AD688
Rev. B | Page 7 of 16
APPLICATIONS
The AD688 can be configured to provide ±10 V reference
outputs as shown in Figure 4. The architecture of the AD688
provides ground sense and uncommitted output buffer
amplifiers which offer the user a great deal of functional
flexibility. The AD688 is specified and tested in the
configuration shown in Figure 4. The user may choose to take
advantage of other configuration options available with the
AD688; however performance in these configurations is not
guaranteed to meet the stringent data sheet specifications.
Unbuffered outputs are available at Pin 6 and Pin 8. Loading of
these unbuffered outputs will impair circuit performance.
Amplifiers A3 and A4 can be used interchangeably. However,
the AD688 is tested (and the specifications are guaranteed) with
the amplifiers connected as indicated in Figure 4. When either
A3 or A4 is unused, its output force and sense pins should be
connected and the input tied to ground.
Two outputs of the same voltage polarity may be obtained by
connecting both A3 and A4 to the appropriate unbuffered
output on Pin 6 or Pin 8. Performance in these dual output
configurations will typically meet data sheet specifications.
R3
R
B
R1
R2
R4
R5
R6
A1
A4
AD688
A3
A2
SYSTEM
GROUND
+10V
+15V SUPPLY
–10V
–15V SUPPLY
SYSTEM
GROUND
0.1
µ
F
0.1
µ
F
7
6
4
3
1
14
2
15
16
5 9
10
8
12
11
13
00815-004
Figure 4. +10 V and −10 V Outputs
CALIBRATION
Generally, the AD688 will meet the requirements of a precision
system without additional adjustment. Initial output voltage
error of 2 mV and output noise specs of 6 µV p-p allow for
accuracies of 12 to 16 bits. However, in applications where an
even greater level of accuracy is required, additional calibration
may be called for. The provision for trimming has been made
through the use of the GAIN ADJ and BAL ADJ pins (Pin 5 and
Pin 12, respectively).
The AD688 provides a precision 20 V span with a center tap
which is used with the buffer and ground sense amplifiers to
achieve the ±10 V output configuration. GAIN ADJ and
BAL ADJ can be used to trim the magnitude of the 20 V span
voltage and the position of the center tap within the span. The
gain adjustment should be performed first. Although the trims
are not interactive within the device, the gain trim will move the
balance trim point as it changes the magnitude of the span.
Figure 5 shows the gain and balance trims of the AD688. A
100 kΩ 20-turn potentiometer is used for each trim. The
potentiometer for the gain trim is connected between Pin 6
(V
HIGH
) and Pin 8 (V
LOW
) with the wiper connected to Pin 5
(GAIN ADJ). The potentiometer is adjusted to produce exactly
20 V between Pin 1 and Pin 15, the amplifier outputs. The
balance potentiometer, also connected between Pin 6 and Pin 8
with the wiper to Pin 12 (BAL ADJ), is then adjusted to center
the span from +10 V to −10 V.
Input impedance on both the GAIN ADJ and the BAL ADJ pins
is approximately 150 kΩ. The gain adjustment trim network
effectively attenuates the 20 V across the trim potentiometer by
a factor of about 1150 to provide a trim range of –5.8 mV to
+12.0 mV with a resolution of approximately 900 µV/turn
(20-turn potentiometer). The balance adjustment trim network
attenuates the trim voltage by a factor of about 1250, providing
a trim range of ±8 mV with a resolution of 800 µV/turn.
Trimming the AD688 introduces no additional errors over
temperature, so precision potentiometers are not required.
When balance adjustment is not necessary, Pin 12 should be left
floating. If gain adjustment is not required, Pin 5 should also be
left floating.
R3
R
B
R1
R2
R4
R5
R6
A1
A4
AD688
A3
A2
SYSTEM
GROUND
+10V
+15V SUPPLY
–10V
–15V SUPPLY
SYSTEM
GROUND
0.1
µ
F
0.1
µ
F
100k
Ω
20T
BALANCE
ADJUST
100kΩ
20T
GAIN ADJUST
+15V
NOISE
REDUCTION
1µF
7
6
4
3
1
14
2
15
16
5
9
10
8
12 11
13
20kΩ
00815-005
Figure 5. Gain and Balance Adjustment with Noise Reduction
