2.5V RIN = 1580 GAIN = 1 1 0 -1 -2 GAIN (dB) -3 -4 -5 -6 320 300 280 260 240 220 200 GROUP DELAY (ns) 12 11 10 9 GAIN (dB) 8 7 6 5
Passband Gain and Group Delay
320 300 280 260 240 220 200 GROUP DELAY (ns)
180 VS = 5V -7 RIN = 1580 160 GAIN = 1 -8 140 TA = 25C -9 120 0.5 0.75 1.0 1.25 1.5 1.75 2.0 2.25 2.5 2.75 3.0 FREQUENCY (MHz)
660025 G02
180 VS = 5V 4 RIN = 402 160 GAIN = 4 140 3 TA = 25C 120 2 0.5 0.75 1.0 1.25 1.5 1.75 2.0 2.25 2.5 2.75 3.0 FREQUENCY (MHz)
660025 G03
Output Impedance vs Frequency (OUT+ or OUT-)
100 110
CMRR
VIN = 1VP-P VS = 5V 100 R = 1580 IN GAIN = 1 90 CMRR (dB) 80 70 60 20 50 10 0 1k 10k 100k 1M FREQUENCY (Hz) 10M 100M
660025 G05
PSRR
90 80 70 60 PSRR (dB) 50 40 30 V+ TO DIFFERENTIAL OUT VS = 3V
OUTPUT IMPEDANCE ()
10
1
0.1 100k
40 1M 10M FREQUENCY (Hz) 100M
660025 G04
1k
10k
100k 1M FREQUENCY (Hz)
10M
100M
660025 G06
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LT6600-2.5 TYPICAL PERFORMANCE CHARACTERISTICS
Distortion vs Frequency
-60 DIFFERENTIAL INPUT, 2ND HARMONIC DIFFERENTIAL INPUT, 3RD HARMONIC SINGLE-ENDED INPUT, 2ND HARMONIC SINGLE-ENDED INPUT, 3RD HARMONIC -60
Distortion vs Frequency
DIFFERENTIAL INPUT, 2ND HARMONIC DIFFERENTIAL INPUT, 3RD HARMONIC SINGLE-ENDED INPUT, 2ND HARMONIC SINGLE-ENDED INPUT, 3RD HARMONIC
-70 DISTORTION (dB)
-70 DISTORTION (dB)
-80
-80
-90 VIN = 2VP-P VS = 3V RL = 800 AT EACH OUTPUT 0.1 1 FREQUENCY (MHz) 10
660025 G07
-90 VIN = 2VP-P VS = 5V RL = 800 AT EACH OUTPUT 0.1 1 FREQUENCY (MHz) 10
660025 G08
-100
-100
-110
-110
Distortion vs Frequency
-60 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 VIN = 2VP-P VS = 5V RL = 800 AT EACH OUTPUT 0.1 1 FREQUENCY (MHz) 10
660025 G09
Distortion vs Signal Level
2ND HARMONIC, DIFFERENTIAL INPUT 3RD HARMONIC, DIFFERENTIAL INPUT 2ND HARMONIC, SINGLE-ENDED INPUT 3RD HARMONIC, SINGLE-ENDED INPUT
-70 DISTORTION (dB)
-80
-90
DISTORTION (dB)
-100 -110 0 1
-110
VS = 3V F = 1MHz RL = 800 AT EACH OUTPUT 2 3 4 INPUT LEVEL (VP-P) 5 6
660025 G10
Distortion vs Signal Level
-40 -50 -60 -70 -80 -90 -100 -110 0 1 2 VS = 5V F = 1MHz RL = 800 AT EACH OUTPUT 3 4 5 6 7 INPUT LEVEL (VP-P) 8 9 2ND HARMONIC, DIFFERENTIAL INPUT 3RD HARMONIC, DIFFERENTIAL INPUT 2ND HARMONIC, SINGLE-ENDED INPUT 3RD HARMONIC, SINGLE-ENDED INPUT -40 -50 -60 -70 -80 -90 -100 -110
Distortion vs Signal Level
2ND HARMONIC, DIFFERENTIAL INPUT 3RD HARMONIC, DIFFERENTIAL INPUT 2ND HARMONIC, SINGLE-ENDED INPUT 3RD HARMONIC, SINGLE-ENDED INPUT
DISTORTION (dB)
DISTORTION (dB)
VS = 5V F = 1MHz RL = 800 AT EACH OUTPUT 0 1 2 3 4 5 6 7 INPUT LEVEL (VP-P) 8 9
660025 G11
660025 G12
660025fb
5
LT6600-2.5 TYPICAL PERFORMANCE CHARACTERISTICS
Distortion vs Input Common Mode Level
-40 DISTORTION COMPONENT (dB) -50 -60 -70 -80 -90 2ND HARMONIC, VS = 3V 3RD HARMONIC, VS = 3V 2ND HARMONIC, VS = 5V 3RD HARMONIC, VS = 5V 2VP-P 1MHz INPUT RIN = 1580 GAIN = 1 -40 DISTORTION COMPONENT (dB) -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
-100 -110 -2 -1 0 1 2 -3 3 INPUT COMMON MODE VOLTAGE RELATIVE TO VMID (V)
660025 G13
-100 -110 2VP-P 1MHz INPUT, RIN = 402, GAIN = 4
-2 -1 0 1 2 -3 3 INPUT COMMON MODE VOLTAGE RELATIVE TO VMID (V)
660025 G14
Distortion vs Output Common Mode Level
-40 DISTORTION COMPONENT (dB) -50 -60 -70 -80 -90 -100 -110 -1.5 -1.0 -0.5 2VP-P 1MHz INPUT, RIN = 1580, GAIN = 1 1.0 1.5 2.0 VOLTAGE VOCM TO VMID (V) 0.5 0 2.5 2ND HARMONIC, VS = 3V 3RD HARMONIC, VS = 3V 2ND HARMONIC, VS = 5V 3RD HARMONIC, VS = 5V 2ND HARMONIC, VS = 5V 3RD HARMONIC, VS = 5V 32 30 TOTAL SUPPLY CURRENT (mA) 28
Supply Current vs Total Supply Voltage
TA = 85C TA = 25C 26 24 22 20 18 16 2 3 4 6 8 5 7 9 TOTAL SUPPLY VOLTAGE (V) 10 TA = -40C
660025 G15
660025 G16
Transient Response Gain = 1
VOUT+ 50mV/DIV
DIFFERENTIAL INPUT 200mV/DIV
500ns/DIV
660025 G17
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LT6600-2.5 PIN FUNCTIONS
(DFN/SO)
IN- and IN+ (Pins 1, 12/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 1580/RIN. NC (Pins 2, 5, 11/NA): No Connection VOCM (Pin 3/Pin 2): DC Common Mode Reference Voltagefor the 2nd Filter Stage. Its value programs the common mode voltage of the differential output of the filter. This is a high impedance input, which can be driven from an external voltage reference, or it can be tied to VMID on the PC board. VOCM should be bypassed with a 0.01F ceramic capacitor unless it is connected to a ground plane. V+ and V- (Pins 4, 8, 9/Pins 3, 6): Power Supply Pins. For a single 3.3V or 5V supply (V- grounded) a quality 0.1F ceramic bypass capacitor is required from the positive supply pin (V+) to the negative supply pin (V-). The bypass
should be as close as possible to the IC. For dual supply applications, bypass V+ to ground and V- to ground with a quality 0.1F ceramic capacitor. OUT+ and OUT- (Pins 6, 7/Pins 4, 5): Output Pins. These are the filter differential outputs. Each pin can drive a 100 and/or 50pF load to AC ground. VMID (Pin 10/Pin 7): The VMID pin is internally biased at mid-supply, see Block Diagram. For single supply operation, the VMID pin should be bypassed with a quality 0.01F ceramic capacitor to V-. For dual supply operation, VMID 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. VMID 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.
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LT6600-2.5 BLOCK DIAGRAM
VIN+ RIN IN+ VMID V+ 11k 1580 11k 800 V- OP AMP PROPRIETARY LOWPASS FILTER STAGE V- OUT-
+
VOCM
800
- +
800
+-
VOCM
-
-+
800 1580
660025 BD
VIN-
RIN
IN-
VOCM
V+
OUT+
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LT6600-2.5 APPLICATIONS INFORMATION
Interfacing to the LT6600-2.5 Note: The referenced pin numbers correspond to the S8 package. See the Pin Functions for the equivalent DFN-12 package pin numbers. The LT6600-2.5 requires two equal external resistors, RIN, to set the differential gain to 1580/RIN. The inputs to the filter are the voltages VIN+ and VIN- presented to the see 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 LT6600-2.5 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 LT6600-2.5 operating with a single 3.3V supply and unity passband gain; the input signal is
V 3 2 1 0 VIN
+
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 is 2VP-P for frequencies below 2.5MHz. The common mode output voltage is determined by the voltage at VOCM. Since VOCM is shorted to VMID, 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 VMID. Figure 2 shows how to AC couple signals into the LT6600-2.5. 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 1580 gain setting resistor form a high pass filter, attenuating signals below 1kHz. Larger values of coupling capacitors will proportionally reduce this highpass 3dB frequency. In Figure 3 the LT6600-2.5 is providing 12dB of gain. The common mode output voltage is set to 2V.
3.3V 0.1F V 3 3
VIN
-
1580
1 7 0.01F 2 8
-
LT6600-2.5
+
4
VOUT+ VOUT-
2 1 0
VOUT+ VOUT- t
660025 F01
VIN VIN- t
+
+
6
-5
1580
Figure 1. (S8 Pin Numbers)
3.3V V 0.1F 2 1 0 -1 VIN
+
0.1F 1580 1 7 0.1F t VIN
+
V 3 VOUT+ VOUT- 2 1 0 VOUT+ VOUT- t
3
-
0.01F 1580
2 8
LT6600-2.5
+
4
+
6
-
5
660025 F02
Figure 2. (S8 Pin Numbers)
5V V 3 2 1 0 500mVP-P (DIFF) VIN+ VIN- t 0.01F VIN
+
0.1F
-
V 3 VOUT+ 2 VOUT- 1 0 VOUT-
402
VIN
1 7 2 8
3
-
LT6600-2.5
+
4
VOUT+
+
6
-
5
402
+ -
2V
660025 F03
t
Figure 3. (S8 Pin Numbers)
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LT6600-2.5 APPLICATIONS INFORMATION
Use Figure 4 to determine the interface between the LT6600-2.5 and a current output DAC. The gain, or "transimpedance," is defined as A = VOUT/IIN. To compute the transimpedance, use the following equation: A= 1580 * R1 () (R1+ R2) Figure 5 is a laboratory setup that can be used to characterize the LT6600-2.5 using single-ended instruments with 50 source impedance and 50 input impedance. For a 12dB gain configuration the LT6600-2.5 requires a402 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 LT6600-2.5 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 LT6600-2.5 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 rail-to-rail output stage of the LT6600-2.5 can process large differential signal levels. On a 3V supply, the output signal can be 5.1VP-P. Similarly, a 5V supply can support signals as large as 8.8VP-P. To prevent excessive power dissipation in the internal circuitry, the user must limit differential signal levels to 9VP-P. The two amplifiers inside the LT6600-2.5 have independent control of their output common mode voltage (see the "Block Diagram" section). The following guidelines will optimize the performance of the filter.
By setting R1 + R2 = 1580, the gain equation reduces to A = R1(). The voltage at the pins of the DAC is determined by R1, R2, the voltage on VMID and the DAC output current. Consider Figure 4 with R1 = 49.9 and R2 = 1540. The voltage at VMID, for VS = 3.3V, is 1.65V. The voltage at the DAC pins is given by: VDAC = VPIN7 * R1 R1* R2 +IIN * R1+ R2 + 1580 R1+ R2 = 26mV +IIN * 48.3
IIN is IIN+ or IIN-. The transimpedance in this example is 49.6. Evaluating the LT6600-2.5 The low impedance levels and high frequency operation of the LT6600-2.5 require some attention to the matching networks between the LT6600-2.5 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 LT6600-2.5 with a network analyzer.
CURRENT OUTPUT DAC IIN- R1 IIN+
660025 F04
2.5V 3.3V 0.1F R2 3 NETWORK ANALYZER SOURCE VOUT+ VOUT- VOUT+ - VOUT- IIN+ - IIN- = 1580 * R1 R1 + R2 -2.5V 50 COILCRAFT TTWB-1010 1:1 388 1 7 53.6 2 8 388 0.1F COILCRAFT TTWB-16A 4:1 402 NETWORK ANALYZER INPUT
3
1 7
-+ +
6
-
4
+ -
6
4
LT6600-2.5 402 5
50
0.01F
2 LT6600-2.5 8 R2
-
5
+
0.1F
R1
660025 F05
Figure 4. (S8 Pin Numbers)
Figure 5. (S8 Pin Numbers)
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LT6600-2.5 APPLICATIONS INFORMATION
VMID can be allowed to float, but it must be bypassed to an AC ground with a 0.01F capacitor or some instability maybe observed. VMID 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 VMID. 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 VMID. VOCM can be shorted to VMID for simplicity. If a different common mode output voltage is required, connect VOCM to a voltage source or resistor network. For 3V and 3.3V supplies the voltage at VOCM must be less than or equal to the mid supply level. For example, voltage (VOCM) 1.65V on a single 3.3V supply. For power supply voltages higher than 3.3V the voltage at VOCM can be set above mid supply, as shown in Table 1. The voltage on VOCM should not exceed 1V below the voltage on VMID. VOCM is a high impedance input.
Table 1. Output Common Range for Various Supplies
SUPPLY VOLTAGE 3V DIFFERENTIAL OUT VOLTAGE SWING 4VP-P 2VP-P 1VP-P 5V 8VP-P 4VP-P 2VP-P 1VP-P 5V 9VP-P 4VP-P 2VP-P 1VP-P OUTPUT COMMON MODE RANGE FOR LOW DISTORTION 1.4V VOCM 1.6V 1V VOCM 1.6V 0.75V VOCM 1.6V 2.4V VOCM 2.6V 1.5V VOCM 3.5V 1V VOCM 3.75V 0.75V VOCM 3.75V -2V VOCM 2V -3.5V VOCM 3.5V -3.75V VOCM 3.75V -4.25V VOCM 3.75V
ply 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 "Electrical Characteristics"). Common Mode DC Currents In applications like Figure 1 and Figure 3 where the LT6600-2.5 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. VMID sets the output common mode voltage of the 1st differential amplifier inside the LT6600-2.5 (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 1580 feedback resistor and the external 402 input resistor. The resulting 1.25mA common mode DC current in each input path,must be absorbed by the sources VIN+ and VIN-. VOCM sets the common mode output voltage of the 2nd differential amplifier inside the LT6600-2.5, and therefore sets the common mode output voltage of the filter. Since, in the example of Figure 3, VOCM differs from VMID by 0.5V, an additional 625A (312A per side) of DC current will flow in the resistors coupling the 1st differential amplifier output stage to filter output. Thus, a total of 3.125mA is used to translate the common mode voltages. A simple modification to Figure 3 will reduce the DC common mode currents by 36%. If VMID is shorted toVOCM the common mode output voltage of both op amp stages will be 2V and the resulting DC current will be 2mA. Of course, by AC coupling the inputs of Figure 3, the common mode DC current can be reduced to 625A.
NOTE: VOCM is set by the voltage at this RIN. The voltage at VOCM should not exceed 1V below the voltage at VMID. To achieve some of the output common mode ranges shown in the table, the voltage at VMID must be set externally to a value below mid supply.
The LT6600-2.5 was designed to process a variety of input signals including signals centered around the mid-sup-
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11
LT6600-2.5 APPLICATIONS INFORMATION
Noise The noise performance of the LT6600-2.5 can be evaluated with the circuit of Figure 6. Given the low noise output of the LT6600-2.5 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.
2.5V 0.1F VIN RIN 3 COILCRAFT TTWB-1010 25 1:1 25 SPECTRUM ANALYZER INPUT NOISE SPECTRAL DENSITY (nVRMS/Hz) 50 100 40 SPECTRAL DENSITY 30 60 80 INTEGRATED NOISE (VRMS)
20
40
10 INTEGRATED 0 0.01
20
0 0.1 1 10
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FREQUENCY (MHz)
1 7 2 8
-+ +
6
4
LT6600-2.5
Figure 7. Input Referred Noise, Gain = 1
50
-
RIN
5 0.1F
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-2.5V
Figure 7 is plot of the noise spectral density as a function of frequency for an LT6600-2.5 with RIN = 1580 using the fixture of Figure 6 (the instrument noise has been subtracted from the results). The noise at each output is comprised of a differential component and a common mode component. Using a transformer or combiner to convert the differential outputs to single-ended signal rejects the common mode noise and gives a true measure of the S/N achievable in the system. Conversely, if each output is measured individually and the noise power added together, the resulting calculated noise level will be higher than the true differential noise. Power Dissipation The LT6600-2.5 amplifiers combine high speed with largesignal currents in a small package. There is a need to ensure that the die's junction temperature does not exceed 150C. The LT6600-2.5 S8 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 totalof 660 square millimeters connected to Pin 6 of theLT6600-2.5 S8 (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
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Figure 6. (S8 Pin Numbers)
Example: With the IC removed and the 25 resistorsgrounded, Figure 6, measure the total integrated noise (eS) of the spectrum analyzer from 10kHz to 2.5MHz. 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 100kHz 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: A Table 2 lists the typical input referred integrated noise for various values of RIN.
Table 2. Noise Performance
PASSBAND GAIN (V/V) 4 2 1 INPUT REFERRED INTEGRATED NOISE 10kHz TO 2.5MHz 18VRMS 29VRMS 51VRMS INPUT REFERRED INTEGRATED NOISE 10kHz TO 5MHz 23VRMS 39VRMS 73VRMS
eIN =
(eO )2 - (eS )2
RIN 402 806 1580
12
LT6600-2.5 APPLICATIONS INFORMATION
to the V- pin to provide a heat sink, the thermal resistance will be around 105C/W. Table 3 can be used as a guide when considering thermal resistance.
Table 3. LT6600-2.5 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
Junction temperature, TJ, is calculated from the ambienttemperature, 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 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 worst-case conditions, estimate the thermal resistance from Table 2, then apply the equation for TJ. For example, using the circuit in Figure 3 with DC differential input voltage of 1V, a differential output voltage of 4V, no load resistance and an ambient temperature of 85C, the supply current (current into V+) measures 37.6mA. 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 * 0.0376 * 100) = 104C When using higher supply voltages or when driving small impedances, more copper may be necessary to keep TJ below 150C.
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13
LT6600-2.5 PACKAGE DESCRIPTION
DF Package 12-Lead Plastic DFN (4mm x 4mm)
(Reference LTC DWG # 05-08-1733 Rev O)
2.50 REF 0.70 0.05
4.50 0.05 3.10 0.05
3.38 0.05 2.65 0.05
PACKAGE OUTLINE 0.25 0.05 0.50 BSC RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS APPLY SOLDER MASK TO AREAS THAT ARE NOT SOLDERED 4.00 0.10 (4 SIDES) 2.50 REF 7 12 0.40 0.10
3.38 0.10 2.65 0.10
PIN 1 TOP MARK (NOTE 6) 6 R = 0.115 TYP 0.75 0.05 1 0.25 0.05 0.50 BSC
PIN 1 NOTCH R = 0.20 TYP OR 0.35 x 45 CHAMFER
(DF12) DFN 0806 REV O
0.200 REF
BOTTOM VIEW--EXPOSED PAD
0.00 - 0.05 NOTE: 1. DRAWING IS PROPOSED TO BE MADE A JEDEC PACKAGE OUTLINE MO-220 VARIATION (WGGD-X)--TO BE APPROVED 2. DRAWING NOT TO SCALE 3. ALL DIMENSIONS ARE IN MILLIMETERS 4. DIMENSIONS OF EXPOSED PAD ON BOTTOM OF PACKAGE DO NOT INCLUDE MOLD FLASH. MOLD FLASH, IF PRESENT, SHALL NOT EXCEED 0.15mm ON ANY SIDE 5. EXPOSED PAD SHALL BE SOLDER PLATED 6. SHADED AREA IS ONLY A REFERENCE FOR PIN 1 LOCATION ON THE TOP AND BOTTOM OF PACKAGE
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14
LT6600-2.5 PACKAGE DESCRIPTION
S8 Package 8-Lead Plastic Small Outline (Narrow .150 Inch)
(Reference LTC DWG # 05-08-1610)
.050 BSC
.045 .005
8
.189 - .197 (4.801 - 5.004) NOTE 3 7 6 5
.245 MIN
.160 .005 .228 - .244 (5.791 - 6.197)
.150 - .157 (3.810 - 3.988) NOTE 3
.030 .005 TYP RECOMMENDED SOLDER PAD LAYOUT
.010 - .020 x 45 (0.254 - 0.508) .008 - .010 (0.203 - 0.254) .016 - .050 (0.406 - 1.270) NOTE: 1. DIMENSIONS IN 0- 8 TYP
1
2
3
4
.053 - .069 (1.346 - 1.752)
.004 - .010 (0.101 - 0.254)
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)
.014 - .019 (0.355 - 0.483) TYP
.050 (1.270) BSC
SO8 0303
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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.
15
LT6600-2.5 TYPICAL APPLICATION
5th Order Lowpass Filter (S8 Pin Numbers Shown)
V+ 0.1F R R
VIN-
1 7 2 8
-
3
C VIN+
LT6600
+ -
4
VOUT+ VOUT-
+
R
R
6
5
C= GAIN =
1 2 * R * 2.5MHz 1580 , MAXIMUM GAIN = 4 2R V-
0.1F
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Amplitude Response
10 0 -10 -20 GAIN (dB) -30 -40 -50 -60 -70 -80 -90 100k 1M FREQUENCY (Hz) 10M 20M
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Transient Response Gain = 1
VS = 2.5V GAIN = 1 R = 787 TA = 25C
VOUT+ 50mV/DIV
DIFFERENTIAL INPUT 200mV/DIV
500ns/DIV
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RELATED PARTS
PART NUMBER LTC(R)1565-31 LTC1566-1 LT1567 LT1568 LTC1992 LTC1992-1 LTC1992-2 LTC1992-5 LTC1992-10 LT6600-10 LT6600-20 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 Low-Power Differential In/Out Amplifier Low-Power Differential In/Out Amplifier Low-Power Differential In/Out Amplifier Low-Power Differential In/Out Amplifier Low-Power Differential In/Out Amplifier Very Low Noise Differential Amplifier and 10MHz Lowpass Filter Very Low Noise Differential Amplifier and 20MHz Lowpass Filter 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 Adjustable Gain, MSOP Package Fixed Gain of 1, Matching 0.3% Fixed Gain of 2, Matching 0.3% Fixed Gain of 5, Matching 0.3% Fixed Gain of 10, Matching 0.3% 82dB S/N with 3V Supply, SO-8 Package 76dB S/N with 3V Supply, SO-8 Package
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16 Linear Technology Corporation
(408) 432-1900 FAX: (408) 434-0507
LT 0408 REV B * PRINTED IN USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417
www.linear.com
(c) LINEAR TECHNOLOGY CORPORATION 2003

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