REV. 0

AD8131

–9–

OPERATIONAL DESCRIPTION

Definition of Terms

AD8131

+IN

–IN

+OUT

–OUT

+OUT

–OUT

Figure 37. 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:

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:

Balance is a measure of how well differential signals are matched

in amplitude and exactly 180 degrees 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 divider’s midpoint with the

magnitude of the differential signal. By this definition, output

balance is the magnitude of the output common-mode voltage

divided by the magnitude of the output differential-mode

voltage:

Output Balance Error

V

V

OUT cm

OUT dm

=

,

,

THEORY OF OPERATION

The AD8131 differs from conventional op amps in that it has

two outputs whose voltages move in opposite directions. Like

an op amp, it relies on high open-loop gain and negative feed-

back to force these outputs to the desired voltages. The AD8131

behaves much like a standard voltage feedback op amp and

makes it easy to perform single-ended-to-differential conversion,

common-mode level-shifting, and amplification of differential

signals.

Previous differential drivers, both discrete and integrated

designs, have been based on using two independent amplifiers,

and two independent feedback loops, one to control each of the

outputs. When these circuits are driven from a single-ended

source, the resulting outputs are typically not well balanced.

Achieving a balanced output has typically required exceptional

matching of the amplifiers and feedback networks.

DC common-mode level-shifting has also been difficult with

previous differential drivers. Level-shifting has required the use

of a third amplifier and feedback loop to control the output

common-mode level. Sometimes the third amplifier has also

been used to attempt to correct an inherently unbalanced

circuit. Excellent performance over a wide frequency range has

proven difficult with this approach.

The AD8131 uses two feedback loops to separately control the

differential and common-mode output voltages. The differential

feedback, set by internal resistors, controls only the differential

output voltage. The common-mode feedback controls only the

common-mode output voltage. This architecture makes it easy

to arbitrarily set the output common-mode level. It is forced, by

internal common-mode feedback, to be equal to the voltage

output voltage.

The AD8131 architecture results in outputs that are very highly

balanced over a wide frequency range without requiring external

components or adjustments. The common-mode feedback loop

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 degrees apart

in phase.

Analyzing an Application Circuit

The AD8131 uses high open-loop gain and negative feedback to

force its differential and common-mode output voltages in such

a way as to minimize the differential and common-mode error

voltages. The differential error voltage is defined as the voltage

between the differential inputs labeled +IN and –IN in Figure

37. For most purposes, this voltage can be assumed to be zero.

Similarly, the difference between the actual output common-

assumed to be zero. Starting from these two assumptions, any

application circuit can be analyzed.

Closed-Loop Gain

The differential mode gain of the circuit in Figure 37 can be

determined to be described by the following equation:

V

V

R

R

OUT dm

IN dm

F

G

,

,

== 2

Ω and R

Ω nominally.

Estimating the Output Noise Voltage

Similar to the case of a conventional op amp, the differential

output errors (noise and offset voltages) can be estimated by

multiplying the input referred terms, at +IN and –IN, by the

circuit noise gain. The noise gain is defined as:

G

R

R

N

F

G

=+











 =

13

The total output referred noise for the AD8131, including the

at 20 MHz.

Calculating an Application Circuit’s Input Impedance

The effective input impedance of a circuit such as that in Figure

being driven by a single-ended or differential signal source. For

impedance becomes: