AD688SQ AD [Analog Devices], AD688SQ Datasheet - Page 4

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

AD688
APPLYING THE AD688
The AD688 can be configured to provide 10 V reference out-
puts as shown in Figure 2a. The architecture of the AD688 pro-
vides 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 2a. 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 Pins 6 and 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 2a. 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 out-
put on Pin 6 or 8. Performance in these dual output configura-
tions will typically meet data sheet specifications.
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 V p-p allow for
accuracies of 12-16 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 ADJUST and BALANCE
ADJUST pins (Pins 5 and 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. The GAIN ADJUST
and BALANCE ADJUST 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 ADJUST should be performed first. Al-
though 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 2b shows the GAIN and BALANCE trims of the
AD688. A 100 k 20-turn potentiometer is used for each trim.
The potentiometer for the GAIN trim is connected between
Pins 6 (V
(GAIN ADJ). The potentiometer is adjusted to produce exactly
HIGH
Figure 2a. +10 V and –10 V Outputs
) and 8 (V
LOW
) with the wiper connected to Pin 5
–4–
20 V between Pins 1 and 15, the amplifier outputs. The BAL-
ANCE potentiometer, also connected between Pins 6 and 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 ADJUST and the BAL-
ANCE ADJUST pins is approximately 150 k . The GAIN
ADJUST 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 approxi-
mately 900 V/turn (20 turn potentiometer). The BALANCE
ADJUST trim network attenuates the trim voltage by a factor of
about 1250, providing a trim range of 8 mV with a resolution
of 800 V/turn.
Trimming the AD688 introduces no additional errors over
temperature, so precision potentiometers are not required.
In cases when BALANCE ADJUST is not necessary, Pin 12
should be left floating. If GAIN ADJUST is not required, Pin 5
should also be left floating.
NOISE PERFORMANCE AND REDUCTION
The noise generated by the AD688 is typically less than 6 V
p-p over the 0.1 Hz to 10 Hz band. Noise in a 1 MHz
bandwidth is approximately 840 V p-p. The dominant source
of this noise is the buried Zener which contributes approxi-
mately 140 nV/ Hz. In comparison, the op amp’s contribution
is negligible. Figure 3 shows the 0.1 Hz to 10 Hz noise of a
typical AD688.
Figure 2b. Gain and Balance Adjust with Noise Reduction
Figure 3. 0.1 Hz to 10 Hz Noise
REV. A