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LT6600-15 Very Low Noise, Differential Amplifier and 15MHz Lowpass Filter FEATURES
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DESCRIPTION
The LT(R)6600-15 combines a fully differential amplifier with a 4th order 15MHz lowpass filter approximating a Chebyshev frequency response. Most differential amplifiers require many precision external components to tailor gain and bandwidth. In contrast, with the LT6600-15, two external resistors program differential gain, and the filter's 15MHz cutoff frequency and passband ripple are internally set. The LT6600-15 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 LT6600-15 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 LT6600-15 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 LT6600-15 is packaged in an SO-8 and is pin compatible with stand alone differential amplifiers.
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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 15MHz Cutoff 76dB S/N with 3V Supply and 2VP-P Output Low Distortion, 2VP-P, 800 Load, VS = 3V 1MHz: 86dBc 2nd, 90dBc 3rd 10MHz: 63dBc 2nd, 69dBc 3rd Fully Differential Inputs and Outputs Compatible with Popular Differential Amplifier Pinouts SO-8 Package
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
LT6600-15 3V 0.1F RIN 536 1 7 0.01F VIN RIN 536 2 8 3 5.6pF 3V V+ DOUT VCM 2.2F GAIN = 536/RIN
660015 TA01a
An 8192 Point FFT Spectrum
LTC2249 0 -10 -20 AMPLITUDE (dB) -30 -40 -50 -60 -70 -80 -90 -100 -110 -120 0 10 20 FREQUENCY (MHz)
660015 TA01b
INPUT 10.7MHz 2VP-P fSAMPLE = 80MHz
-
VMID VOCM
+ -
4
25 5.6pF 5.6pF
+
AIN
25 5
+
6
-
V-
30
40
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LT6600-15 ABSOLUTE MAXIMUM RATINGS
(Note 1)
PIN CONFIGURATION
TOP VIEW IN- 1 VOCM 2 V+ 3 OUT+ 4 8 7 6 5 IN+ VMID V- OUT-
Total Supply Voltage .................................................11V 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
S8 PACKAGE 8-LEAD PLASTIC SO TJMAX = 150C, JA = 100C/W
ORDER INFORMATION
LEAD FREE FINISH LT6600CS8-15#PBF LT6600IS8-15#PBF LEAD BASED FINISH LT6600CS8-15 LT6600IS8-15 TAPE AND REEL LT6600CS8-15#TRPBF LT6600IS8-15#TRPBF TAPE AND REEL LT6600CS8-15#TR LT6600IS8-15#TR PART MARKING 660015 600I15 PART MARKING 660015 600I15 PACKAGE DESCRIPTION 8-Lead Plastic SO 8-Lead Plastic SO PACKAGE DESCRIPTION 8-Lead Plastic SO 8-Lead Plastic SO TEMPERATURE RANGE -40C to 85C -40C to 85C TEMPERATURE RANGE -40C to 85C -40C to 85C
Consult LTC Marketing for parts specified with wider operating temperature ranges. 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 = 536, and RLOAD = 1k.
PARAMETER Filter Gain, VS = 3V CONDITIONS VIN = 2VP-P, fIN = DC to 260kHz VIN = 2VP-P, fIN = 1.5MHz (Gain Relative to 260kHz) VIN = 2VP-P, fIN = 7.5MHz (Gain Relative to 260kHz) VIN = 2VP-P, fIN = 12MHz (Gain Relative to 260kHz) VIN = 2VP-P, fIN = 15MHz (Gain Relative to 260kHz) VIN = 2VP-P, fIN = 45MHz (Gain Relative to 260kHz) VIN = 2VP-P, fIN = 75MHz (Gain Relative to 260kHz) Filter Gain, VS = 5V VIN = 2VP-P, fIN = DC to 260kHz VIN = 2VP-P, fIN = 1.5MHz (Gain Relative to 260kHz) VIN = 2VP-P, fIN = 7.5MHz (Gain Relative to 260kHz) VIN = 2VP-P, fIN = 12MHz (Gain Relative to 260kHz) VIN = 2VP-P, fIN = 15MHz (Gain Relative to 260kHz) VIN = 2VP-P, fIN = 45MHz (Gain Relative to 260kHz) VIN = 2VP-P, fIN = 75MHz (Gain Relative to 260kHz) Filter Gain, VS = 5V Filter Gain, RIN = 133 VIN = 2VP-P, fIN = DC to 260kHz VOUT = 2VP-P, fIN = DC to 260kHz, VS = 3V VOUT = 2VP-P, fIN = DC to 260kHz, VS = 5V VOUT = 2VP-P, fIN = DC to 260kHz, VS = 5V
l l l l l l l l l l l l
ELECTRICAL CHARACTERISTICS
MIN - 0.5 -0.1 -0.3 -0.3 - 0.7
TYP 0.1 0 0 0.2 0 - 29 -46
MAX 0.5 0.1 0.4 1.0 1.0 -25 0.5 0.1 0.3 0.9 0.9 -25 0.4 12.5 12.5 12.4
UNITS dB dB dB dB dB dB dB dB dB dB dB dB dB dB dB dB dB dB
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- 0.5 - 0.1 -0.4 -0.4 -0.8
0 0 0 0.1 0 - 29 - 46
- 0.6 11.5 11.5 11.4
-0.1 12.0 12.0 11.9
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LT6600-15 ELECTRICAL CHARACTERISTICS
PARAMETER Filter Gain Temperature Coefficient (Note 2) Noise Distortion (Note 4) CONDITIONS fIN = 250kHz, VIN = 2VP-P Noise BW = 10kHz to 15MHz 1MHz, 2VP-P, RL = 800, VS = 3V 10MHz, 2VP-P, RL = 800, VS = 3V Differential Output Swing Input Bias Current Input Referred Differential Offset Measured Between Pins 4 and 5 Average of Pin 1 and Pin 8 RIN = 536 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 VS = 3V VOCM = VMID= VS/2 VS = 5V VS = 3V VS = 3V, VS = 5V VS = 3V VS = 5V VS = 5V 2nd Harmonic 3rd Harmonic 2nd Harmonic 3rd Harmonic VS = 5V VS = 3V
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 = 536, and RLOAD = 1k.
MIN TYP 780 109 86 90 63 69 3.80 3.75 - 90 4.75 4.50 - 35 5 10 10 5 5 5 10 Differential Input = 500mVP-P, RIN = 133 Differential Input = 2VP-P, Pin 7 at Mid-Supply Common Mode Voltage at Pin 2 0.0 0.0 -2.5 1.0 1.5 -1.0 -35 -40 -55 2.45 4.3 -10 -10 5 0 -10 64 2.50 1.50 5.7 -2 -2 35 38 3 39 44 45 48 11 2.55 7.7 1.5 3.0 1.0 1.5 3.0 2.0 40 40 35 25 30 35 15 17 20 MAX UNITS ppm/C VRMS dBc dBc 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 k A A mA mA mA mA V
RIN = 133
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
Power Supply Voltage 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 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-15 is guaranteed functional over the operating temperature range -40C to 85C. Note 7: The LT6600C-15 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-15 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.
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LT6600-15 TYPICAL PERFORMANCE CHARACTERISTICS
Amplitude Response
10 0 -10 GAIN (dB) -20 -30 -40 -50 VS = 5V GAIN = 1 TA = 25C 1 10 FREQUENCY (MHz) 100
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Passband Gain and Phase
1 0 -1 -2 GAIN (dB) -3 -4 -5 -6 -7 -8 -9 0 5 15 10 FREQUENCY (MHz) 20 25
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Passband Gain and Delay
VS = 5V GAIN = 1 TA = 25C 225 180 135 90 PHASE (DEG) 1 0 -1 -2 GAIN (dB) -3 -4 -5 -6 -7 -8 -9 0 5 15 10 FREQUENCY (MHz) 20 25
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GAIN
GAIN
VS = 5V GAIN = 1 TA = 25C
50 45 40 35 DELAY (ns)
PHASE
45 0 -45 -90 -135 -180 -225
DELAY
30 25 20 15 10 5 0
-60 0.1
Passband Gain and Delay
14 12 10 8 GAIN (dB) 6 4 2 0 -2 -4 -6 0 5 15 10 FREQUENCY (MHz) 20 25
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Output Impedance
VS = 5V GAIN = 4 TA = 25C 50 45 OUTPUT IMPEDANCE () 40 35 DELAY (ns) 100 VS = 5V GAIN = 1 TA = 25C 80 75 70 65 CMRR (dB) 60 55 50 45 40 35 0.1 0.1 30 1 10 FREQUENCY (MHz) 100
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Common Mode Rejection Ratio
VIN = 1VP-P VS = 5V GAIN = 1 TA = 25C
GAIN
10
DELAY
30 25 20 15 10 5 0
1
0.1
1 10 FREQUENCY (MHz)
100
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Power Supply Rejection Ratio
80 70 60 DISTORTION (dB) PSRR (dB) 50 40 30 20 10 VS = 3V VIN = 200mVP-P TA = 25C V+ TO DIFFOUT 1 10 FREQUENCY (MHz) 100
600015 G07
Distortion vs Frequency
-50 -60 -70 -80 -90 VIN = 2VP-P VS = 3V RL = 800 AT EACH OUTPUT GAIN = 1 TA = 25C -40
Distortion vs Signal Level
3RD HARMONIC VS = 3V 10MHz INPUT RL = 800 AT -50 EACH OUTPUT GAIN = 1 -60 TA = 25C 2ND HARMONIC 10MHz INPUT 2ND HARMONIC 1MHz INPUT 0 1 2 3 3RD HARMONIC 1MHz INPUT
DISTORTION (dB) 100
660015 G08
-70 -80 -90
-100 -110 0.1 1 10 FREQUENCY (MHz)
-100 -110
0 0.1
4
5
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INPUT LEVEL (VP-P)
DIFFERENTIAL INPUT, 2ND HARMONIC DIFFERENTIAL INPUT, 3RD HARMONIC SINGLE-ENDED INPUT, 2ND HARMONIC SINGLE-ENDED INPUT, 3RD HARMONIC
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LT6600-15 TYPICAL PERFORMANCE CHARACTERISTICS
Distortion vs Signal Level
-40 VS = 5V RL = 800 AT EACH OUTPUT -50 GAIN = 1 TA = 25C -60 2ND HARMONIC, 10MHz INPUT -70 -80 -90 -100 -110 0 1 3RD HARMONIC, 10MHz INPUT 2ND HARMONIC, 1MHz INPUT 3RD HARMONIC, 1MHz INPUT 2 3 4 5
660015 G10
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 GAIN = 1 RL = 800 AT EACH OUTPUT TA = 25C 2VP-P 1MHz INPUT -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
DISTORTION (dB)
-100 -110 -2 -1 0 1 2 -3 3 INPUT COMMON MODE VOTLAGE RELATIVE TO PIN 7 (V)
66002 G11
-100
GAIN = 4, RL = 800 AT EACH OUTPUT TA = 25C, 500mVP-P 1MHz INPUT
INPUT LEVEL (VP-P)
-3 3 -2 -1 0 1 2 INPUT COMMON MODE VOTLAGE RELATIVE TO PIN 7 (V)
660015 G12
Distortion vs Output Common Mode
-40 DISTORTION COMPONENT (dB) -50 -60 -70 -80 -90 -100 -110 -1.5 -1 -0.5 2ND HARMONIC, VS = 3V 3RD HARMONIC, VS = 3V 2ND HARMONIC, VS = 5V 3RD HARMONIC, VS = 5V 2ND HARMONIC, VS = 5V 3RD HARMONIC, VS = 5V 2VP-P 1MHz INPUT GAIN = 1, RL = 800 AT EACH OUTPUT TA = 25C 2.5
0 0.5 1 1.5 2 VOLTAGE PIN 2 TO PIN 7 (V)
660015 G13
Total Supply Current vs Total Supply Voltage
50 TOTAL SUPPLY CURRENT (mA) 45 40 35 TA = -40C 30 25 20 2 10 4 6 8 TOTAL SUPPLY VOLTAGE (V) 12
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Transient Response
OUT- 200mV/DIV
TA = 85C TA = 25C OUT+ 200mV/DIV
IN- IN+ 500mV/DIV 100ns/DIV DIFFERENTIAL GAIN = 1 SINGLE-ENDED INPUT DIFFERENTAL OUTPUT
660015 G15
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LT6600-15 PIN FUNCTIONS
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 536/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 DIAGRAM
VIN+ RIN 8 IN+ VMID 7 V+ 11k 536 11k 200 V- OP AMP V- 6 OUT- 5
PROPRIETARY LOWPASS FILTER STAGE
+
VOCM
200
- +
200
+-
VOCM
-
-+
536
200
1 VIN- RIN IN-
2 VOCM
3 V+
4 OUT+
660015 BD
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LT6600-15 APPLICATIONS INFORMATION
Interfacing to the LT6600-15 The LT6600-15 requires two equal external resistors, RIN, to set the differential gain to 536/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 LT6600-15 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 LT6600-15 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- t
+
output voltage is 1.65V, and the differential output voltage is 2VP-P for frequencies below 15MHz. 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 section). Figure 2 shows how to AC couple signals into the LT6600-15. 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 536 gain setting resistor form a high pass filter, attenuating signals below 3kHz. Larger values of coupling capacitors will proportionally reduce this highpass 3dB frequency.
3.3V 0.1F V 3 3
VIN
-
536
1 7 0.01F 2 8
-
LT6600-15
+
4
VOUT+ VOUT-
2 1 0
VOUT+ VOUT- t
660015 F01
+
6
-5
536
Figure 1
3.3V V 0.1F 2 1 0 -1 VIN+ 0.1F t
+ VIN
0.1F 536 1 7 0.01F 536 2 8 3
V 3 VOUT+ VOUT- 2 1 0 VOUT+ VOUT-
-
LT6600-15
+
4
+
6
-
5
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Figure 2
62pF V 3 2 1 0 500mVP-P (DIFF) VIN+ VIN- t 62pF 0.01F VIN
+
5V 0.1F V 3 VOUT+ 2 VOUT- 1 0 VOUT- 3
VIN
-
133
1 7 2 8
-
LT6600-15
+
4
VOUT+
+
6
-
5
133
+ -
2V
660015 F03
t
Figure 3
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LT6600-15 APPLICATIONS INFORMATION
In Figure 3 the LT6600-15 is providing 12dB of gain. The gain resistor has an optional 62pF in parallel to improve the passband flatness near 15MHz. The common mode output voltage is set to 2V. Use Figure 4 to determine the interface between the LT6600-15 and a current output DAC. The gain, or "transimpedance," is defined as A = VOUT/IIN. To compute the transimpedance, use the following equation: A= 536 * R1 () (R1+ R2) amples where impedance must be considered is the evaluation of the LT6600-15 with a network analyzer. Figure 5 is a laboratory setup that can be used to characterize the LT6600-15 using single-ended instruments with 50 source impedance and 50 input impedance. For a unity gain configuration the LT6600-15 requires a 536 source resistance yet the network analyzer output is calibrated for a 50 load resistance. The 1:1 transformer, 52.3 and 523 resistors satisfy the two constraints above. The transformer converts the single-ended source into a differential stimulus. Similarly, the output of the LT6600-15 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 LT6600-15 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 LT6600-15 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 LT6600-15 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 CURRENT OUTPUT DAC IIN- R1 IIN+ R1 R2 0.011F R2 1 7 2 8 0.1F 3.3V 0.1F 3 NETWORK ANALYZER SOURCE VOUT+ VOUT- 50 COILCRAFT TTWB-1010 1:1 523 1 7 52.3 2 8 523 COILCRAFT TTWB-16A 4:1 402 NETWORK ANALYZER INPUT
By setting R1 + R2 = 536, 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 = 487. The voltage at Pin 7 is 1.65V. The voltage at the DAC pins is given by:
VDAC = VPIN7 * R1 R1* R2 + IIN * R1+ R2 + 536 R1+ R2 = 77mV + IIN * 45.3
IIN is IIN+ or IIN-. The transimpedance in this example is 49.8. Evaluating the LT6600-15 The low impedance levels and high frequency operation of the LT6600-15 require some attention to the matching networks between the LT6600-15 and other devices. The previous examples assume an ideal (0) source impedance and a large (1k) load resistance. Among practical ex-
3
-
-+ +
6
+ -
6
4
4
LT6600-15
50 402
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LT6600-15
-
+
5
5
0.1F
660015 F04
-2.5V
Figure 4
Figure 5
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LT6600-15 APPLICATIONS INFORMATION
20 0 OUTPUT LEVEL (dBV) -20 -40 -60 -80 -100 0 1 4 3 5 2 1MHz INPUT LEVEL (VP-P) 6 7 3RD HARMONIC 85C 3RD HARMONIC 25C 2ND HARMONIC 85C 2ND HARMONIC, 25C 1dB COMPRESSION POINTS 25C 85C
The LT6600-15 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 section). Common Mode DC Currents In applications like Figure 1 and Figure 3 where the LT6600-15 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 LT6600-15 (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 536 feedback resistor and the external 133 input resistor. The resulting 3.7mA 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 LT6600-15, 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 9.9mA is used to translate the common mode voltages. A simple modification to Figure 3 will reduce the DC common mode currents by 40%. 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 6mA. Of course, by AC coupling the inputs of Figure 3, the common mode DC current can be reduced to 2.5mA.
660015 F06
Figure 6. Output Level vs Input Level, Differential 1MHz Input, Gain = 1
supply rails, the input/output behavior of the IC shown in Figure 6 is relatively independent of the power supply voltage. The two amplifiers inside the LT6600-15 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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LT6600-15 APPLICATIONS INFORMATION
Noise The noise performance of the LT6600-15 can be evaluated with the circuit of Figure 7. Given the low noise output of the LT6600-15 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 15MHz. 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:
45 40 NOISE DENSITY (nVRMS/Hz) 35 30 25 20 15 10 5 0 0.01 0.1 1 10 FREQUENCY (MHz) VIN RIN 1 7 2 8 RIN 2.5V 0.1F 3 COILCRAFT TTWB-1010 25 1:1 25 SPECTRUM ANALYZER INPUT
-+ +
6
4
LT6600-15
50
-
5 0.1F
660015 F07
-2.5V
Figure 7
NOISE DENSITY, GAIN = 1x NOISE DENSITY, GAIN = 4x INTEGRATED NOISE, GAIN = 1x INTEGRATED NOISE, GAIN = 4x 180 160 140 120 100 80 60 40 20 0 100
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INTEGRATED NOISE (V)
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 LT6600-15 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 15MHz 36VRMS 62VRMS 109VRMS INPUT REFERRED INTEGRATED NOISE 10kHz TO 30MHz 51VRMS 92VRMS 169VRMS
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 LT6600-15 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 LT6600-15 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 LT6600-15 (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.
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RIN 133 267 536
The noise at each output is comprised of a differential component and a common mode component. Using a transformer or combiner to convert the differential outputs to single-ended signal rejects the common mode noise and gives a true measure of the S/N achievable in the system. Conversely, if each output is measured individually and the
10
LT6600-15 APPLICATIONS INFORMATION
Table 2. LT6600-15 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 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 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 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 50mA. 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 * 0.05 * 100) = 110C When using higher supply voltages or when driving small impedances, more copper may be necessary to keep TJ below 150C.
PACKAGE DESCRIPTION
S8 Package 8-Lead Plastic Small Outline (Narrow .150 Inch)
(Reference LTC DWG # 05-08-1610)
.045 .005
8 .189 - .197 (4.801 - 5.004) NOTE 3 7 6 5
.050 BSC
.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) 0- 8 TYP
1
2
3
4
.053 - .069 (1.346 - 1.752)
.004 - .010 (0.101 - 0.254)
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)
.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.
11
LT6600-15 TYPICAL APPLICATION
Dual Matched I and Q Lowpass Filter and ADC (Typical Phase Matching 1 Degree)
3V 0.1F
VCMA 3V 0.1F RIN 536 3 5.6pF 2.2F
1 7 2 8
-
I 0.1F RIN 536
LT6600-15
+
4
25 5.6pF 5.6pF LTC2299 INA
+
6 3V
-
25 5
0.1F RIN 536 3 5.6pF
1 7 2 8
-
Q 0.1F RIN 536
LT6600-15
+
4
25 5.6pF INB
+
6
-
25 5
5.6pF GAIN = 536/RIN 2.2F
660015 TA02
VCMB
RELATED PARTS
PART NUMBER LTC 1565-31 LTC1566-1 LT1567 LT1568 LT1993-X LT1994 LT6600-2.5 LT6600-5 LT6600-10 LT6600-20
(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
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
Low Distortion, Low Noise Differential Amplifier/ADC Driver Fixed Gain of 6dB, 12dB and 20dB Low Distortion, Low Noise Differential Amplifier/ADC Driver Adjustable, Low Power, VS = 2.375V to 12.6V Very Low Noise Differential Amplifier and 2.5MHz Lowpass Filter Very Low Noise Differential Amplifier and 5MHz Lowpass Filter Very Low Noise Differential Amplifier and 10MHz Lowpass Filter Very Low Noise Differential Amplifier and 20MHz Lowpass Filter 86dB S/N with 3V Supply, SO-8 82dB S/N with 3V Supply, SO-8 82dB S/N with 3V Supply, SO-8 76dB S/N with 3V Supply, SO-8
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12 Linear Technology Corporation
(408) 432-1900 FAX: (408) 434-0507
LT 0408 REV A * PRINTED IN USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417
www.linear.com
(c) LINEAR TECHNOLOGY CORPORATION 2005

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