AD8137YCPZ-R2 datasheet(22/32 Pages) AAVID

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AD8137

Data Sheet

Rev. E | Page 22 of 32

THEORY OF OPERATION

The AD8137 is a low power, low cost, fully differential voltage

feedback amplifier that features a rail-to-rail output stage,

common-mode circuitry with an internally derived common-

mode reference voltage, and bias shutdown circuitry. The amplifier

uses two feedback loops to separately control differential and

common-mode feedback. The differential gain is set with external

resistors as in a traditional amplifier, and the output common-

mode voltage is set by an internal feedback loop, controlled by

an external VOCM input. This architecture makes it easy to set

arbitrarily the output common-mode voltage level without

affecting the differential gain of the amplifier.

–OUT

+IN

ACM

VOCM

+OUT

–IN CN

Figure 62. Block Diagram

From Figure 62, the input transconductance stage is an H-bridge

whose output current is mirrored to high impedance nodes CP

and CN. The output section is traditional H-bridge driven circuitry

with common emitter devices driving nodes +OUT and −OUT.

The 3 dB point of the amplifier is defined as

where:

gm is the transconductance of the input stage.

CC is the total capacitance on node CP/CN (capacitances CP

and CN are well matched).

For the AD8137, the input stage gm is ~1 mA/V and the

capacitance CC is 3.5 pF, setting the crossover frequency of the

amplifier at 41 MHz. This frequency generally establishes an

amplifier’s unity gain bandwidth, but with the AD8137, the

closed-loop bandwidth depends upon the feedback resistor

value as well (see Figure 17). The open-loop gain and phase

simulations are shown in Figure 63.

FREQUENCY (MHz)

100

–60

–40

–20

–120

–100

–80

–200

–180

–160

–140

0.0001

0.01

0.001

0.1

100

OPEN-LOOP GAIN (dB)

PHASE (DEGREES)

Figure 63. Open-Loop Gain and Phase

In Figure 62, the common-mode feedback amplifier ACM

samples the output common-mode voltage, and by negative

feedback forces the output common-mode voltage to be equal

to the voltage applied to the VOCM input. In other words, the

feedback loop servos the output common-mode voltage to the

voltage applied to the VOCM input. An internal bias generator

sets the VOCM level to approximately midsupply; therefore, the

output common-mode voltage is set to approximately midsupply

when the VOCM input is left floating. The source resistance of the

internal bias generator is large and can be overridden easily by an

external voltage supplied by a source with a relatively small output

resistance. The VOCM input can be driven to within approximately

1 V of the supply rails while maintaining linear operation in the

common-mode feedback loop.

The common-mode feedback loop inside the AD8137 produces

outputs that are highly balanced over a wide frequency range

without the requirement of tightly matched external components,

because it forces the signal component of the output common-

mode voltage to be zeroed. The result is nearly perfectly balanced

differential outputs of identical amplitude and exactly 180°

apart in phase.

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