
AD8132
Rev. F | Page 27 of 32
FREQUENCY (Hz)
10
10k
0
–10
–20
–30
–40
–50
–60
–70
–80
–90
100k
1M
10M
100M
V
O
/
I
0
Figure 78. Frequency Response of 1 MHz Low-Pass Filter
HIGH COMMON-MODE OUTPUT IMPEDANCE
AMPLIFIER
Changing the connection to V
OCM
(Pin 2) can change the common-
mode from low impedance to high impedance. If V
OCM
is actively
set to a particular voltage, the AD8132 tries to force V
OUT, 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.
R
G
348
R
F
348
R
F
348
R
G
348
10
10
1k
1k
49.9
49.9
0
Figure 79. High Common-Mode, Output Impedance, Differential Amplifier
V
OCM
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).
V
DIFF
V
OCM
0
Figure 80. Transformer with Low Output
Impedance Secondary Set at V
OCM
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 V
OCM
. This, then, drives V
OUT, cm
to the same level. At higher
frequencies, it is important to minimize the capacitance on the
V
OCM
node; otherwise, phase shifts can compromise the perfor-
mance. The voltage divider resistances can also be lowered for
better frequency response.
V
DIFF
NC
0
Figure 81. Transformer with High Output Impedance Secondary