Datasheet

AD8137 Data Sheet
Rev. E | Page 22 of 32
THEORY OF OPERATION
The AD8137 is a low power, low cost, fully differential voltage
feedback amplifier that features a rail-to-rail output stage,
common-mode circuitry with an internally derived common-
mode reference voltage, and bias shutdown circuitry. The amplifier
uses two feedback loops to separately control differential and
common-mode feedback. The differential gain is set with external
resistors as in a traditional amplifier, and the output common-
mode voltage is set by an internal feedback loop, controlled by
an external V
OCM
input. This architecture makes it easy to set
arbitrarily the output common-mode voltage level without
affecting the differential gain of the amplifier.
–OUT +IN
A
CM
V
OCM
C
C
C
C
CP +OUT–IN CN
04771-0-017
Figure 62. Block Diagram
From Figure 62, the input transconductance stage is an H-bridge
whose output current is mirrored to high impedance nodes CP
and CN. The output section is traditional H-bridge driven circuitry
with common emitter devices driving nodes +OUT and −OUT.
The 3 dB point of the amplifier is defined as
C
m
C
g
BW
×π
=
2
where:
g
m
is the transconductance of the input stage.
C
C
is the total capacitance on node CP/CN (capacitances CP
and CN are well matched).
For the AD8137, the input stage g
m
is ~1 mA/V and the
capacitance C
C
is 3.5 pF, setting the crossover frequency of the
amplifier at 41 MHz. This frequency generally establishes an
amplifier’s unity gain bandwidth, but with the AD8137, the
closed-loop bandwidth depends upon the feedback resistor
value as well (see Figure 17). The open-loop gain and phase
simulations are shown in Figure 63.
FREQUENCY (MHz)
100
80
–60
–40
–20
0
20
40
60
–120
–100
–80
–200
–180
–160
–140
0.0001 0.010.001 0.1 1 10 100
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OPEN-LOOP GAIN (dB)
PHASE (DEGREES)
Figure 63. Open-Loop Gain and Phase
In Figure 62, the common-mode feedback amplifier A
CM
samples the output common-mode voltage, and by negative
feedback forces the output common-mode voltage to be equal
to the voltage applied to the V
OCM
input. In other words, the
feedback loop servos the output common-mode voltage to the
voltage applied to the V
OCM
input. An internal bias generator
sets the V
OCM
level to approximately midsupply; therefore, the
output common-mode voltage is set to approximately midsupply
when the V
OCM
input is left floating. The source resistance of the
internal bias generator is large and can be overridden easily by an
external voltage supplied by a source with a relatively small output
resistance. The V
OCM
input can be driven to within approximately
1 V of the supply rails while maintaining linear operation in the
common-mode feedback loop.
The common-mode feedback loop inside the AD8137 produces
outputs that are highly balanced over a wide frequency range
without the requirement of tightly matched external components,
because it 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°
apart in phase.