AD8132ARZ-R7 datasheet(26/32 Pages) AD

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AD8132

Rev. F | Page 26 of 32

499Ω

523Ω

1kΩ

10µF

+5V

AD8132

0.1µF

49.9Ω

50Ω

SOURCE

0.1µF

10µF

–5V

49.9Ω

TWISTED

PAIR

100Ω

AD830

0.1µF

10µF

–5V

10µF

+5V

0.1µF

VOUT

Figure 74. Balanced Line Driver and Receiver Using AD8132 and AD830

Any imbalance in the differential drive signal appears 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 acts as an antenna and radiates. Thus, to

minimize radiation when driving differential twisted pair cables,

make sure the differential drive signal is well balanced.

The common-mode feedback loop in the AD8132 helps to min-

imize the amount of common-mode voltage at the output, and

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

driver. Figure 74 shows an application that uses an AD8132 as

a balanced line driver and an AD830 as a differential receiver con-

figured for unity gain. This circuit was operated with 10 meters of

Category 5 cable.

TRANSMIT EQUALIZER

Any length of transmission line attenuates the signals it carries.

This effect is worse at higher frequencies than at lower frequencies.

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 a higher frequency, the gain can be increased at

a high frequency. Figure 75 shows the gain of a two-line driver

that has its RG resistors shunted by 10 pF capacitors. The effect

of this is shown in the frequency response plot of Figure 76.

249Ω

49.9Ω

10pF

499Ω

10pF

249Ω

24.9Ω

VIN

49.9Ω

499Ω

49.9Ω

100Ω

VOUT

Figure 75. Frequency Boost Circuit

1000

–10

–20

–30

–40

–50

–60

–70

–80

100

FREQUENCY (MHz)

Figure 76. Frequency Response for Transmit Boost Circuit

LOW-PASS DIFFERENTIAL FILTER

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

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

ential outputs that can provide an antialias function when driving

a differential ADC.

Figure 77 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 78.

33pF

2.15kΩ

953Ω

33pF

2.15kΩ

100pF

2kΩ

24.9Ω

49.9Ω

549Ω

54Ω

200pF

VIN

VOUT

Figure 77. 1 MHz, 3-Pole Differential Output,

Low-Pass, Multiple Feedback Filter

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