DEMO CIRCUIT DC987 QUICK START GUIDE
DEMO CIRCUIT 987
LOW NOISE/DISTORTION DIFFERENTIAL ADC DRIVER
QUICK START GUIDE
LTC6400 and LTC6401
Low Noise/Distortion
Differential ADC Drivers
DESCRIPTION
Demonstration circuit 987 features the LTC6400 and
LTC6401 ADC Drivers. It incorporates a variety of passive
components to support configurations for varied applica-
tions. Single-ended or differential input and output configu-
rations are possible, and there is a calibration circuit to null
out the effects of other circuit components.
The LTC6400/1 is a high-speed differential amplifier with
superior distortion and noise performance, perfect for de-
manding communications transceivers, cellular base-
stations, and other high-speed signal chain applications.
The LTC6400/1 accepts single-ended or differential inputs
with almost no difference in distortion performance. One
standout feature of the LTC6400/1 is the ability to indepen-
dently adjust the output common-mode voltage without any
additional components.
Design files for this circuit board are available. Call
the LTC factory.
, LTC and LT are registered trademarks of Linear Technology Corporation.
QUICK START PROCEDURE
Table 1 shows the function of each SMA connector on the
board. Refer to Figure 1 and Table 1, and follow the proce-
dure below:
1.
Connect the power supply VCC. The power labels of VCC,
EN, and GND on the board directly correspond to the
VCC, EN, and VEE pins of the LTC6400/1, respectively.
Jumper JP1 enables or shuts down the LTC6400/1.
ance-matched to 50Ω, suitable for the input of a network
or spectrum analyzer.
Table 1: DC987 SMA Connector Descriptions
CONNECTOR
FUNCTION
J1 (+IN)
Differential Input. Use this connector to supply an input
to the DC987. Drive from a 50ohm signal source, no
external termination necessary.
J2 (-IN)
Differential Input.
Not connected by default.
Resistor
R6 can installed and R2 removed to drive the DC987
differentially.
Output Common-Mode Adjust.
By default, 1.25V is
supplied to this pin by two resistors (VCC=3V).
Over-
ride this voltage with any DC voltage source.
Output. Impedance-matched to 50 ohms, can be used
to drive a 50ohm network/spectrum analyzer input.
Differential Output.
Not connected by default.
Resistor
R12 can be installed and R14 removed for differential
output drive.
Calibration network input. Use with network analyzer to
calibrate out effects of board trace parasitics, transfor-
mers, and SMA connectors.
2.
For network analyzer use, calibrate out board and trans-
former effects using J6 and J7. There is a 1:1 impedance
transfer from J6 to J7, so external termination may be
necessary for a 50Ω match.
TP53 (VOCM)
3.
Apply an input signal to J1. A low-distortion, low
noise signal source with an external high-order filter
will yield the best performance. DC987’s input is
impedance-matched to 50Ω; no external termination
is necessary.
4.
Observe the output via J4. By default, the unfiltered out-
puts of the LTC6400/1 are used. The output is imped-
J4 (+OUT)
J5 (-OUT)
J6 (TEST IN)
J7 (TEST OUT) Calibration network output.
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DEMO CIRCUIT DC987 QUICK START GUIDE
LOW NOISE/DISTORTION DIFFERENTIAL ADC DRIVER
Picture of DC987 Top Silkscreen
ADDITIONAL INFORMATION
Although the DC987 demo board is ready to use out of the
box, it has features that you can access by adding, remov-
ing or changing components on the board. These are de-
scribed below.
50Ω signal source. The input to the DC987 will still be
matched to 50Ω (differential) when driven in this manner.
DRIVING THE INPUTS WITH DC COUPLING
It is possible to drive the DC987 inputs differentially with DC
coupling. Capacitors C1 and C2 should be shorted or re-
placed with 0Ω resistors. As a warning, the low input im-
pedance of the LTC6400/1 can cause large input bias cur-
rents if DC987 is driven DC coupled. If transformer T1 is a
transmission-line transformer (the transformer included on-
board is a TLT), DC voltages and currents will propagate
through. See the LTC6400/1 datasheet for additional infor-
mation about DC coupling and input bias currents.
DIFFERENTIAL OUTPUTS
To use the both outputs of the DC987, remove resistor R13
and install R11. The output now appears differentially
DEMO BOARD VERSIONS
DC987 has eight versions to support all of the different gain
options of the LTC6400/1. DC987B-A through DC987B-D
contain the LTC6400-8, LTC6400-14, LTC6400-20, and
LTC6400-26, respectively. DC987B-E through DC987B-H
contain the LTC6401-8, LTC6401-14, LTC6401-20, and
LTC6401-26. See the schematic (last page of this guide) for
component differences between the versions.
DIFFERENTIAL INPUTS
To drive the inputs differentially, remove resistor R1 and
install R5. J1 and J2 can now be driven differentially from a
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DEMO CIRCUIT DC987 QUICK START GUIDE
LOW NOISE/DISTORTION DIFFERENTIAL ADC DRIVER
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
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.
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
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DEMO CIRCUIT DC987 QUICK START GUIDE
LOW NOISE/DISTORTION DIFFERENTIAL ADC DRIVER
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