AD8129-EVAL Analog Devices, AD8129-EVAL Datasheet - Page 21

• Manufacturer Part Number: AD8129-EVAL
• Description: Low-cost 270 MHz differential receiver amplifier
• Manufacturer: Analog Devices

APPLICATIONS
Basic Gain Circuits
The gain of the AD8129/AD8130 can be set with a pair of feed-
back resistors. The basic configuration is shown in Figure 5.
The gain equation is the same as that of a conventional op amp:
G = 1 + R
R
(See Figure 6.) The AD8129 is compensated to operate at gains
of 10 and higher, so shorting the feedback path to obtain unity
gain will cause oscillation.
The input signal can be applied either differentially or single-
endedly—all that matters is the magnitude of the differential
signal between the two inputs. For single-ended input applica-
tions, applying the signal to the +IN with –IN grounded will
create a noninverting gain, while reversing these connections
will create an inverting gain. Since the two inputs are high-
impedance and matched, both of these conditions will 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.
F
can be set to zero (short circuit), and R
F
/R
V
V
G
IN
IN
. For unity gain applications using the AD8130,
AD8129/
AD8130
AD8130
R
G
R
F
+
+
+
+
PD
PD
–V
–V
–V
–V
S
S
+V
+V
0.1 F 10 F
+V
+V
S
S
0.1 F
0.1 F
0.1 F
10 F
V
V
G
OUT
OUT
can be removed.
10 F
10 F
Twisted-Pair Cable, Composite Video Receiver with Equal-
ization Using an AD8130
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. Cat-
egory 5 type cables are now very common in office settings and
are extensively used for data transmission. These same cables
can also be used for the analog transmission of signals like video.
These long cables will pick up noise from the environment they
pass through. This noise will not favor one conductor over an-
other, and will therefore be 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.
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 of the
feedback network and the termination network as there would
be in conventional op amp-type receivers.
Another issue to be dealt with on long cables is the attenuation
of the signal at longer distances. This attenuation is a function of
frequency and increases as roughly as the square root of frequency.
For good fidelity of video circuits, the overall frequency response
of the transmission channel should be flat versus frequency. Since
the cable attenuates the high frequencies, a frequency-selective
boost circuit can be used to undo this effect. These circuits
are called equalizers.
An equalizer uses frequency-dependent elements (Ls and Cs) in
order to create a frequency response that is the opposite of the
rest of the channel’s response in order 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 particu-
lar makes this easier than other circuits, because, once again, the
feedback path is totally independent of the input path and there
is no interaction.
The circuit in Figure 7 was developed as a receiver/equalizer for
transmitting composite video over 300 m of Category 5 cable. This
cable has an attenuation of approximately 20 dB at 10 MHz
for 300 m. At 100 MHz, the attenuation is approximately
60 dB. (See Figure 8.)
200pF
100
C1
R1
V
IN
100
AD8130
R
499
G
+
+
1k
R
F
PD
–V
–V
AD8129/AD8130
S
+V
+V
S
0.1 F
0.1 F
10 F
V
OUT
10 F