AD688

Rev. B | Page 12 of 16

10V

0V

1k

Ω

10V

10V

A4

A3

+

–

+

–

Figure 27. Load Crosstalk Test Circuit

100

90

10

0%

1mV

5V

2

µs

Figure 28. Load Crosstalk

Attempts to drive a large capacitive load (in excess of 1000 pF)

may result in ringing or oscillation, as shown in the step

response photo (Figure 29). This is due to the additional pole

formed by the load capacitance and the output impedance of

the amplifier, which consumes phase margin. The recom-

mended method of driving capacitive loads of this magnitude is

shown in Figure 30. The 150 Ω resistor isolates the capacitive

load from the output stage, while the 10 kΩ 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 31.

100

90

10

0%

10V

1V

200

µs

+

–

10k

Ω

1

µF

1

µF

150

Ω

Figure 30. Compensation for Capacitive Loads

100

90

10

0%

10V

1V

200

µs

Figure 31. Output Amplifier Step Response Using Figure 30 Compensation

BRIDGE DRIVER CIRCUIT

The Wheatstone bridge is a common transducer. In its simplest

form, a bridge consists of four 2-terminal elements connected to

form a quadrilateral, a source of excitation connected along one

of the diagonals and a detector comprising the other diagonal.

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 32, 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.

R3

R1

R2

R4

R5

R6

A1

A4

AD688

A3

A2

2N3904

220

Ω

+15V

–15V

220

Ω

2N3906

+

–

7

6

4

3

14

2

15

16

5

9

10

8

12

11

13

1

Figure 32. Bipolar Bridge Drive