AD688SQ AD [Analog Devices], AD688SQ Datasheet - Page 8

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

AD688
Attempts to drive a large capacitive load (in excess of 1,000 pF)
may result in ringing or oscillation, as shown in the step re-
sponse photo (Figure 14a). This is due to the additional pole
formed by the load capacitance and the output impedance of the
amplifier, which consumes phase margin. The recommended
method of driving capacitive loads of this magnitude is shown in
Figure 14b. The 150
the output stage, while the 10 k resistor provides a dc feedback
path and preserves the output accuracy. The 1 F capacitor
provides a high frequency feedback loop. The performance of
this circuit is shown in Figure 14c.
Figure 14c. Output Amplifier Step Response Using Figure
14b Compensation
BRIDGE DRIVER CIRCUIT
The Wheatstone bridge is a common transducer. In its simplest
form, a bridge consists of 4 two-terminal elements connected to
form a quadrilateral, a source of excitation connected along one
of the diagonals and a detector comprising the other diagonal.
Figure 14a. Output Amplifier Step Response, C
Figure 14b. Compensation for Capacitive Loads
resistor isolates the capacitive load from
L
= 1 F
–8–
In this unipolar drive configuration, the output voltage of the
bridge is riding on a common-mode voltage signal equal to
approximately V
necessarily be limited to high common-mode rejection
techniques such as instrumentation or isolation amplifiers.
However, if the bridge is driven from a pair of bipolar supplies,
then the common-mode voltage is ideally eliminated and the
restrictions on any processing elements that follow are relaxed.
As shown in Figure 15, the AD688 is an excellent choice for the
control element in a bipolar bridge driver scheme. Transistors
Q1 and Q2 serve as series pass elements to boost the current
drive capability to the 57 mA required by the typical 350
bridge. A differential gain stage may still be required if the
bridge balance is not perfect.
Figure 15. Bipolar Bridge Drive
Dimensions shown in inches and (mm).
IN
/2. Further processing of this signal may
OUTLINE DIMENSIONS
REV. A