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Data Sheet

AD8476

Rev. B | Page 17 of 24

THEORY OF OPERATION

OVERVIEW

The AD8476 is a fully differential amplifier, with integrated laser-

trimmed resistors, that provides a precision gain of 1. The

internal differential amplifier of the AD8476 differs from

conventional operational amplifiers in that it has two outputs

whose voltages are equal in magnitude, but move in opposite

directions (180° out of phase).

The AD8476 is designed to greatly simplify single-ended-to-

differential conversion, common-mode level shifting and

precision driving of differential signals into low power,

differential input ADCs. The VOCM input allows the user to

set the output common-mode voltage to match with the input

range of the ADC. Like an operational amplifier, the VOCM

function relies on high open-loop gain and negative feedback to

force the output nodes to the desired voltages.

10kΩ

NOTES

1. NC = NO CONNECT.

DO NOT CONNECT TO THIS PIN.

AD8476

Figure 51. Block Diagram

CIRCUIT INFORMATION

The AD8476 amplifier uses a voltage feedback topology;

therefore, the amplifier exhibits a nominally constant gain

bandwidth product. Like a voltage feedback operational

amplifier, the AD8476 also has high input impedance at its

internal input terminals (the summing nodes of the internal

amplifier) and low output impedance.

The AD8476 employs two feedback loops, one each to control

the differential and common-mode output voltages. The differen-

tial feedback loop, which is fixed with precision laser-trimmed

on-chip resistors, controls the differential output voltage.

Output Common-Mode Voltage (VOCM)

The internal common-mode feedback controls the common-

mode output voltage. This architecture makes it easy for the

user to set the output common-mode level to any arbitrary

value independent of the input voltage. The output common-

mode voltage is forced by the internal common-mode feedback

loop to be equal to the voltage applied to the VOCM input. The

VOCM pin can be left unconnected, and the output common-

mode voltage self-biases to midsupply by the internal feedback

control.

Due to the internal common-mode feedback loop and the fully

differential topology of the amplifier, the AD8476 outputs are

precisely balanced over a wide frequency range. This means that

the amplifier’s differential outputs are very close to the ideal of

being identical in amplitude and exactly 180° out of phase.

DC PRECISION

The dc precision of the AD8476 is highly dependent on the

accuracy of its integrated gain resistors. Using superposition to

analyze the circuit shown in Figure 52, the following equation

shows the relationship between the input and output voltages of

the amplifier:

(

)

(

)

(

)

(

)

OUT

−

where:

RGP

RFP

RP =

RGN

RFN

RN =

−

)

(

The differential closed-loop gain of the amplifier is

OUT

and the common rejection of the amplifier is

(

)

OUT

−

RFP

RFN

RGP

RGN

VON

VOP

VOCM

Figure 52. Functional Circuit Diagram of the AD8476 at a Given Gain

The preceding equations show that the gain accuracy and the

common-mode rejection (CMRR) of the AD8476 are deter-

mined primarily by the matching of the feedback networks

(resistor ratios). If the two networks are perfectly matched, that

is, if RP and RN equal RF/RG, then the resistor network does not

generate any CMRR errors and the differential closed loop gain

of the amplifier reduces to

OUT

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