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AD8138

Data Sheet

Rev. G | Page 16 of 24

OPERATIONAL DESCRIPTION

DEFINITION OF TERMS

AD8138

+IN

–IN

+OUT

–OUT

+DIN

–DIN

VOCM

VOUT, dm

RL, dm

Figure 42. 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)

where 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

voltages. The output common-mode voltage is defined as

VOUT, cm = (V+OUT + V−OUT)/2

Balance is a measure of how well differential signals are

matched in amplitude and exactly 180° apart in phase. Balance

is most easily determined by placing a well-matched resistor

divider between the differential voltage nodes and comparing

the magnitude of the signal at the midpoint of the divider with

the magnitude of the differential signal (see Figure 41). By this

definition, output balance is the magnitude of the output

common-mode voltage divided by the magnitude of the output

differential mode voltage:

OUT

Error

Balance

Output

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