AD8137(2004) Ver la hoja de datos (PDF) - Analog Devices

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AD8137
(Rev.:2004)

Low Cost, Low Power, Differential ADC Driver

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 ampli-

fier 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 while the output

common-mode voltage is set by an internal feedback loop,

controlled by an external VOCM input. This architecture makes it

easy to arbitrarily set the output common-mode voltage level

without affecting the differential gain of the amplifier.

–OUT

CP +IN

CC

VOCM

ACM

–IN CN

CC

+OUT

Figure 61. Block Diagram

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

BW

=

gm

2π × CC

where gm is the transconductance of the input stage and CC is

the total capacitance on node CP/CN (capacitances CP and CN

are well matched). For the AD8137, the input stage gm is

~1 mA/V and the capacitance CC 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 19). The open-loop

gain and phase simulations are shown in Figure 62.

AD8137

100

80

60

40

20

OPEN-LOOP GAIN (dB)

0

–20

–40

–60

–80

–100

–120

PHASE (DEGREES)

–140

–160

–180

–200

0.0001

0.001

0.01

0.1

1

FREQUENCY (MHz)

10

100

Figure 62. Open-Loop Gain and Phase

In Figure 61, the common-mode feedback amplifier ACM

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 VOCM input. In other words, the

feedback loop servos the output common-mode voltage to the

voltage applied to the VOCM input. An internal bias generator

sets the VOCM level to approximately midsupply, therefore, the

output common-mode voltage will be set to approximately

midsupply when the VOCM input is left floating. The source resis-

tance of the internal bias generator is large and can be overrid-

den easily by an external voltage supplied by a source with a

relatively small output resistance. The VOCM input can be driven

to within approximately 1 V of the supply rails while maintain-

ing 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.

Rev. A | Page 17 of 24

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