DEMO CIRCUIT DC1299 QUICK START GUIDE
LOW NOISE/DISTORTION DUAL DIFFERENTIAL ADC DRIVER
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DESCRIPTION
Demonstration circuit 1299 features the LTC6420 and
LTC6421 Dual ADC Drivers. It incorporates a variety of pas-
sive components to support configurations for varied appli-
cations. Single-ended or differential input and output confi-
gurations are possible, and there is a calibration circuit to
null out the effects of other circuit components.
The LTC6420/1 is a high-speed dual differential amplifier
with superior distortion and noise performance, perfect for
demanding communications transceivers, cellular base-
stations, and other high-speed signal chain applications.
The LTC6420/1 accepts single-ended or differential inputs
with almost no difference in distortion performance. Two
standout features of the LTC6420/1 are the ability to inde-
pendently adjust the output common-mode voltage of both
amplifiers independently without any additional components
and the ability to shut each amplifier down into a low-power
state independently of the other.
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 in Figure 1. The
power labels of V+ and GND directly correspond to the
V+, and V- pins of the LTC6420/1. Jumpers JP1 and JP2
shut down the amplifiers within the LTC6420/1.
2.
Apply input signals to J1 and J7. A network ana-
lyzer output or RF signal source with an external
bandpass filter will yield the best performance.
DC1299’s input is impedance-matched to 50Ω; no
external termination is necessary.
3.
Observe the outputs via J3 and J8. The outputs are im-
pedance-matched to 50Ω, suitable for the input of a net-
work or spectrum analyzer. Note that the schematic cir-
cuit includes impedance transformers and series resis-
tance that lowers the gain of the overall circuit.
4.
The VOCMA and VOCMB turrets may be used to adjust
the common-mode output voltages of the LTC6420/1.
See the LTC6420/1 datasheet for more information.
Table 1: DC1299 Board Connector Descriptions
CONNECTOR FUNCTION
J1,J7
(+INA,+INB)
Board Inputs. Use this connector to supply an input to
the DC1299. Drive from a 50Ω signal source, no exter-
nal termination necessary.
J2 (-INA) Differential Input. Not connected by default. Capacitor
C25 can be installed and C26 removed to drive this
channel differentially.
J3,J8
(+OUTA,+OUTB)
Board Outputs. Impedance-matched to 50
Ω
, can be
used to drive a network/spectrum analyzer input.
J4 (-OUTA)
Differential Output. Not connected by default. Capaci-
tor C24 can be installed and C28 removed to receive
this output differentially.
J5 (-INB)
Differential Input. Not connected by default. Capacitor
C31 can be installed and C30 removed to drive this
channel differentially.
J6 (-OUTB)
Differential Output. Not connected by default. Capaci-
tor C35 can be installed and C33 removed to receive
this output differentially.
VOCMA,
VOCMB
Turrets can be driven from a DC voltage source to set
the input and output common-mode levels of the
LTC6420/1. Input impedance is approximately 600Ω
DEMO CIRCUIT 129
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QUICK START GUIDE
LTC6420 and LTC6421
Low Noise/Distortion
Dual Differential ADC Driver
s
DEMO CIRCUIT DC1299 QUICK START GUIDE
LOW NOISE/DISTORTION DUAL DIFFERENTIAL ADC DRIVER
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ADDITIONAL INFORMATION
Although the DC1299 demo board is ready to use out of
the box, it has features that you can access by adding, re-
moving or changing components on the board. These are
described below.
DEMO BOARD VERSIONS
DC1299 has versions to support the different options of the
LTC6420/1 family. DC1299A-A contains the LTC6420-20,
and DC1299A-B contains the LTC6421-20. See the sche-
matic for component differences between the versions.
DIFFERENTIAL INPUTS
To drive the inputs differentially, remove the capacitors
C26/C30 and install them in locations C25/C31. J1/J2 and
J7/J5 can now be driven differentially from 50Ω signal
sources. The inputs to the DC1299 will still be matched to
50Ω (differentially) when driven in this manner.
DRIVING THE INPUTS WITH DC COUPLING
It is possible to drive the DC1299 inputs differentially with
DC coupling. The input capacitors (C22/C25 and C31/C36)
should be shorted or replaced with 0Ω resistors. As a
warning, the low input impedance of the LTC6420/1 can
cause large input bias currents if DC1299 is driven DC
coupled. If transformers T1/T3 are transmission-line trans-
formers (the transformers included on-board are), DC vol-
tages and currents will propagate through. See the
LTC6420/1 datasheet for additional information about DC
coupling and input bias currents.
DIFFERENTIAL OUTPUTS
To use the outputs of DC1299 differentially, move capaci-
tors C28/C33 to positions C24/C35. The outputs now ap-
pear differentially across J3/J4 and J8/J6. The outputs of
Figure 1: Proper Measurement Equipment Setup
DEMO CIRCUIT DC1299 QUICK START GUIDE
LOW NOISE/DISTORTION DUAL DIFFERENTIAL ADC DRIVER
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the DC1299 will still be matched to 50Ω (differentially)
when used in this manner.
CHANGING THE OUTPUT COMMON-MODE VOLTAGE
The turrets marked VOCMA and VOCMB control the output
(and input, if AC-coupled) common-mode voltages of
DC1299. This function can be used to level-shift the DC
output voltages for optimum system performance. Howev-
er, if used for this purpose, output capacitors C20/C24 and
C40/C35 can be shorted or replaced by 0Ω resistors to
allow the DC bias to reach the output. Transformers T2/T4
are transmission-line type, which means DC voltages will
couple from input to output of the transformer. By default,
on-board resistors supply 1.25V voltage to the common-
mode pins (with V+=3.0V). The VOCM voltage of the two
amplifiers can be set independently.
OUTPUT DRIVE
Since the outputs of the LTC6420/1 are low-impedance
operational amplifier outputs, performance will degrade
when the output is directly driving low impedances such as
50Ω. That is the reason for the series resistors and 4:1
impedance transformers, which give the outputs a more
benign 400Ω load. See the LTC6420/1 datasheet for more
information on distortion performance versus load imped-
ance.
ENABLE (SHUTDOWN)
Jumpers JP1 and JP2 control the enable function of the
LTC6420/1. When set to the EN 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 current. The two amplifiers can be enabled or
shut down independently.
NOTE.
The VOCM bias resistors R1/R2 and R16/R18 draw
current from V+ to GND, and must be accounted for when
measuring the current of the LTC6420/1, especially in
shutdown.
GROUND AND V+ CONNECTIONS
DC1299’s GND turrets are not only connected to V- pins of
the LTC6420/1, they also serve 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 V+ and GND connectors on DC1299 will yield
the best performance from the LTC6420/1.
SCHEMATIC NOTES
Transformers T1/T3 at the inputs of the LTC6420/1 are
impedance-matching and/or balun transformers to ensure
that the amplifiers see a fully differential input. The trans-
formers will have approximately 1dB of attenuation that will
affect the measured gain of the demo board.
Transformers T2/T4 and the series 88.7Ω resistors at the
LTC6420/1 outputs create a 13dB attenuation that will af-
fect the measured gain of the demo board. This is done so
that the output of DC1299 can plug directly into a 50Ω ana-
lyzer input while the amplifier sees a more benign 400Ω
load resistance.
DEMO CIRCUIT DC1299 QUICK START GUIDE
LOW NOISE/DISTORTION DUAL DIFFERENTIAL ADC DRIVER
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