AD688
Rev. B | Page 12 of 16
10V
0V
V
L
1kΩ
10V
V
OUT
10V
A4
A3
+
–
+
–
00815-028
Figure 27. Load Crosstalk Test Circuit
00815-029
100
90
10
0%
1mV
5V
2µs
V
OUT
V
L
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.
00815-030
100
90
10
0%
10V
1V
200µs
V
IN
V
OUT
Figure 29. Output Amplifier Step Response, C
L
= 1 µF
V
IN
V
OUT
+
–
10k
Ω
1
µ
F
C
L
1
µ
F
150
Ω
00815-031
Figure 30. Compensation for Capacitive Loads
00815-032
100
90
10
0%
10V
1V
200µs
V
IN
V
OUT
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
approximately V
IN
/2. Further processing of this signal may
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
R
B
R1
R2
R4
R5
R6
–V
S
+V
S
A1
A4
AD688
A3
A2
Q
1
=
2N3904
220Ω
+15V
–15V
220Ω
Q
2
=
2N3906
E
O
+
–
7
6
4
3
14
2
15
16
5
9
10
8
12 11
13
1
00815-033
Figure 32. Bipolar Bridge Drive
