DEMO CIRCUIT DC987 QUICK START GUIDE
LOW NOISE/DISTORTION DIFFERENTIAL ADC DRIVER
3
across J4 and J5. The output of the DC987 will still be
matched to 50Ω (differentially) when used in this manner.
CHANGING THE OUTPUT COMMON-MODE VOLTAGE
Turret TP5 controls the output common-mode voltage of
DC987. This function can be used to level-shift the DC out-
put voltage for optimum system performance. However, if
used for this purpose, AC-coupling capacitors C3 and C4
can be shorted or replaced by 0Ω resistors to allow the DC
bias to reach the output. Transformer T2 is a transmission-
line type, which means DC voltages will couple from input
to output of the transformer. By default, resistors R19 and
R20 supply 1.25V voltage to the common-mode pin (with
VCC=3.0V).
OUTPUT DRIVE
Since the output of the LTC6400/1 is a low-impedance op-
erational amplifier output, performance will degrade when
the output is directly driving low impedances such as 50Ω.
See the LTC6400/1 datasheet for more information.
ENABLE (SHUTDOWN)
Jumper JP1 controls the enable function of DC987. When
set to the ENBL position, the part will be on and draw
quiescent current. In the DIS position, the part will be in
shutdown and draw a very small amount of quiescent cur-
rent.
NOTE.
The output common-mode bias resistors R19 and
R20 draw current from VCC to GND, and must be
accounted for when measuring the current of the
LTC6400/1, especially in shutdown.
GROUND AND VCC CONNECTIONS
DC987’s GND connector is not only connected to VEE of the
LTC6400/1, it also serves as the board ground. For best
performance, a low-impedance return path to the power
supply from GND is crucial. Short, low impedance wires to
the VCC and GND connectors of DC987 will yield the best
performance from the LTC6400/1.
CABLES AND CONNECTORS
Long BNC or SMA cables are transmission lines, and for
best circuit performance should be kept as short as possi-
ble. BNC or SMA cables or barrels should be characterized
(or nulled out through calibration) for good high-frequency
performance.
Connectors also fall under the category of circuit elements,
and must be of good quality and well characterized to en-
sure predictable results.
SCHEMATIC NOTES
The schematic included with this Quick Start Guide includes
approximate signal levels seen at various points along
DC987’s signal chain. The signal levels shown (SL1-SL3)
assume that the signal level at the input(s) is 0dB, and that
there is a 50Ω load at the output(s), from an oscilloscope,
spectrum/network analyzer, et cetera. If the output load is a
high-impedance load, the signal levels at SL2-SL4 will vary
from the schematic.
Note that transformer T2 and the series resistors at the
LTC6400/1 outputs create a 13dB attenuation that will affect
the measured gain of the demo board. This is done so that
the output of DC987 can plug directly into a 50Ω analyzer
input, but the amplifier sees a more benign 400Ω load re-
sistance.
The calibration circuit, as shown in the schematic, contains
two extra resistors R21-R22. These resistors are included
for added circuit flexibility, and are not typically populated.