AD688

REV. A

–3–

ORDERING GUIDE

Part

Initial

Temperature

Temperature

Package

Number

1

Error

Coefficient

Range - C

Option

2

AD688AQ

5 mV

3 ppm/

°C

–40 to +85

Q-16

AD688BQ

2 mV

3 ppm/

°C

–40 to +85*

Q-16

AD688SQ

5 mV

6 ppm/

°C

–55 to +125

Q-16

NOTES

refer to the Analog Devices Military Products Databook or current AD688/883B data

sheet.

*Temperature coefficient specified from 0 to +70

°C.

PIN CONFIGURATION

THEORY OF OPERATION

The AD688 consists of a buried Zener diode reference, amplifi-

ers and associated thin-film resistors as shown in the block

diagram of Figure 1. The temperature compensation circuitry

provides the device with a temperature coefficient of 1.5 ppm/

°C

or less.

Amplifier A1 performs several functions. A1 primarily acts to

amplify the Zener voltage to the required 20 volts. In addition,

A1 also provides for external adjustment of the 20 V output

through Pin 5, the GAIN ADJUST. Using the bias compensa-

tion resistor between the Zener output and the noninverting

input to A1, a capacitor can be added at the NOISE

REDUCTION pin (Pin 7) to form a low pass filter and reduce

the noise contribution of the Zener to the circuit. Two matched

12 k

Ω nominal thin-film resistors (R4 and R5) divide the 20 V

output in half.

Ground sensing for the circuit is provided by Amplifier A2. The

noninverting input (Pin 9) senses the system ground and forces

the midpoint of resistors R4 and R5 to be a virtual ground. Pin

12 (BALANCE ADJUST) can be used for fine adjustment of

this midpoint transfer.

Amplifiers A3 and A4 are internally compensated and are used

to buffer the voltages at Pins 6 and 8 as well as to provide a full

Kelvin output. Thus, the AD688 has a full Kelvin capability by

providing the means to sense a system ground and provide

forced and sensed outputs referenced to that ground.

Figure 1. AD688 Functional Block Diagram