AD8132_15 Datasheet, PDF (29/33 Pages) Analog Devices – Low Cost, High Speed Differential Amplifier

AD8132
The actual −3 dB frequency was measured to be 1.12 MHz, as
shown in Figure 78.
10
0
–10
–20
–30
–40
–50
–60
–70
–80
–90
10k
100k
1M
10M
FREQUENCY (Hz)
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, some applications benefit from high common-mode
output impedance. This is accomplished with the circuit shown
in Figure 79.
R_G
348Ω
R_F
348Ω
10Ω
1kΩ
49.9Ω
R_G
348Ω
R_F
348Ω
1kΩ
10Ω
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 a 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_OCM V_DIFF
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 high common-mode output impedance. 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.
NC V_DIFF
Figure 81. Transformer with High Output Impedance Secondary
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 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
performance. The voltage divider resistances can also be lowered
for better frequency response.
Rev. I | Page 28 of 32