AD8146/AD8147/AD8148

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THEORY OF OPERATION

Each differential driver differs from a conventional op amp 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

feedback to force these outputs to the desired voltages. The

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, were 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 was also

used to attempt to correct an inherently unbalanced circuit.

Excellent performance over a wide frequency range has proven

difficult with this approach.

Each of the drivers uses two feedback loops to separately

control the differential and common-mode output voltages.

The differential feedback, set by the internal resistors, controls

only the differential output voltage. The internal common-

mode feedback loop controls only the common-mode output

voltage. This architecture makes it easy to transmit signals over

the common-mode voltage channels by simply applying the

voltage is forced, by internal common-mode feedback, to equal

differential output voltage.

The driver architecture results in 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 exactly 180° apart in phase.

DEFINITION OF TERMS

Differential Voltage

Differential voltage refers to the difference between two node

voltages that are balanced with respect to each other. For

example, in Figure 28 the output differential voltage (or

equivalently output differential mode voltage) is defined as

Common-Mode Voltage

Common-mode voltage refers to the average of two node

voltages with respect to a common reference. The output

common-mode voltage is defined as

Output Balance

Output balance is a measure of how well the differential output

signals are matched in amplitude and how close they are to

exactly 180° apart in phase. Balance is most easily determined

by placing a well-matched resistor divider between the differential

output 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 error is the magnitude

of the change in output common-mode voltage divided by the

magnitude of the change in output differential mode voltage in

response to a differential input signal.

dm

OUT

cm

OUT

V

V

Error

Balance

Output

,

,

Δ

Δ

=

ANALYZING AN APPLICATION CIRCUIT

The drivers use high open-loop gain and negative feedback to

force their differential and common-mode output voltages to

minimize the differential and common-mode input error

voltages. The differential input error voltage is defined as the

Figure 28. 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 28 can be

described by

G

F

dm

IN,

dm

OUT,

R

R

V

V

=

where:

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+

–

Figure 28. Internal Architecture and Signal Name Definitions