AD8137 Analog Devices, AD8137 Datasheet - Page 17

• Manufacturer Part Number: AD8137
• Description: Low Cost, Low Power 12-Bit Differential ADC Driver
• Manufacturer: Analog Devices

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 differ-
ential and common-mode feedback. The differential gain is set
with external resistors as in a traditional amplifier while the
output common-mode voltage is set by an internal feedback
loop, controlled by an external V
makes it easy to arbitrarily set the output common-mode
voltage level without affecting the differential gain of the
amplifier.
From Figure 61, 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
where g
the total capacitance on node CP/CN (capacitances CP and CN
are well matched). For the AD8137, the input stage g
~1 mA/V and the capacitance C
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 feed-
back resistor value as well (see Figure 19). The open-loop gain
and phase simulations are shown in Figure 62.
BW
–OUT
m
is the transconductance of the input stage and C
=
2
π
g
×
m
C
C
CP
Figure 61. Block Diagram
C
+IN
C
A
CM
V
C
OCM
OCM
is 3.5 pF, setting the crossover
input. This architecture
–IN CN
C
C
m
+OUT
is
C
is
Rev. B | Page 17 of 24
In Figure 61, the common-mode feedback amplifier A
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
feedback loop servos the output common-mode voltage to the
voltage applied to the V
sets the V
output common-mode voltage will be set to approximately
midsupply when the V
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
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 compo-
nents, 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.
–100
–120
–140
–160
–180
–200
100
–20
–40
–60
–80
80
60
40
20
0.0001
0
OCM
level to approximately midsupply; therefore, the
PHASE (DEGREES)
0.001
Figure 62. Open-Loop Gain and Phase
OCM
0.01
OCM
OPEN-LOOP GAIN (dB)
FREQUENCY (MHz)
input is left floating. The source
input. An internal bias generator
OCM
0.1
input. In other words, the
1
OCM
10
input can
AD8137
CM
100