Evaluation Board User Guide

UG-190

Rev. A | Page 3 of 8

EVALUATION BOARD HARDWARE

POWER SUPPLIES

Power is applied to the board through P1, a Molex® 3-pin

header, Part Number 22-11-2032 (see Figure 1 and Figure 3).

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. TP2

connects to the positive supply, TP3 connects to the negative

supply, and TP5 through TP10 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 (see Figure 3). C10 and C12 can also

be used for additional power supply bypassing. 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 3), 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, the ground plane

under and around Pin 1 and Pin 8 of the DUT and the copper

that connects to them have been removed.

R6 and R7 are included as input termination resistors for

applications that have single-ended inputs. Having a place for

a shunt resistor on each input makes it simple to match the two

feedback 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 differential 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.

potentiometer that spans the power supplies. For the dc case,

C9 is provided at the wiper for decoupling.

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.

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

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

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 back side 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.