QUICK START GUIDE FOR DEMONSTRATION CIRCUIT DC800
LOW DISTORTION, LOW NOISE DIFFERENTIAL AMPLIFIER / ADC DRIVER
3
DIFFERENTIAL OUTPUTS
To use the both outputs of the DC800, remove resistor
R14. The output now appears differentially across J4 and
J5. The output of the DC800 will still be matched to 50Ω
(differential) when used in this manner.
CHANGING THE OUTPUT COMMON-MODE VOLTAGE
Driving connector J3 controls the output common-mode
voltage of DC800. This function can be used to level-shift
the DC output voltage for optimum system performance.
However, if used for this purpose, AC-coupling capacitors
C3 and C4 should 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.
DISCRETE OUTPUT FILTERING
DC800 provides some additional component pads for
building discrete output filters. In addition to the internal
output low-pass filters built into the LT1993-X, a simple
LC bandpass filter can be placed on pads L1 and C8 to
filter a narrow-band signal. Other types of discrete filters
can be built by replacing components on the output of the
LT1993-X: R7-R15, C3-C4, C11, C16, and T2. For best
performance, care must be taken to maintain proper im-
pedance matching on the DC800 output. Also, since the
output of the LT1993-X is a low-impedance operational
amplifier output, performance will degrade when the out-
put is directly driving low impedances such as 50Ω. See
the LT1993-X datasheet for more information.
ENABLE (SHUTDOWN)
To drive the EN function of the LT1993-X with an external
voltage source, switch S1 must be disabled by removing
resistors R17 and R18. The LT1993-X will now be con-
trolled via the EN pin. To enable the part, pull the pin low
to within 0.8V of the GND pin on the DC800.
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
LT1993-X in shutdown.
GROUND AND VCC CONNECTIONS
DC800’s GND connector is not only connected to VEE of
the LT1993-X, 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 DC800 will yield the best
performance from the LT1993-X.
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 ele-
ments, and must be of good quality and well characterized
to ensure predictable results.
SCHEMATIC NOTES
The schematic included at the end of this Quick Start
Guide includes approximate signal levels seen at various
points along DC800’s signal chain. The signal levels
shown (SL1-SL4) assume that the signal level at the in-
put(s) is 0dB, and that there is a 50Ω load at the out-
put(s), from an oscilloscope, analyzer, or another circuit
component. If the output load is a high-impedance load,
the signal levels at SL2-SL4 will vary from the schematic.
The calibration circuit, as shown in the schematic, con-
tains two extra resistors R21-R22. These resistors are in-
cluded for added circuit flexibility, as well as for RF scat-
tering parameter calibration purposes.