AD8132ARZ-R7 datasheet(27/32 Pages) AD

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AD8132

Rev. F | Page 27 of 32

FREQUENCY (Hz)

10k

–10

–20

–30

–40

–50

–60

–70

–80

–90

100k

10M

100M

Figure 78. Frequency Response of 1 MHz Low-Pass Filter

HIGH COMMON-MODE OUTPUT IMPEDANCE

AMPLIFIER

Changing the connection to VOCM (Pin 2) can change the common-

mode from low impedance to high impedance. If VOCM is actively

set to a particular voltage, the AD8132 tries to force VOUT, cm to

the same voltage with a relatively low output impedance. All the

previous analysis assumed that this output impedance is arbitrarily

low enough to drive the load condition in the circuit.

However, there are some applications that benefit from a high

common-mode output impedance. This is accomplished with

the circuit shown in Figure 79.

348Ω

10Ω

1kΩ

49.9Ω

Figure 79. High Common-Mode, Output Impedance, Differential Amplifier

VOCM is driven by a resistor divider that measures the output

common-mode voltage. Thus, the common-mode output voltage

takes on the value that is set by the driven circuit. In this case,

it comes from the center point of the termination at the receive

end of a 10 meter length of Category 5 twisted pair cable.

If the receive end common-mode voltage is set to ground, it is

well defined at the receive end. Any common-mode signal that

is picked up over the cable length due to noise appears at the

transmit end and must be absorbed by the transmitter. Thus, it is

important that the transmitter have adequate common-mode

output range to absorb the full amplitude of the common-mode

signal coupled onto the cable and therefore prevent clipping.

Another way to look at this is that the circuit performs what

is sometimes called transformer action. One main difference

is that the AD8132 passes dc while transformers do not.

A transformer can also be easily configured to have either a high or

low common-mode output impedance. If the transformers center

tap is connected to a solid voltage reference, it sets the common-

mode voltage on the secondary side of the transformer. In this case,

if one of the differential outputs is grounded, the other output has

half of the differential output signal. This keeps the common-mode

voltage at ground, where it is required to be due to the center tap

connection. This is analogous to the AD8132 operating with a low

output impedance common-mode (see Figure 80).

VDIFF

VOCM

Figure 80. Transformer with Low Output

Impedance Secondary Set at VOCM

If the center tap of the secondary of a transformer is allowed

to float as shown in Figure 81 (or if there is no center tap),

the transformer has a high common-mode output imped-

ance. This means that the common mode of the secondary

is determined by what it is connected to and not by anything

to do with the transformer itself.

If one of the differential ends of the transformer is grounded,

the other end swings with the full output voltage. This means

that the common mode of the output voltage is one-half of the

differential output voltage. However, this shows that the common

mode is not forced via a low impedance to a given voltage. The

common-mode output voltage can be easily changed to any voltage

through its other output terminals.

The AD8132 can exhibit the same performance when one of the

outputs in Figure 79 is grounded. The other output swings at the

full differential output voltage. The common-mode signal is

measured by the voltage divider across the outputs and input

to VOCM. This, then, drives VOUT, cm to the same level. At higher

frequencies, it is important to minimize the capacitance on the

VOCM node; otherwise, phase shifts can compromise the perfor-

mance. The voltage divider resistances can also be lowered for

better frequency response.

VDIFF

Figure 81. Transformer with High Output Impedance Secondary

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