AD688

REV. A

–8–

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

Ω resistor isolates the capacitive load from

the output stage, while the 10 k

Ω resistor provides a dc feedback

path and preserves the output accuracy. The 1

µF capacitor

provides a high frequency feedback loop. The performance of

this circuit is shown in Figure 14c.

Figure 14b. Compensation for Capacitive Loads

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.

OUTLINE DIMENSIONS

Dimensions shown in inches and (mm).

In this unipolar drive configuration, the output voltage of the

bridge is riding on a common-mode voltage signal equal to

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

Ω

bridge. A differential gain stage may still be required if the

bridge balance is not perfect.

Figure 15. Bipolar Bridge Drive