AD8129/AD8130
APPLICATIONS
BASIC GAIN CIRCUITS
TWISTED-PAIR CABLE, COMPOSITE VIDEO
RECEIVER WITH EQUALIZATION USING AN AD8130
The gain of the AD8129/AD8130 can be set with a pair of
feedback resistors. The basic configuration is shown in Figure 132.
The gain equation is the same as that of a conventional op amp:
G = 1 + RF/RG. For unity-gain applications using the AD8130,
RF can be set to 0 (short circuit), and RG can be removed (see
Figure 133). The AD8129 is compensated to operate at gains of
10 and higher; therefore, shorting the feedback path to obtain
unity gain causes oscillation.
The AD8130 has excellent common-mode rejection at its
inputs. This makes it an ideal candidate for a receiver for signals
that are transmitted over long distances on twisted-pair cables.
Category 5 cables are very common in office settings and are
extensively used for data transmission. These cables can also be
used for the analog transmission of signals such as video.
These long cables pick up noise from the environment they pass
through. This noise does not favor one conductor over another
and therefore is a common-mode signal. A receiver that rejects
the common-mode signal on the cable can greatly enhance the
signal-to-noise ratio performance of the link.
+V
AD8129/
AD8130
10μF
0.1μF
3
7
1
8
+V
+
+
PD
S
V
IN
The AD8130 is also very easy to use as a differential receiver,
because the differential inputs and the feedback inputs are
entirely separate. This means that there is no interaction
between the feedback network and the termination network,
as there would be in conventional op amp types of receivers.
6
V
OUT
4
5
–V
S
2
R
F
R
G
10μF
0.1μF
–V
Another issue with long cables is that there is more attenuation
of the signal at longer distances. Attenuation is also a function
of frequency; it increases to roughly the square root of frequency.
Figure 132. Basic Gain Circuit: VOUT = VIN (1 + RF/RG)
+V
For good fidelity of video circuits, the overall frequency
response of the transmission channel should be flat vs.
frequency. Because the cable attenuates the high frequencies, a
frequency-selective boost circuit can be used to undo this effect.
These circuits are called equalizers.
AD8130
10μF
0.1μF
3
7
+V
1
8
+
+
PD
S
V
IN
6
V
OUT
4
5
–V
An equalizer uses frequency-dependent elements (Ls and Cs) to
create a frequency response that is the opposite of the rest of the
channel’s response to create an overall flat response. There are
many ways to create such circuits, but a common technique is to
put the frequency-selective elements in the feedback path of an
op amp circuit. The AD8130 in particular makes this easier
than other circuits, because, once again, the feedback path is
completely independent of the input path and there is no
interaction.
S
2
10μF
0.1μF
–V
Figure 133. An AD8130 with Unity Gain
The input signal can be applied either differentially or in a
single-ended fashion—all that matters is the magnitude of the
differential signal between the two inputs. For single-ended
input applications, applying the signal to the +IN with −IN
grounded creates a noninverting gain, while reversing these
connections creates an inverting gain. Because the two inputs
are high impedance and matched, both of these conditions
provide the same high input impedance. Thus, an advantage of
the active feedback architecture is the ability to make a high
input impedance inverting op amp. If conventional op amps are
used, a high impedance buffer followed by an inverting stage is
needed. This requires two op amps.
The circuit in Figure 134 was developed as a receiver/equalizer
for transmitting composite video over 300 meters of Category 5
cable. This cable has an attenuation of approximately 20 dB at
10 MHz for 300 meters. At 100 MHz, the attenuation is
approximately 60 dB (see Figure 135).
Rev. C | Page 33 of 40
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