AD8132ARM datasheet(14/20 Pages) AD

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AD8132

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When unequal feedback ratios are used, the two gains associated

with VOUT,dm become nonzero. This signiﬁcantly complicates

the mathematical analysis along with any intuitive understand-

ing of how the part operates. Some of these conﬁgurations will

be in another section.

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 conventional op amp. For its operation, the AD8132 makes

use of two feedback loops as compared to the single loop of

conventional op amps. While 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 is controllable by adding passive feedback networks. How-

ever, 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 as

a result of having 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, which

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

•

The input impedances are arbitrarily large and their loading

effect can be ignored.

•

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, which drives the

ampliﬁer to a state where the input differential voltage is

effectively zero.

•

Offset voltages are assumed to be zero.

While it is possible to operate the AD8132 with a purely differ-

ential 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 undriven

input will be tied to a reference voltage. For now this is ground,

and other conditions will be discussed later. Also, the voltage at

VOCM, and hence VOUT,cm will be assumed to be ground for now.

Figure 4 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,

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

opposite-sign input. These terms are:

β1 = RG1/(RG1+ RF1)

β2 = RG2/(RG2 + RF2)

The feedback factor

β1 is for the side that is driven, while the

feedback factor

β2 is for the side that is tied to a reference volt-

age, (ground for now). Note also that each feedback factor can

vary anywhere between 0 and 1.

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

which is true for all values of

β1 and β2:

G = 2

× (1–β1)/(β1 + β2)

This expression is not very intuitive, but some further examples

can provide better understanding of its implications. One obser-

vation that can be made right away is that a tolerance error in

β1

does not have the same effect on gain as the same tolerance

error in

β2.

Resistorless Differential Ampliﬁer (High Input Impedance

Inverting Ampliﬁer)

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

any resistors and is shown in Figure 7. In this circuit,

β1 is equal

to zero, and

β2 is equal to one. The gain is equal to two.

A more intuitive means to ﬁgure the gain is by simple inspec-

tion. +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. Since –OUT

has to swing in the opposite direction from +OUT due to the

common-mode constraint, its effect will double the output

signal and produce a gain of two.

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 has been assumed to be at midsupply. Since there is

still the constraint from the above discussion that +VOUT must

equal VIN, changing the VOCM voltage will not change +VOUT

(= VIN). Therefore, all of the effect of changing VOCM must

show up at –OUT.

For example, if VOCM is raised by 1 V, then –VOUT must go up

by 2 V. This makes VOUT,cm also go up by 1 V, since it is deﬁned

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

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

Other

2 = 1 Circuits

The above simple conﬁguration with

β2 = 1 and its gain-of-two

is the highest gain circuit that can be made under this condition.

Since

β1 was equal to zero, only higher β1 values are possible.

All of these circuits with higher values of

β1 will have gains lower

than two. However, circuits with

β1 equal to one are not practical,

because they have no effective input, and result in a gain of 0.

To increase

β1 from zero, it is necessary to add two resistors in

a feedback network. A generalized circuit that has

β1 with a

value higher than zero is shown in Figure 6. A couple of differ-

ent convenient gains that can be created are a gain of 1, when

β1 is equal to 1/3, and a gain of 0.5 when β1 equals 0.6.

In all of these circuits with

β2 equal to 1, V

OCM serves as the

reference voltage from which to measure the input voltage and

the individual output voltages. In general, when VOCM is varied

in these circuits, a differential output signal will be generated in

addition to VOUT,cm changing the same amount as the voltage

change of VOCM.

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