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REV. 0

AD8132

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Low-Pass Differential Filter

Similar to an op amp, various types of active ﬁlters can be cre-

ated with the AD8132. These can have single-ended inputs and

differential outputs, which can provide an antialias function

when driving a differential A/D converter.

Figure 14 is a schematic of a low-pass, multiple-feedback ﬁlter.

The active section contains two poles, and an additional pole is

added at the output. The ﬁlter was designed to have a –3 dB

frequency of 1 MHz. The actual –3 dB frequency was measured

to be 1.12 MHz as shown in Figure 15.

33pF

2.15k

953

33pF

2.15k

100pF

24.9

49.9

549

200pF

VIN

VOUT

Figure 14. 1 MHz, 3-Pole Differential Output Low-Pass

Multiple Feedback Filter

FREQUENCY – Hz

10k

–10

–20

–30

–40

–50

–60

–70

–80

–90

100k

10M

100M

Figure 15. 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 will try

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, the are some applications that beneﬁt from a high

common-mode output impedance. This can be accomplished

with the circuit shown in Figure 16.

348

49.9

Figure 16. High Common-Mode Output Impedance Differ-

ential Ampliﬁer

The complementary electrical ﬁelds will mostly be conﬁned to

the space between the two twisted conductors and will not sig-

niﬁcantly radiate out from the cable. The current in the cable will

create magnetic ﬁelds that will radiate to some degree. However,

the amount of radiation is mitigated by the twists, because for

each twist, the two adjacent twists will have an opposite polarity

magnetic ﬁeld. If the twist pitch is tight enough, these small

magnetic ﬁeld loops will contain most of the magnetic flux,

and the magnetic far-ﬁeld strength will be negligible.

Any imbalance in the differential drive signal will appear as a

common-mode signal on the cable. This is the equivalent of a

single wire that is driven with the common-mode signal. In this

case, the wire will act as an antenna and radiate. Thus, in order

to minimize radiation when driving differential twisted pair

cables, the differential drive signal should be very well balanced.

The common-mode feedback loop in the AD8132 helps to

minimize the amount of common-mode voltage at the output,

and can therefore be used to create a well-balanced differential

line driver. Figure 11 shows an application that uses an AD8132

as a balanced line driver and AD830 as a differential receiver

conﬁgured for unity gain. This circuit was operated with 10 m

of Category 5 cable.

Transmit Equalizer

Any length of transmission line will attenuate the signals it carries.

This effect is worse at higher frequencies than at low frequen-

cies. One way to compensate for this is to provide an equalizer

circuit that boosts the higher frequencies in the transmitter

circuit, so that at the receive end of the cable, the attenuation

effects are diminished.

By lowering the impedance of the RG component of the feed-

back network at higher frequency, the gain can be increased at

high frequency. Figure 12 shows a gain-of-two line driver that

has its RGs shunted by 10 pF resistors. The effect of this is shown

in the frequency response plot of Figure 13.

249

49.9

10pF

499

10pF

249

24.9

VIN

49.9

499

49.9

100

VOUT

Figure 12. Frequency Boost Circuit

1000

–10

–20

–30

–40

–50

–60

–70

–80

100

FREQUENCY – MHz

Figure 13. Frequency Response for Transmit Boost Circuit

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