AD7982 datasheet(17/24 Pages) AD | Precision, Selectable Gain, Fully Differential Funnel Amplifier

17 / 24 page

AD8475

Rev. B | Page 17 of 24

THEORY OF OPERATION

OVERVIEW

The AD8475 is a fully differential amplifier, with integrated laser-

trimmed resistors, that provides precision attenuating gains of

0.4 and 0.8. The internal differential amplifier of the AD8475

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). An additional input,

VOCM, sets the output common-mode voltage. Like an opera-

tional amplifier, it relies on high open-loop gain and negative

feedback to force the output nodes to the desired voltages. The

AD8475 is designed to greatly simplify single-ended-to-differential

conversion, common-mode level shifting and precision attenua-

tion of large signals so that they are compatible with low voltage,

differential input ADCs.

1kΩ

1.25kΩ

AD8475

–IN 0.8x –IN 0.4x

+VS

VOCM

+OUT

+IN 0.8x +IN 0.4x

–VS

–OUT

1.25kΩ

Figure 49. Block Diagram

CIRCUIT INFORMATION

The AD8475 amplifier uses a voltage feedback topology;

therefore, the amplifier exhibits a nominally constant gain

bandwidth product. Like a voltage feedback operational

amplifier, the AD8475 also has high input impedance at its

internal input terminals (the summing nodes of the internal

amplifier) and low output impedance.

The AD8475 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 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 AD8475 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 AD8475 is highly dependent on the

accuracy of its internal resistors. Using superposition to analyze

the circuit shown in Figure 50, the following equation shows the

relationship between the input and output voltages of the

amplifier:

()

OUT

−

where,

RGP

RFP

P =

RGN

RFN

N =

−

)

(

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 50. Functional Circuit Diagram of the AD8475 at a Given Gain

The preceding equations show that the gain accuracy and the

common-mode rejection (CMRR) of the AD8475 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

The AD8475’s integrated resistors are precision wafer-laser-

trimmed to guarantee a minimum CMRR of 86dB (50μV/V),

and gain error of less that 0.05%. To achieve equivalent precision

and performance using a discrete solution, resistors must be

matched to 0.01% or better.

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