AD688

Rev. B | Page 6 of 16

THEORY OF OPERATION

The AD688 consists of a buried Zener diode reference,

amplifiers and associated thin-film resistors as shown in

Figure 3. 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 V. In addition, A1

also provides for external adjustment of the 20 V output

through Pin 5 (GAIN ADJ). Using the bias compensation

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 kΩ 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 (BAL ADJ) 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 Pin 6 and Pin 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 forced

and sensed outputs referenced to that ground.

R3

R1

R2

R4

R5

R6

GAIN

ADJ

GND

SENSE

+IN

NC

ADJ

NC

A4 IN

–10V OUT

FORCE

–10V OUT

SENSE

+10V OUT

FORCE

+10V OUT

SENSE

A3 IN

NOISE

REDUCTION

A1

A4

A3

A2

AD688

7

6

4

3

1

14

2

15

16

5

9

10

8

12

11

13

NC = NO CONNECT

Figure 3. Functional Block Diagram