AD830
D iffer ential Line Receiver
D iffer ence Am plifier with Gain > 1
T he AD830 was specifically designed to perform as a differen-
tial line receiver. T he circuit in Figure 33 shows how simple it is
to configure the AD830 for this function. T he signal from sys-
tem “A” is received differentially relative to A’s common, and
that voltage is exactly reproduced relative to the common in sys-
tem B. T he common-mode rejection versus frequency, shown in
Figure 1, is excellent, typically 100 dB at low frequencies. T he
high input impedance permits the AD830 to operate as a bridg-
ing amplifier across low impedance terminations with negligible
loading. T he differential gain and phase specifications are very
good as shown in Figure 7 for 500 Ω and Figure 10 for 150 Ω.
T he input and output common should be separated to achieve
the full CMR performance of the AD830 as a differential ampli-
fier. However, a common return path is necessary between sys-
tems A and B.
T he AD830 can provide instrumentation amplifier style differ-
ential amplification at gains greater than 1. T he input signal is
connected differentially and the gain is set via feedback resistors
as shown in Figure 35. T he gain, G = (R2 + R1)/R2. T he AD830
can provide either inverting or noninverting differential amplifi-
cation. T he polarity of the gain is established by the polarity of
the connection at the input. Feedback resistors R2 should gener-
ally be R2 ≤ 1 kΩ to maintain closed-loop 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. T he gain may be as high as 100.
V
P
0.1µF
AD830
8
V
1
2
3
4
1
V
G
G
CM
M
V
P
INPUT
SIGNAL
V
OUT
0.1µF
7
6
5
V
2
AD830
V
8
1
A=1
1
V
G
G
CM
Z
M
CM
INPUT
SIGNAL
R
R
2
1
V
OUT
C
2
3
4
7
6
5
V
2
M
A=1
0.1µF
COMMON IN
SYSTEM A
Z
CM
C
R1
M
V
N
0.1µF
R2
V
N
V
= (V – V ) (1+R /R )
1 2 1 2
OUT
V
= V – V
1
2
COMMON IN
SYSTEM
OUT
B
Figure 35. Gain of G Differential Am plifier, G > 1
O ffsetting the O utput with Gain
Figure 33. Differential Line Receiver
Wide Range Level Shifter
Some applications, such as A/D drivers, require that the signal
be amplified and also offset, typically to accommodate the input
range of the device. T he AD830 can offset the output signal
very simply through Pin 3 even with gain > 1. T he voltage ap-
plied to Pin 3 must be attenuated by an appropriate factor so
that V3 ϫ G = desired offset. In Figure 36, a resistive divider
from a voltage reference is used to produce the attenuated offset
voltage.
T he wide common-mode range and accuracy of the AD830 al-
lows easy level shifting of differential signals referred to an input
common-mode voltage to any new voltage defined at the out-
put. T he inputs may be referenced to levels as high as 10 V at
the inputs with a ±2 V swing about 10 V. In the circuit of Fig-
ure 34, the output voltage, VOUT , is defined by the simple equa-
tion shown below. T he excellent linearity and low distortion are
preserved over the full input and output common-mode range.
T he voltage sources need not be of low impedance, since the
high input resistance and modest input bias current of the
AD830 V-to-I converters permit the use of resistive voltage di-
viders as reference voltages.
V
P
0.1µF
AD830
V
1
8
7
1
V
G
CM
M
INPUT
SIGNAL
V
OUT
2
3
4
V
2
A=1
V
P
Z
CM
R
R
2
1
0.1µF
6
5
AD830
C
G
V
1
8
7
6
5
1
M
G
G
M
INPUT
SIGNAL
0.1µF
V
OUT
2
3
4
V
2
V
REF
R
A=1
1
INPUT
COMMON
V
N
R
C
2
M
R
3
0.1µF
V
3
V
= (V – V ) (1+R /R )
1 2 1 2
OUT
R
4
V
N
V
3
V
= V – V + V
1 2 3
OUT
OUTPUT
Figure 36. Offsetting the Output with Differential Gain > 1
COMMON
Figure 34. Differential Am plification with Level Shifting
REV. A
–13–
ADI [ ADI ]
