AD8139

Rev. A | Page 18 of 24

THEORY OF OPERATION

The AD8139 is a high speed, low noise differential amplifier

fabricated on the Analog Devices second generation eXtra Fast

Complementary Bipolar (XFCB) process. It is designed to

provide two closely balanced differential outputs in response to

either differential or single-ended input signals. Differential

gain is set by external resistors, similar to traditional voltage-

feedback operational amplifiers. The common-mode level of the

pendent of the input common-mode voltage. The AD8139 has

an H-bridge input stage for high slew rate, low noise, and low

distortion operation and rail-to-rail output stages that provide

maximum dynamic output range. This set of features allows for

convenient single-ended-to-differential conversion, a common

need to take advantage of modern high resolution ADCs with

differential inputs.

TYPICAL CONNECTION AND DEFINITION OF

TERMS

Figure 57 shows a typical connection for the AD8139, using

terminal sets the output common-mode voltage. The two

balanced fashion in response to an input signal.

+

–

–

+

AD8139

Figure 57. Typical Connection

The differential output voltage is defined as

ON

OP

dm

O,

V

V

V

−

=

(1)

Common-mode voltage is the average of two voltages. The

output common-mode voltage is defined as

2

ON

OP

cm

O,

V

V

V

+

=

(2)

Output Balance

matched in amplitude and how precisely they are 180 degrees

out of phase with each other. It is the internal common-mode

feedback loop that forces the signal component of the output

common-mode towards zero, resulting in the near perfectly

balanced differential outputs of identical amplitude and exactly

180 degrees out of phase. The output balance performance does not

require tightly matched external components, nor does it require

that the feedback factors of each loop be equal to each other. Low

frequency output balance is limited ultimately by the mismatch

of an on-chip voltage divider, which is trimmed for optimum

performance.

Output balance is measured by placing a well matched resistor

divider across the differential voltage outputs and comparing

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

differential output. By this definition, output balance is equal to

the magnitude of the change in output common-mode voltage

divided by the magnitude of the change in output differential-

mode voltage:

dm

O,

cm

O,

V

V

Balance

Output

Δ

Δ

=

(3)

The block diagram of the AD8139 in Figure 58 shows the

internal common-mode feedback loop (voltage divider across

equal to each other.

AP

AN

V

V

=

(4)

The common-mode feedback loop drives the output common-

mode voltage, sampled at the midpoint of the two 500 Ω resistors,

2

dm

O,

OCM

OP

V

V

V

+

=

(5)

and

2

dm

O,

OCM

ON

V

V

V

−

=

(6)

10pF

10pF

500

Ω

500

Ω

MIDSUPPLY

+

+

Figure 58. Block Diagram