AD708
Rev. C | Page 12 of 16
BRIDGE SIGNAL CONDITIONER
The AD708 can be used in the circuit shown in Figure 30 to
produce an accurate and inexpensive dynamic bridge condi-
tioner. The low offset voltage match and low offset voltage drift
match of the AD708 combine to achieve circuit performance
better than all but the best instrumentation amplifiers. The
outstanding specifications of the AD708, such as open-loop
gain, input offset currents, and low input bias currents, do not
limit circuit accuracy.
As configured, the circuit only requires a gain resistor, R
G
, of
suitable accuracy and a stable, accurate voltage reference. The
transfer function is
V
O
= V
REF
[∆R/(R + ∆R)][R
G
/R]
The only significant errors due to the AD708S are
V
OS_OUT
= (V
OS_MATCH
)(2R
G
/R) = 30 mV
V
OS_OUT
(T) = (V
OS_DRIFT
)(2R
G
/R) = 0.3 mV/°C
To achieve high accuracy, Resistor R
G
should be 0.1% or better
with a low drift coefficient.
5789-030
R
R
R = 350
R + R
1/2
AD708
V
O
–15V
R
G
175k
1/2
AD708
887
V
REF
+15
AD580
2.5V
Figure 30. Bridge Signal Conditioning Circuit
05789-031
IN459
1
IN459
1
NOTE
1
LOW LEAKAGE DIODES
1/2
AD708
1/2
AD708
5k
10k
IN
V
O
= |V
IN
|
10k 5k
3.75k
10k 10k
Figure 31. Precision Absolute Value Circuit
PRECISION ABSOLUTE VALUE CIRCUIT
The AD708 is ideally suited to the precision absolute value
circuit shown in
Figure 31. The low offset voltage match of the
AD708 enables this circuit to accurately resolve the input signal.
In addition, the tight offset voltage drift match maintains the
resolution of the circuit over the full military temperature
range. The high dc open-loop gain and exceptional gain
linearity allows the circuit to perform well at both large and
small signal levels.
In this circuit, the only significant dc errors are due to the offset
voltage of the two amplifiers, the input offset current match of
the amplifiers, and the mismatch of the resistors. Errors
associated with the AD708S contribute less than 0.001% error
over −55°C to +125°C.
Maximum error at 25°C
ppm4VV/1040
V10
nA1k10V30
==
Maximum error at +125°C or −55°C
C125@ppm7
V10
k10nA2V50
°+=
Figure 32 shows V
OUT
vs. V
IN
for this circuit with a ±3 mV input
signal at 0.05 Hz. Note that the circuit exhibits very low offset at
the zero crossing. This circuit can also produce V
OUT
= −|V
IN
| by
reversing the polarity of the two diodes.
05789-032
V
IN
= 1mV/DIV
V
OUT
= 1mV/DI
1mV1mV
Figure 32. Absolute Value Circuit Performance
(Input Signal = 0.05 Hz)
SELECTION OF PASSIVE COMPONENTS
Use high quality passive components to take full advantage of
the high precision and low drift characteristics of the AD708.
Discrete resistors and resistor networks with temperature
coefficients of less than 10 ppm/°C are available from Vishay,
Caddock, Precision Replacement Parts (PRP), and others.
