QUICK START GUIDE FOR DEMONSTRATION CIRCUIT DC954
LOW DISTORTION, LOW NOISE DIFFERENTIAL AMPLIFIER / ADC DRIVER
LT6402-6/-12/-20
DESCRIPTION
Demonstration circuit 954 features the LT6402 Differential
Amplifier and ADC Driver. It incorporates a variety of pas-
sive components to support configurations for varied ap-
plications. Single-ended or differential input and output
configurations are possible, and there is a calibration cir-
cuit to null out the effects of other circuit components.
DC954A-A contains the LT6402-6, DC954A-B contains
the LT6402-12, and DC954A-C contains the LT6402-20.
The LT6402 is a high-speed differential amplifier with su-
perior distortion and noise performance, perfect for de-
manding communications transceiver applications and
other high-speed mixed signal applications. The LT6402
accepts single-ended or differential inputs with almost no
difference in distortion performance. Another standout
feature of the LT6402 is the ability to independently adjust
the output common-mode voltage, which allows output
level shifting without any additional components. See the
LT6402 datasheet for more information.
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 for the connection diagram and
follow the procedure below:
1.
Connect the power supply as shown. The power labels
Table 1: DC954 SMA Connector Descriptions
CONNECTOR
FUNCTION
J1 (-IN)
of VCC, EN, and GND directly correspond to the VCC,
EN, and VEE pins of the LT6402, respectively. Voltages
other than +5V may be used as long as the voltage be-
tween VCC and GND is between 4.5V and 5.5V. Switch
SW1 enables or shuts down the LT6402; to enable the
part externally, see the Enable (Shutdown) section of
this Quick Start Guide.
2.
For network analyzer use, calibrate out board and trans-
Differential Input.
Connected to GND through resistor
R2 by default.
Resistor can be removed to drive the
DC954 differentially.
Differential Input. Use this connector to supply an in-
put to the DC954. Drive from a 50ohm signal source,
no external termination necessary.
Output Common-Mode Pin.
By default, 0.44•VCC is
supplied to this pin by a 14k resistor and an 11k re-
sistor.
Override this voltage with any low-impedance
DC voltage source.
Differential Output.
Connected to GND through 0Ω
Ω
resistor R14 by default.
Resistor can be removed for
differential output drive.
Differential Output. Resistively impedance-matched to
50 ohms, can be used to drive a 50ohm network ana-
lyzer input.
Calibration network input. Use with network analyzer
to calibrate out effects of board trace parasitics, trans-
formers, and SMA connectors.
J2 (+IN)
J3 (VOCM)
J4 (-OUT)
former effects using J6 and J7. There is a 1:1 imped-
ance transfer from J6 to J7, so external termination
may be necessary for a 50Ω match.
3.
Apply an input signal to J2. A low-distortion, low noise
J5 (+OUT)
signal source with an external high-order filter will yield
the best performance.
NOTE:
J6 (TEST IN)
DC954’s input is impedance-matched to 50Ω. No
external termination is necessary.
J7 (TEST OUT) Calibration network output.
4.
Observe the output via J5. By default, the unfiltered
outputs of the LT6402 are used. The output is imped-
ance-matched to 50Ω, suitable for the input of a net-
work or spectrum analyzer.
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QUICK START GUIDE FOR DEMONSTRATION CIRCUIT DC954
LOW DISTORTION, LOW NOISE DIFFERENTIAL AMPLIFIER / ADC DRIVER
Figure 1.
Proper Measurement Equipment Setup
ADDITIONAL INFORMATION
Although the DC954 demo board is ready to use out of the
box (See “Quick Start Procedure” above), it has features
that you can access by adding, removing or changing
components on the board. Here are some ways that you
can change the configuration to take advantage of these
features.
DIFFERENTIAL INPUTS
To drive the inputs differentially, remove resistor R2. J1
and J2 can now be driven differentially from a 50Ω signal
source. The input to the DC954 will still be matched to
50Ω (differential) when driven in this manner.
DRIVING THE INPUTS WITH DC COUPLING
It is possible to drive the DC954 inputs differentially with
DC coupling. Capacitors C1 and C2 should be shorted or
replaced with 0Ω resistors. As a warning, the low input
impedance of the LT6402 can cause large input bias cur-
rents if DC954 is driven DC coupled. Since transformer T1
is a transmission-line transformer, DC voltages (and cur-
rents) will propagate through. Removing T1 would delete
the simple impedance transformation, but free up space to
place extra components on T1’s pads. See the LT6402
datasheet for additional information about DC coupling
and input bias currents.
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QUICK START GUIDE FOR DEMONSTRATION CIRCUIT DC954
LOW DISTORTION, LOW NOISE DIFFERENTIAL AMPLIFIER / ADC DRIVER
NOTE:
DIFFERENTIAL OUTPUTS
To use both the outputs of the DC954, remove resistor
R14. The output now appears differentially across J4 and
J5. The output of the DC954 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 DC954. 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
DC954 provides some additional component pads for
building discrete output filters. In addition to the internal
output low-pass filters built into the LT6402, 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
LT6402: R7-R15, C3-C4, C11, C16, and T2. For best per-
formance, care must be taken to maintain proper imped-
ance matching on the DC954 output. Also, since the out-
put of the LT6402 is a low-impedance operational amplifier
output, performance will degrade when the output is di-
rectly driving low impedances such as 50Ω. See the
LT6402 datasheet for more information.
ENABLE (SHUTDOWN)
To drive the EN function of the LT6402 with an external
voltage source, switch S1 must be disabled by removing
resistors R17 and R18. The LT6402 will now be controlled
via the EN pin. To enable the part, pull the pin low to within
0.8V of the GND pin on the DC954.
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
LT6402 in shutdown.
GROUND AND VCC CONNECTIONS
DC954’s GND connector is not only connected to VEE of
the LT6402, 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 DC954 will yield the best
performance from the LT6402.
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 DC954’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.
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QUICK START GUIDE FOR DEMONSTRATION CIRCUIT DC954
LOW DISTORTION, LOW NOISE DIFFERENTIAL AMPLIFIER / ADC DRIVER
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