ANALOG DEVICES EVAL-ADDIFFAMP Service Manual

Differential Driver

FEATURES

Flexible board layout Accomodates AD8131/AD8132/AD8138/AD8139 Accomodates various circuit configurations:

4-resistor Two different feedback loops Input and output transformers Filters

Evaluation Board

EVAL-ADDIFFAMP

GENERAL DESCRIPTION

An Analog Devices differential driver evaluation board makes it easy for designers to get quick performance results for their particular differential driver application circuits. The board layout is very flexible and allows for many circuit configurations, including traditional 4-resistor circuits, circuits with two different feedback loops, circuits with input and output transformers, filters, and many others. Most resistors and capacitors are in 1206 packages.

The board accommodates the

AD8139. Use the data sheets for these devices with this

evaluation board data sheet.

AD8131 is a special case because it has internal feedback

The networks but can, nonetheless, be evaluated on the board by:

• Omitting the R19 and R20 feedback resistors and C3 and

C4 capacitors.

• Using 0 Ω resistors for Gain Resistor R17 and Gain

Resistor R18.

• Ignoring all further references to these components.

AD8131, AD8132, AD8138, and

DIFFERENTIAL DRIVER EVALUATION BOARD SCHEMATIC

R22

V–

R24

V+

V–

V+ V–

JP2

JP1

Rev. A

Information furnished by Analog Devices is believed to be accurate and reliable. However, no responsibility is assumed by Anal og Devices for its use, nor for any infringements of patents or ot her rights of third parties that may result from its use. Specifications subject to change without notice. No license is granted by implication or otherwise under any patent or patent rights of Analog Devices. Trademarks and registered trademarks are the property of their respective owners.

T1, T3

R23

R17

R18

TP4

C12

C10

8 2 1

V+

C8 TP2

DUT

OCM

C11

TP3

V–

Figure 1.

TP10TP5 TP6 TP7 TP8 TP9

R20

R19

R21

TP1

R15

R11

C13

R12

R16

PR1

R13

R14

R10

JP3

JP4

T2, T4

02838-001

EVAL-ADDIFFAMP

HARDWARE

POWER SUPPLIES

Power is applied to the board through P1, a Molex® 22-11-2032 3-pin header. 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 singlesupply operation, simply 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 positions C8 and C11. 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 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 4-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.

INPUT

OCM

The V

input can be set to a dc level by adjusting Poten-

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tiometer R1 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

via TP4 (referenced

OCM

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

input from an external ac

OCM

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

, connect the center conductor of a coaxial cable to TP4

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and ground its shield at TP10.

R21 is provided for the high common-mode output impedance application illustrated in Figure 77 of the

AD8132 data sheet.

MEASURING OUTPUT COMMON-MODE VOLTAGE

The internal common-mode feedback loop used in the differential drivers forces the output common-mode voltage to be equal to the voltage applied to the V

input, thereby

OCM

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 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. The 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 incorporate transformers on its input and output. This capability can be especially useful when connecting to singleended 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.

Rev. A | Page 2 of 4

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