AD830
Rev. C | Page 16 of 20
stability and also keep bias current induced offsets low. Highest
CMRR and lowest dc offsets are preserved by including a
compensating resistor in series with Pin 3. The gain may be as
high as 100.
0.1µF
0.1µF
P
V
OUT
V
N
AD830
1
2
3
4
8
7
6
5
A = 1
C
G
M
G
M
INPUT
SIGNAL
CM
Z
CM
V
1
V
2
R
1
R
2
R
2
R
1
V
OUT
= (V
1
– V
2
)(1 + R
1
/R
2
)
00881-040
Figure 40. Gain of G Differential Amplifier, G>1
Offsetting the Output With Gain
Some applications, such as ADCs, require that the signal be
amplified and also offset, typically to accommodate the input
range of the device. The AD830 can offset the output signal
very simply through Pin 3 even with gain > 1. The voltage
applied to Pin 3 must be attenuated by an appropriate factor so
that V
3
× G = desired offset. In Figure 41, a resistive divider
from a voltage reference is used to produce the attenuated offset
voltage.
P
V
OUT
V
N
AD830
1
2
3
4
8
7
6
5
A = 1
G
M
G
M
C
0.1µF
0.1µF
INPUT
SIGNAL
V
CM
Z
CM
V
1
V
2
R
1
R
2
R
2
R
1
R
3
R
4
V
3
V
REF
V
OUT
= (V
1
– V
2
)(1 + R
1
/R
2
)
0881-041
Figure 41. Offsetting the Output with Differential Gain >1
Loop Through or Line Bridging Amplifier (Figure 42)
The AD830 is ideally suited for use as a video line bridging
amplifier. The video signal is tapped from the conductor of the
cable relative to its shield. The high input impedance of the
AD830 provides negligible loading on the cable. More significantly,
the benign loading is maintained while the AD830 is powered
down. Coupled with its good video load driving performance,
the AD830 is well suited for video cable monitoring applications.
P
V
OUT
V
N
AD830
1
2
3
4
8
7
6
5
A = 1
G
M
C
G
M
R
G
249Ω
OPTIONAL C
C
499Ω
499Ω
75Ω
75Ω
0.1µF
0.1µF
0881-042
Figure 42. Cable Tap Amplifier
Resistorless Summing
Direct, two input, resistorless summing is easily realized from
the general unity gain mode. By grounding V
X2
and applying the
two inputs to V
X1
and V
Y1
, the output is the exact sum of the
applied voltages, V
1
and V
3
, relative to common; V
OUT
= V
1
+ V
3
.
A diagram of this simple but potent application is shown below
in Figure 43. The AD830 summing circuit possesses several
virtues not present in the classic op amp based summing circuits.
It has high impedance inputs, no resistors, very precise summing,
high reverse isolation, and noninverting gain. Achieving this
function and performance with op amps requires significantly
more components.
P
OUT
V
N
AD830
1
2
3
4
8
7
6
5
A = 1
G
M
C
G
M
V
1
V
3
V
OUT
= V
1
+V
3
0881-043
Figure 43. Resistorless Summing Amplifier
2× Gain Bandwidth Line Driver
A gain of two, without the use of resistors, is possible with the
AD830. This is accomplished by grounding V
X2
, tying the V
X1
and V
Y1
inputs together, and applying the input, V
IN
, to this
wired connection. The output is exactly twice the applied
voltage, V
IN
; V
OUT
= 2 × V
IN
. Figure 44 shows the connections
for this highly useful application. The most notable characteristic of
this alternative gain of +2 is that there is no loss of bandwidth as
in a voltage feedback op amp based gain of +2 where the
bandwidth is halved; therefore, the gain bandwidth is doubled.
In addition, this circuit is accurate without the need for any
precise valued resistors, as in the op amp equivalents, and it
possesses excellent differential gain and phase performance, as
shown in Figure 45 and Figure 46.
