AD8141/AD8142

Rev. 0 | Page 15 of 24

THEORY OF OPERATION

The differential drivers contained in the AD8141 and AD8142

differ from conventional op amps in that they have two outputs

whose voltages move in opposite directions. Like op amps, they

rely on high open-loop gain and negative feedback to force these

outputs to the desired voltages. The AD8141 and AD8142

drivers make 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 generally 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.

Each AD8141/AD8142 driver uses two feedback loops to

separately control the differential and common-mode output

voltages. The differential feedback, set by the internal resistors,

controls the differential output voltage only. The internal common-

mode feedback loop controls the common-mode output voltage

only. This architecture makes it easy to arbitrarily set the output

input. The output common-mode voltage is forced, by internal

input, while simultaneously balancing the differential output voltage.

signals on the three output common-mode voltages.

The overall driver architecture produces outputs that are 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 that are 180° apart in phase.

ANALYZING AN APPLICATION CIRCUIT

The drivers use two negative feedback loops, each with high

open-loop gain, to force their differential and common-mode

output voltages in such a way as to minimize the differential

and common-mode input error voltages. The differential input

error voltage is defined as the voltage between the differential

voltage can be assumed to be zero. Similarly, the difference between

the actual output common-mode voltage and the voltage applied to

assumptions, any application circuit can be analyzed.

CLOSED-LOOP GAIN

The differential mode gain of the circuit in Figure 36 can be

described by

2

=

=

G

F

dm

IN,

dm

OUT,

R

R

V

V

+

–

Figure 36. Circuit Definitions

CALCULATING AN APPLICATION CIRCUIT’S INPUT

IMPEDANCE

The effective input impedance of a circuit such as that in Figure 36

by a single-ended or differential signal source. For balanced

impedance becomes

()

kΩ

5

.

1

2

1

=

⎟

⎟

⎟

⎟

⎠

⎞

⎜

⎜

⎜

⎜

⎝

⎛

+

×

−

=

F

G

F

G

IN

R

R

R

R

R

The input impedance of the circuit is higher than for a

conventional op amp connected as an inverter because a

fraction of the differential output voltage appears at the inputs

as a common-mode signal, partially bootstrapping the voltage