Datasheet

AD588
Rev. L | Page 6 of 20
THEORY OF OPERATION
The AD588 consists of a buried Zener diode reference,
amplifiers used to provide pin programmable output ranges,
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 from 6.5 V to the required 10 V
output. In addition, A1 provides for external adjustment of the
10 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 10 kΩ nominal thin-film resistors (R4 and R5) divide
the 10 V output in half. Pin V
CT
(Pin 11) provides access to the
center of the voltage span and BAL ADJ (Pin 12) can be used
for fine adjustment of this division.
Ground sensing for the circuit is provided by Amplifier A2. The
noninverting input (Pin 9) senses the system ground, which is
transferred to the point on the circuit where the inverting input
(Pin 10) is connected. This can be Pin 6, Pin 8, or Pin 11. The
output of A2 drives Pin 8 to the appropriate voltage. Thus, if
Pin 10 is connected to Pin 8, the V
LOW
pin is the same voltage as
the system ground. Alternatively, if Pin 10 is connected to the
V
CT
pin, it is a ground; and Pin 6 and Pin 8 are +5 V and −5 V,
respectively.
Amplifier A3 and Amplifier A4 are internally compensated and
are used to buffer the voltages at Pin 6, Pin 8, and Pin 11, as well
as to provide a full Kelvin output. Thus, the AD588 has a full
Kelvin capability by providing the means to sense a system
ground and provide forced and sensed outputs referenced to
that ground.
Note that both positive and negative supplies are required for
operation of the AD588.
R3
R
B
R1
R2
R4
R5
R6
GAIN
ADJ
GND
SENSE
+IN
GND
SENSE
–IN
V
LOW
BAL
ADJ
V
CT
A4 IN
–V
S
+V
S
A4 OUT
FORCE
A4 OUT
SENSE
A3 OUT
FORCE
A3 OUT
SEN
SE
A3 IN
V
HIGH
NOISE
REDUCTION
A1
A4
AD588
A2
00531-003
A3
13111281095
1
14
15
2
16
3467
Figure 3. AD588 Functional Block Diagram