DC962A - Demo Manual

LT6600-2.5, LTC6600-5,
LT6600-10, LT6600-15 and LT6600-20
Fully Differential 4th Order Lowpass Filter
Demonstration circuit 962 features the LT6600-XX series
of a fully differential4th order low pass filter approximating a Chebyshev frequency response. The cutoff frequencies (Fc) of LT6600-XX are:2.5M H z, 5M H z, 10M H z,
15M H z and 20M H z (see Table 1). The LT6600-XX combines a low pass filter w ith a fully differential
amplifier for driving the differential input of an analog to Digital Converter (ADC). An LT6600-XX also
provides the necessary levelshifting to set its output
common mode voltage to accommodate the reference voltage requirements of an ADC. An LT6600-XX
filter IC requires only tw o external input resistors to set
the filter’s differential gain. A DC962 board contains an
LT6600-XX low pass filter configured w ith input resistors
as a unity gain differentialamplifier. G ains greater than
one require changing the input resistors to a low er value
(refer to DC962 schematic).
Connection to the differential input and output of a
DC962 is through SM A connectors. On board jumpers
configure the DC962 for dualor single supply operation.
The differentialinput of a DC962 is AC coupled and can
be configured for DC coupling by shorting the input capacitors w ith zero ohms surface-mount resistor jumpers. In addition, the DC962 has surface-mount pads to
add input passive components for input signal filtering
and DC biasing.
Design files for this circuit board are available.
Callthe LTC factory.
, LTC and LT are registered trademarks ofLinear Technology Corporation.
Table 1. The DC962 Assem blies
Figure 1. DC962A
LT6600-2.5, Fc=2.5M H z
LT6600-5, Fc=5M H z
LT6600-10, Fc=10M H z
LT6600-15, Fc=15M H z
LT6600-20, Fc=20M H z
Figure 2. LT6600-X Am plitude Response (Gain=1)
Figure 3. Single-Ended Input To DifferentialOutput Quick Test Set-U p
DC962 Jum per Settings:
LT6600-XX, VM ID pin to gnd or open and
V- pin to V- or gnd.
LT6600-XX, VOCM pin shorted to pin 7 or
A. Single-Ended Input To DifferentialOutput
1. Connect to a DC962 a dual pow er supply, a function
generator and an oscilloscope as show n in Figure 3 (JP1
to DU AL and JP2 to M ID).
Note 1: The 50 termination on J2 input is used to balance the 50 generator impedance on J1 input. The additional 50 input impedance is in series w ith the input
resistors of the LT6600-XX (R 2 and R 3) therefore the
single-ended to differential gain is less than one. Example: the feedback resistors in the LT6600-10 or -20 are
402 (see Figure 6) and for gain equalto one, the input
resistors should be also 402 . H ow ever, in Figure 3,
50 is in series w ith the 402 input resistors and the
gain is [402/(402+50)] w hich equals to 0.8894.
2. Set the function generator for a 1Vp-p, 100 kH z sinew ave and turn-on the pow er supply.
3. The channel 2 input of the oscilloscope is in phase
w ith the DC962 input and the channel1 input is 180 degrees out ofphase w ith the DC962 input. The singleended output show n on channel 1 or 2, is a 0.5Vp-p
sinew ave (a 1Vp-p differentialoutput).
Note 2: The LT6600-XX can directly drive at least a 50pF
capacitive load at each output. How ever an LT6600-XX
can drive directly a low frequency sinew ave (100 kH z or
less) into a capacitive load of up to 100pF. In this “Q uick
Test” procedure, the output signal is a sinew ave and
each LT6600-XX output drives the capacitance of a 24
inch or less cable plus the input capacitance of the oscilloscope input, a capacitive load of 70pF (30pF per foot
for the coax cable and 10pF for the oscilloscope input).
For testing the transient response of an LT6600-XX to a
squarew ave, use a 10x low capacitance oscilloscope
probe to monitor the DC962 output at J3 or J4.
B. DC Coupled InOputs and Output Com m on M ode Voltage Adjustm ent
Figure 4. Input and Output Com m on M ode Quick Test Set-Up
DC962 Jum per Settings:
LT6600-XX, VM ID pin to gnd or open and
V- pin to V- or gnd.
LT6600-XX, VOCM pin shorted to pin 7 or
1. On the DC962, install0603 zero ohm resistors at R 1 and R 4 to short input capacitors C6 and C9 respectively
(see DC962 schematic).
2. Connect DC962 as show n in Figure 4.
3. Apply an input signalw ith a DC offset (VinCM ) (see Table 2).
The output common mode (VOCM ) can be set independently ofthe VinCM (see Table 2).
This adjustment is made by applying a DC voltage at the EXT VOCM connection.
Table 2. Input and Output Com m on M ode Voltage Range (Gain=1)
Input Com m on M ode (VinCM )
3V (VM ID=1.5V) 0V – 2.5V
5V (VM ID=2.5V) 0V – 5V
Output Com m on M ode (VOCM )
0.75V - 1.75V
1.50V – 3.75V
C. Driving the Analog Inputs ofan ADC
W hen using a DC962 to drive the differentialinput of an ADC (analog-to-digitalconverter), the boards output resistors
R 6 and R7 and capacitors C15, C17 and C18 should be configured to the values required for the ADC input. Figure 5
show s the optimum values for the DC962 output components w hen driving an ADC.
LTC2227, LTC2228 and LTC2229 is a 12-bit 40M sps, 65M sps and 80M sps ADC respectively and
LTC2247, LTC2248 and LTC2249 is a 14-bit 40M sps, 65M sps and 80M sps ADC respectively.
Figure 5. DC962 Output Com ponent Values
Figure 6. Setting the DC962 DifferentialGain w ith the Input Resistors
(dualor single supply operation).