AD8132ARZ-R7 datasheet(21/32 Pages) AD

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AD8132

Rev. F | Page 21 of 32

THEORY OF OPERATION

The AD8132 differs from conventional op amps by the external

presence of an additional input and output. The additional input,

VOCM, controls the output common-mode voltage. The additional

output is the analog complement of the single output of a conven-

tional op amp. For its operation, the AD8132 uses two feedback

loops as compared to the single loop of conventional op amps.

Although this provides significant freedom to create various

novel circuits, basic op amp theory can still be used to analyze

the operation.

One of the feedback loops controls the output common-mode

voltage, VOUT, cm. Its input is VOCM (Pin 2) and the output is the

common-mode, or average voltage, of the two differential outputs

(+OUT and −OUT). The gain of this circuit is internally set to

unity. When the AD8132 is operating in its linear region, this

establishes one of the operational constraints: VOUT, cm = VOCM.

The second feedback loop controls the differential operation.

Similar to an op amp, the gain and gain shaping of the transfer

function can be controlled by adding passive feedback networks.

However, only one feedback network is required to close the

loop and fully constrain the operation, but depending on the

function desired, two feedback networks can be used. This is

possible because there are two outputs that are each inverted

with respect to the differential inputs.

GENERAL USAGE OF THE AD8132

Several assumptions are made here for a first-order analysis; they

are the typical assumptions used for the analysis of op amps:

•

The input bias currents are sufficiently small so they can

be neglected.

•

The output impedances are arbitrarily low.

•

The open-loop gain is arbitrarily large, and drives the

amplifier to a state where the input differential voltage is

effectively 0.

•

Offset voltages are assumed to be 0.

Though it is possible to operate the AD8132 with a purely

differential input, many of its applications call for a circuit

that has a single-ended input with a differential output.

For a single-ended-to-differential circuit, the RG of the input

that is not driven is tied to a reference voltage. This is ground.

Other conditions are discussed in the following sections. In

addition, the voltage at VOCM, and therefore VOUT,cm, is assumed

to be ground. Figure 67 shows a generalized schematic of such a

circuit using an AD8132 with two feedback paths.

For each feedback network, a feedback factor can be defined as

the fraction of the output signal that is fed back to the opposite

sign input. These terms are

(

)

β1

(

)

β2

The feedback factor, β1, is for the side that is driven, and the

feedback factor, β2, is for the side that is tied to a reference voltage

(ground). Note that each feedback factor can vary anywhere

between 0 and 1.

A single-ended-to-differential gain equation can be derived

(this is true for all values of β1 and β2):

(

)

()

β2

β1

−

This expression is not very intuitive, but some further examples can

provide better understanding of its implications. One observation

that can be made immediately is that a tolerance error in β1 does

not have the same effect on gain as the same tolerance error in β2.

DIFFERENTIAL AMPLIFIER WITHOUT RESISTORS

(HIGH INPUT IMPEDANCE INVERTING AMPLIFIER)

The simplest closed-loop circuit that can be made does not

require any resistors and is shown in Figure 70. In this circuit,

β1 is equal to 0, and β2 is equal to 1. The gain is equal to 2.

A more intuitive method to figure the gain is by simple inspection.

+OUT is connected to −IN, whose voltage is equal to the voltage at

+IN under equilibrium conditions. Thus, +VOUT is equal to VIN,

and there is unity gain in this path. Because −OUT has to swing

in the opposite direction from +OUT due to the common-mode

constraint, its effect doubles the output signal and produces a

gain of 2.

One useful function that this circuit provides is a high input

impedance inverter. If +OUT is ignored, there is a unity-gain,

high input impedance amplifier formed from +IN to −OUT.

Most traditional op amp inverters have relatively low input

impedances, unless they are buffered with another amplifier.

VOCM is assumed to be at midsupply. Because there is still the

constraint that +VOUT must equal VIN, changing the VOCM voltage

does not change +VOUT (equal to VIN). Therefore, the effect of

changing VOCM must show up at −OUT.

For example, if VOCM is raised by 1 V, then −VOUT must increase

by 2 V. This makes VOUT,cm also increase by 1 V, because it is defined

as the average of the two differential output voltages. This means

that the gain from VOCM to the differential output is 2.

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