Data Sheet AD8134
Rev. B | Page 13 of 20
THEORY OF OPERATION
Each differential driver in the AD8134 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 AD8134 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, are 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.
Each of the AD8134 drivers 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 common-mode level by simply
applying a voltage to the V
OCM
input. The output common-
mode voltage is forced, by internal common-mode feedback, to
equal the voltage applied to the V
OCM
input, without affecting the
differential output voltage. The V
OCM
inputs are not available to
the user but are internally connected to the sync-on-common-
mode circuitry.
The AD8134 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 30, the output differential voltage (or
equivalently output differential mode voltage) is defined as
V
OUT, dm
= (V
OP
− V
ON
)
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
2
)(
,
ONOP
cmOUT
VV
V
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 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
dmOUT
cmOUT
V
V
ErrorBalanceOutput
,
,
ANALYZING AN APPLICATION CIRCUIT
The AD8134 uses high open-loop gain and negative feedback
to force its 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
voltage between the differential inputs labeled V
AP
and V
AN
in
Figure 30. For most purposes, this voltage can be assumed to be
zero. Similarly, the difference between the actual output
common-mode voltage and the voltage applied to V
OCM
can also
be 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 30 can be
described by
2
G
F
IN,dm
OUT,dm
R
R
V
V
where R
F
= 1.5 kΩ and R
G
= 750 Ω nominally.
R
G
V
AP
V
AN
V
IP
V
IN
+
IN, dm
–
V
OCM
V
ON
V
OP
V
OUT, dm
R
G
R
F
R
F
R
L, dm
04770-005
Figure 30. Circuit Definitions
