AD588
Data Sheet
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.
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 VLOW pin is the same voltage as
the system ground. Alternatively, if Pin 10 is connected to the
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 VCT
(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.
VCT 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.
NOISE
REDUCTION
A3 OUT
SENSE
V
HIGH
A3 IN
4
7
6
3
A3 OUT
FORCE
A3
1
R
B
A1
R1
A4 OUT
SENSE
14
15
R4
R5
A4 OUT
FORCE
A4
R2
+V
S
2
R6
R3
AD588
A2
16 –V
S
5
9
10
8
12
11
13
GAIN
ADJ
GND
SENSE SENSE
+IN –IN
GND
V
BAL
ADJ
V
A4 IN
LOW
CT
Figure 3. AD588 Functional Block Diagram
Rev. M | Page 6 of 20
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