AD688SQ 데이터 시트보기 (PDF) - Analog Devices
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AD688SQ Datasheet PDF : 16 Pages
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AD688
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.
NOISE
REDUCTION
7
VHIGH
6
+10V OUT
A3 IN SENSE
4
3
A3
RB
A1
AD688
R1
R4
R2
A4
R5
R3
R6
A2
5
GAIN
ADJ
9
GND
SENSE
+IN
10
8
12
11
NC
VLOW BAL NC
ADJ
NC = NO CONNECT
13
A4 IN
Figure 3. Functional Block Diagram
1
+10V OUT
FORCE
14
–10V OUT
SENSE
15
–10V OUT
FORCE
2 +VS
16 –VS
Rev. B | Page 6 of 16
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