AD708

Data Sheet

Rev. D | 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, RG, of

suitable accuracy and a stable, accurate voltage reference. The

transfer function is

VO = VREF [∆R/(R + ∆R)][RG/R]

The only significant errors due to the AD708S are

VOS_OUT = (VOS_MATCH)(2RG/R) = 30 mV

VOS_OUT (T) = (VOS_DRIFT)(2RG/R) = 0.3 mV/°C

To achieve high accuracy, Resistor RG should be 0.1% or better

with a low drift coefficient.

R

R

R = 350Ω

R +

ΔR

1/2

AD708

VO

–15V

RG

175kΩ

1/2

AD708

887Ω

VREF

+15V

AD580

2.5V

Figure 30. Bridge Signal Conditioning Circuit

IN4591

IN4591

NOTE

1LOW LEAKAGE DIODES

1/2

AD708

1/2

AD708

5kΩ

10kΩ

VIN

VO = |VIN|

10kΩ

5kΩ

3.75kΩ

10kΩ

10kΩ

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

(

)( )

ppm

4

μV

μV/10

40

V

10

nA

1

kΩ

10

μV

30

=

=

+

Maximum error at +125°C or −55°C

( )(

)

C

125

@

ppm

7

V

10

kΩ

10

nA

2

μV

50

°

+

=

+

Figure 32 shows VOUT vs. VIN 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 VOUT = −|VIN| by

reversing the polarity of the two diodes.

VIN = 1mV/DIV

1mV

1mV

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.