Evaluation Board User Guide UG-130
Rev. B | Page 3 of 8
EVALUATION BOARD HARDWARE
POWER SUPPLIES
Power is applied to the board through P1, a Molex® 3-pin
header, Part 22-11-2032. Pin 1 (square footprint) is for the
positive supply, Pin 3 is for the negative supply, and Pin 2 is
connected to the ground plane of the board. Alternatively,
looped test points can be used; Test Point TP2 connects to the
positive supply, TP3 connects to the negative supply, and TP7
and TP8 connect to the ground plane. The TP5, TP6, TP9, and
TP10 test points also connect to the ground plane.
The board accommodates single or dual supplies. For single-
supply operation, connect the negative supply to the ground plane.
It is very important that the power supply pins of the device
under test (DUT) have broadband decoupling circuitry. The
board layout facilitates this with footprints for two 1206 ceramic
capacitors on each supply. At frequencies beyond the resonant
frequency of the first capacitor and its associated internal and
external inductance, the second capacitor provides the required
low impedance return current path. For optimum performance,
place the smaller of the two capacitances closest to the DUT, in
the C8 and C11 positions. C13 provides the user with the option
of adding differential decoupling between the supplies. Bulk
decoupling is provided by C1 and C2; 10 μF tantalum capacitors
are recommended.
FEEDBACK NETWORKS AND INPUT/OUTPUT
TERMINATIONS
R19 and R17 compose the upper resistive feedback loop (see
Figure 1), and R20 and R18 compose the lower feedback loop.
C3 and C4 are included across the feedback resistors to provide
frequency-dependent feedback, typically used to introduce a
real-axis pole in the closed-loop frequency response.
To minimize summing node capacitances, void the ground
plane under and around Pin 1 and Pin 8 of the DUT and the
copper that connects to them.
R6 and R7 are included as input termination resistors for applica-
tions that have single-ended inputs. Having a place for a shunt
resistor on each input makes it simple to match the two feed-
back factors. A common example of how this is used is when
the input signal originates from an unbalanced 50 Ω source. In
this case, the single-ended termination resistance is 50 Ω and
the Thevenin equivalent resistance seen looking back to the
source is 25 Ω. For the traditional four-resistor configuration,
where R19 = R20 and R17 = R18, the feedback networks are
matched by making the shunt resistor on the input leg opposite
the termination resistor equal to 25 Ω. R5 is provided for differ-
ential termination.
R15 and R16 series termination resistors are provided on each
of the outputs for impedance matching, analog-to-digital
converter (ADC) driving, and other system requirements.
V
OCM
INPUT
The V
OCM
input can be set to a dc level by adjusting the R1
potentiometer that spans the power supplies. For the dc case,
C9 is provided at the wiper for decoupling.
An external voltage can be applied to V
OCM
via TP4 (referenced
to the ground plane of the board). In ADC driving applications,
it is convenient to apply the ADC dc reference voltage output
directly to TP4.
It is also possible to drive the V
OCM
input from an external ac
source. In this case, omit C9 or reduce it to a value that allows
the desired signal to be passed. For high frequency signals on
V
OCM
, connect the center conductor of a coaxial cable to TP4
and ground its shield at TP10.
R21 is provided for the high common-mode output impedance
application illustrated in the AD8132 data sheet.
MEASURING OUTPUT COMMON-MODE VOLTAGE
The internal common-mode feedback loop used in the differen-
tial drivers forces the output common-mode voltage to be equal
to the voltage applied to the V
OCM
input, thereby providing
excellent output balance. R11 and R12 form a voltage divider
across the differential output, and the voltage at the divider tap
is equal to the output common-mode voltage, provided that
R11 and R12 are exactly matched in value. If R11 and R12 are
used to evaluate the output common-mode voltage, they should
be measured and matched to better than 300 ppm to obtain results
commensurate with the DUT output balance error performance
of −70 dB. Test Point PR1 accepts coaxial-type oscilloscope test
points, such as the Berg Electronics 33JR135-1.
INPUT/OUTPUT TRANSFORMERS
The board has the added flexibility of allowing the user to incorpo-
rate transformers on its input and output. This capability can be
especially useful when connecting to single-ended test equipment.
Because both input and output transformers have dual, nested
footprints, the user can select from a wide array of transformers
available from companies such as Mini-Circuits® and Coilcraft®.
The layout provides footprints for connecting resistors to ground
on the primary and secondary transformer center taps, offering
the user a number of options with regard to the common-mode
properties of the evaluation circuit.
JP1, JP2, JP3, and JP4 are jumpers on the backside of the board
that provide direct shunts across their associated transformers.
When not using a transformer, bypassing the transformer is a
simple matter of shorting the appropriate jumpers. When using
a transformer, it is a good idea to verify that the associated
jumpers are open.
