AD8129-EVAL Analog Devices, AD8129-EVAL Datasheet - Page 23

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

A way around this is to apply the offset voltage to a voltage
divider whose attenuation factor matches the gain of the ampli-
fier, and then apply this voltage to the high-impedance REF
input. This circuit will first divide the desired offset voltage by
the gain, and the amplifier will multiply it back up to unity. (See
Figure 13.)
Resistorless Gain-of-Two
The voltage applied to the REF input (Pin 4) can also be a high
bandwidth signal. If a unity-gain AD8130 has both +IN and
REF driven with the same signal, there will be unity gain from
V
of two, and requires no resistors. (See Figure 14.)
IN
V
V
OFFSET
OFFSET
and unity gain from V
AD8129/
AD8130
AD8129/
AD8130
V
R
V
IN
R
F
IN
V
G
R
IN
G
R
G
AD8130
+
+
+
+
R
R
F
F
+
+
PD
PD
REF
–V
–V
–V
–V
PD
S
. Thus, the circuit will have a gain
+V
S
+V
+V
+V
–V
–V
S
S
S
+V
+V
0.1 F
0.1 F
S
0.1 F
0.1 F
0.1 F
V
V
0.1 F
V
V
OUT
IN
10 F
10 F
OUT
IN
V
=
10 F
OUT
(1+ R
10 F
10 F
=
(1+R
10 F
F
F
/R
/R
G
G
) +V
) + V
OFFSET
OFFSET
Summer
A general summing circuit can be made by the above technique.
A unity-gain configured AD8130 has one signal applied to +IN,
while the other signal is applied to REF. The output will be the
sum of the two input signals. (See Figure 15.)
This circuit offers several advantages over a conventional op
amp inverting summing circuit. First, the inputs are both high-
impedance and the circuit is noninverting. It would require
significant additional circuitry to make an op amp summing
circuit that has high input impedance and is noninverting.
Another advantage is that the AD8130 circuit still preserves the
full bandwidth of the part. In a conventional summing circuit,
the noise gain is increased for every additional input, so the
bandwidth response decreases accordingly. By this technique,
four signals can be summed by applying them to two AD8130s,
and then summing the two outputs by a third AD8130.
Cable-Tap Amplifier
It is often desirable to have a video signal drive several different
pieces of equipment. However, the cable should only be termi-
nated once at its end point, so it is not appropriate to have a
termination at each device. A “loop-through” connection allows
a device to tap the video signal while not disturbing it by any
excessive loading.
Such a connection, also referred to as a cable-tap amplifier, can
be simply made with an AD8130. (See Figure 16.) The circuit is
configured with unity gain, and if no output offset is desired,
the REF pin is grounded. The negative differential input is
connected directly to the shield of the cable (or an associated
connector) at the point at which it wants to be “tapped.”
VIDEO
IN
V1
V2
75
AD8130
AD8130
75
+
+
PD
+
+
–V
–V
S
+V
+V
PD
S
AD8129/AD8130
–V
–V
0.1 F
S
+V
+V
S
0.1 F
0.1 F
V
OUT
10 F
0.1 F
= V1 + V2
10 F
V
10 F
OUT
10 F