AD688SQ AD [Analog Devices], AD688SQ Datasheet - Page 3

• Manufacturer Part Number: AD688SQ
• Description: High Precision -+10 V Reference
• Manufacturer: AD [Analog Devices]

REV. A
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 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.
Part
Number
AD688AQ
AD688BQ
AD688SQ
NOTES
1
2
*Temperature coefficient specified from 0 to +70 C.
For details on grade and package offerings screened in accordance with MIL-STD-883,
refer to the Analog Devices Military Products Databook or current AD688/883B data
sheet.
Q = Cerdip.
1
Initial
Error
5 mV
2 mV
5 mV
PIN CONFIGURATION
ORDERING GUIDE
Temperature
Coefficient
3 ppm/ C
3 ppm/ C
6 ppm/ C
–3–
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.
Temperature
Range - C
–40 to +85
–40 to +85*
–55 to +125
Figure 1. AD688 Functional Block Diagram
Package
Option
Q-16
Q-16
Q-16
2
AD688