DC962
DEMO CIRCUIT 962A
QUICK STARTLT1994
GUIDE
LT6600-2.5, LTC6600-5,
LT6600-10, LT6600-15 and LT6600-20
Fully Differential 4
th
Order Lowpass Filter
one require changing the input resistors to a l er val
ow
ue
(ref to DC962 schematic).
er
Connection to the dif erential input and output of a
f
DC962 is through SM A connectors. On board j
umpers
conf
igure the DC962 f dualor singl suppl operation.
or
e
y
The dif erentialinput of a DC962 is AC coupl and can
f
ed
be conf
igured f DC coupl by shorting the input ca-
or
ing
pacitors w ith zero ohms surf
ace-mount resistor j
ump-
ers. I addition, the DC962 has surf
n
ace-mount pads to
add input passive components f input signal f tering
or
il
and DC biasing.
Design f es f this circuit board are avail e.
il or
abl
Cal the LTC f
l
actory.
, LTC and LT are registered trademarks ofLinear Technol Corporation.
ogy
DESCRIPTION
Demonstration circuit 962 f
eatures the LT6600-XX series
of a f l dif erential4
th
order l pass f ter approximat-
uly f
ow
il
f re-
ing a Chebyshev f
requency response. The cutof f
quencies (Fc) of LT6600-XX are:2.5M H z, 5M H z, 10M H z,
15M H z and 20M H z (see Tabl 1). The LT6600-XX com-
e
bines a l pass f ter w ith a f l dif erential
ow
il
uly
f
ampl ier f driving the dif erential input of an ana-
if or
f
l to Digital Converter (ADC). An LT6600-XX al
og
so
provides the necessary l
evelshif
ting to set its output
common mode vol
tage to accommodate the ref
er-
ence vol
tage requirements of an ADC. An LT6600-XX
f ter I requires onl tw o external input resistors to set
il C
y
the f ter’ dif erential gain. A DC962 board contains an
il s f
LT6600-XX l pass f ter conf
ow
il
igured w ith input resistors
as a unity gain dif erentialampl ier. G ains greater than
f
if
Tabl 1. The DC962 Assem bl
e
ies
ASSEM BLY
DC962A-A
DC962A-B
DC962A-C
DC962A-D
DC962A-E
U1
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 1. DC962A
Figure 2. LT6600-X Am pl
itude Response (Gain=1)
1
DC962
QUI TEST PROCEDURE
CK
Figure 3. Singl
e-Ended I
nput To Dif erentialOutput Quick Test Set-U p
f
LT6600-X
DC962 Jum per Settings:
JP1
LT6600-XX, VM I pin to gnd or open and
D
V- pin to V- or gnd.
JP2
LT6600-XX, VOCM pin shorted to pin 7 or
an EXT VOCM .
A. Singl
e-Ended I
nput To Dif erentialOutput
f
1. Connect to a DC962 a dual pow er suppl a f
y, unction
generator and an osciloscope as show n in Figure 3 (JP1
l
to DU AL and JP2 to M I
D).
Note 1:
The 50 termination on J2 input is used to bal
-
ance the 50 generator impedance on J1 input. The ad-
ditional 50 input impedance is in series w ith the input
resistors of the LT6600-XX (R 2 and R 3) theref the
ore
singl
e-ended to dif erential gain is l than one. Exam-
f
ess
pl the f
e:
eedback resistors in the LT6600-10 or -20 are
402 (see Figure 6) and f gain equalto one, the input
or
resistors shoul be al 402 . H ow ever, in Figure 3,
d
so
50 is in series w ith the 402 input resistors and the
gain is [402/
(402+50)] w hich equal to 0.8894.
s
2. Set the f
unction generator f a 1Vp-p, 100 kH z sine-
or
w ave and turn-on the pow er suppl
y.
3. The channel 2 input of the osciloscope is in phase
l
w ith the DC962 input and the channel1 input is 180 de-
grees out ofphase w ith the DC962 input. The singl
e-
ended output show n on channel 1 or 2, is a 0.5Vp-p
sinew ave (a 1Vp-p dif erentialoutput).
f
Note 2:
The LT6600-XX can directl drive at l a 50pF
y
east
capacitive l at each output. How ever an LT6600-XX
oad
can drive directl a l f
y ow requency sinew ave (100 kH z or
l
ess) into a capacitive l of up to 100pF. I this “Q uick
oad
n
Test” procedure, the output signal is a sinew ave and
each LT6600-XX output drives the capacitance of a 24
inch or l cabl pl the input capacitance of the oscil
ess
e us
-
l
oscope input, a capacitive l of 70pF (30pF per f
oad
oot
f the coax cabl and 10pF f the osciloscope input).
or
e
or
l
For testing the transient response of an LT6600-XX to a
squarew ave, use a 10x l capacitance osciloscope
ow
l
probe to monitor the DC962 output at J3 or J4.
2
DC962
B. DC Coupl I
ed nOputs and Output Com m on M ode Vol
tage Adj
ustm ent
Figure 4. I
nput and Output Com m on M ode Quick Test Set-Up
LT6600-X
DC962 Jum per Settings:
JP1
LT6600-XX, VM I pin to gnd or open and
D
V- pin to V- or gnd.
JP2
LT6600-XX, VOCM pin shorted to pin 7 or
an EXT VOCM .
1. On the DC962, instal 0603 zero ohm resistors at R 1 and R 4 to short input capacitors C6 and C9 respectivel
l
y
(see DC962 schematic).
2. Connect DC962 as show n in Figure 4.
3. Appl an input signalw ith a DC of set (VinCM ) (see Tabl 2).
y
f
e
The output common mode (VOCM ) can be set independentl ofthe VinCM (see Tabl 2).
y
e
This adj
ustment is made by appl
ying a DC vol
tage at the EXT VOCM connection.
Tabl 2. I
e
nput and Output Com m on M ode Vol
tage Range (Gain=1)
V+
I
nput Com m on M ode (VinCM )
3V (VM I
D=1.5V) 0V – 2.5V
5V (VM I
D=2.5V) 0V – 5V
Output Com m on M ode (VOCM )
0.75V - 1.75V
1.50V – 3.75V
3
DC962
C. Driving the Anal I
og nputs ofan ADC
LT6600-X
W hen using a DC962 to drive the dif erentialinput of an ADC (anal
f
og-to-digitalconverter), the boards output resistors
R 6 and R7 and capacitors C15, C17 and C18 shoul be conf
d
igured to the val required f the ADC input. Figure 5
ues
or
show s the optimum val f the DC962 output components w hen driving an ADC.
ues or
LTC2227, LTC2228 and LTC2229 is a 12-bit 40M sps, 65M sps and 80M sps ADC respectivel and
y
LTC2247, LTC2248 and LTC2249 is a 14-bit 40M sps, 65M sps and 80M sps ADC respectivel
y.
Figure 5. DC962 Output Com ponent Val
ues
Figure 6. Setting the DC962 Dif erentialGain w ith the I
f
nput Resistors
(dualor singl suppl operation).
e
y
4
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