AD708

REV. B

–8–

suitable accuracy and a stable, accurate voltage reference. The

transfer function is:

and the only significant errors due to the AD708S are:

°C

better and have a low drift coefficient.

Figure 29. Bridge Signal Conditioning Circuit

Figure 30. Precision Absolute Value Circuit

PRECISION ABSOLUTE VALUE CIRCUIT

The AD708 is ideally suited to the precision absolute value

circuit shown in Figure 30. 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 AD708’s 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 ampliliers, the input offset current match of

the amplifiers, and the mismatch of the resistors. Errors associ-

ated with the AD708S contribute less than 0.001% error over

–55

°C to +125°C.

Maximum error at 25

°C

30

µV + 10 kΩ

()

10V

= 40 µV/10V = 4 ppm Maximum

error at +125

°C or –55°C

50

µV + 2 nA

()

10V

= 7 ppm @ +125°C

±3mV

input signal at 0.05 Hz. Note that the circuit exhibits very low

Figure 31. Absolute Value Circuit Performance

(Input Signal = 0.05 Hz)

SELECTION OF PASSIVE COMPONENTS

To takc full advantagc of thc high precision and low drift

characteristics of the AD708, high quality passive components

must be used. Discrete resistors and resistor networks with

temperature coefficients of less than 10 ppm/

°C are available

from Vishay, Caddock, PRP and others.

OUTLINE DIMENSIONS

Dimensions shown in inches and (mm).

TO-99 (H) Package

Cerdip (Q) Package

Mini-DIP (N) Package