Datasheet

AD8132
Rev. I | Page 27 of 32
499Ω
523Ω
1kΩ
1kΩ
10µF
+
+5
V
AD8132
0.1µF
49.9Ω
50Ω
S
OURCE
0.1µF
+
0.1µF
10µF
–5V
49.9Ω
49.9Ω
TWISTED
PAIR
100Ω
1
2
3
4
7
5
AD830
+
0.1µF
10µF
–5V
10µF
+
+5V
0.1µF
V
OUT
01035-072
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. Therefore, 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
minimize 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
configured 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 R
G
component of the feedback
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 R
G
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
V
IN
49.9Ω
499Ω
49.9Ω
100Ω
V
OUT
01035-073
Figure 75. Frequency Boost Circuit
1
1000
20
10
0
–10
–20
–30
–40
–50
–60
–70
–80
V
OUT
/
V
IN
(dB)
10 100
FREQUENCY (MHz)
01035-074
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 differential outputs that can provide an antialias function
when driving a differential ADC.
33pF
2.15k
Ω
953Ω
953Ω
33pF
2.15kΩ
100pF
100pF
2kΩ
2kΩ
24.9Ω
49.9Ω
549Ω
549Ω
200pF
200pF
V
IN
V
OUT
01035-075
Figure 77. 1 MHz, 3-Pole Differential Output,
Low-Pass, Multiple Feedback Filter
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.