AD8132 datasheet(13/20 Pages) AD

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REV. 0

AD8132

–13–

OPERATIONAL DESCRIPTION

Deﬁnition of Terms

AD8132

+IN

–IN

+DIN

VOCM

–DIN

RL, dm

+OUT

VOUT, dm

–OUT

Figure 3. Circuit Deﬁnitions

Differential voltage refers to the difference between two node

voltages. For example, the output differential voltage (or

equivalently output differential-mode voltage) is deﬁned as:

VOUT,dm = (V+OUT – V–OUT)

V+OUT and V–OUT refer to the voltages at the +OUT and –OUT

terminals with respect to a common reference.

Common-mode voltage refers to the average of two node volt-

ages. The output common-mode voltage is deﬁned as:

VOUT,cm = (V+OUT + V–OUT)/2

Basic Circuit Operation

One of the more useful and easy to understand ways to use the

AD8132 is to provide two equal-ratio feedback networks. To

match the effect of parasitics, these networks should actually be

comprised of two equal-value feedback resistors, RF and two

equal-value gain resistors, RG. This circuit is diagrammed in

Figure 3.

Like a conventional op amp, the AD8132 has two differential

inputs that can be driven with both a differential-mode input

voltage, VIN,dm, and a common-mode input voltage, VIN,cm.

There is another input, VOCM, which is not present on conven-

tional op amps, but provides another input to consider on the

AD8132. It is totally separate from the above inputs.

There are two complementary outputs whose response can be

deﬁned by a differential-mode output, VOUT,dm and a common-

mode output, VOUT,cm.

TEMPERATURE – C

–0.5

–2.5

–40

–20

100

20406080

–1.0

–1.5

–2.0

VS = 5V

TPC 59. Differential Offset Voltage vs. Temperature

Table I indicates the gain from any type of input to either type

of output.

Table I. Differential and Common-Mode Gains

Input

VOUT,dm

VOUT,cm

VIN,dm

RF/RG

0 (By Design)

VIN,cm

0 (By Design)

VOCM

1 (By Design)

The differential output (VOUT,dm) is equal to the differential

input voltage (VIN,dm) times RF/RG. In this case, it does not

matter if both differential inputs are driven, or only one output

is driven and the other is tied to a reference voltage, like ground.

As can be seen from the two zero entries in the ﬁrst column,

neither of the common-mode inputs has any effect on this gain.

The gain from VIN,dm to VOUT,cm is 0 and to ﬁrst order does not

depend on the ratio matching of the feedback networks. The

common-mode feedback loop within the AD8132 provides a

corrective action to keep this gain term minimized. The term

“balance error” describes the degree to which this gain term

differs from zero.

The gain from VIN,cm to VOUT,dm does directly depend on the

matching of the feedback networks. The analogous term for this

transfer function, which is used in conventional op amps, is

“common-mode rejection ratio” or CMRR. Thus, if it is desirable

to have a high CMRR, the feedback ratios must be well matched.

The gain from VIN,cm to VOUT,cm is also ideally 0, and is ﬁrst-

order independent of the feedback ratio matching. As in the

case of VIN,dm to VOUT,cm, the common-mode feedback loop

keeps this term minimized.

The gain from VOCM to VOUT,dm is ideally 0 only when the feed-

back ratios are matched. The amount of differential output

signal that will be created by varying VOCM is related to the

degree of mismatch in the feedback networks.

VOCM controls the output common-mode voltage VOUT,cm with a

unity-gain transfer function. With equal-ratio feedback networks

(as assumed above), its effect on each output will be the same,

which is another way to say that the gain from VOCM to VOUT,dm

is zero. If not driven, the output common-mode will be at mid-

supplies. It is recommended that a 0.1

µF bypass capacitor be

connected to VOCM.

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