AD811
Driving Capacitive Loads
100
90
80
70
60
50
40
30
20
10
0
The feedback and gain resistor values in Table I will result in
very flat closed-loop responses in applications where the load
capacitances are below 10 pF. Capacitances greater than this
will result in increased peaking and overshoot, although not
necessarily in a sustained oscillation.
G = +2
= ؎15V
V
S
R
VALUE SPECIFIED
S
IS FOR FLATTEST
FREQUENCY RESPONSE
There are at least two very effective ways to compensate for this
effect. One way is to increase the magnitude of the feedback
resistor, which lowers the 3 dB frequency. The other method is
to include a small resistor in series with the output of the ampli-
fier to isolate it from the load capacitance. The results of these
two techniques are illustrated in Figure 32. Using a 1.5 kΩ
feedback resistor, the output ripple is less than 0.5 dB when driv-
ing 100 pF. The main disadvantage of this method is that it
sacrifices a little bit of gain flatness for increased capacitive load
drive capability. With the second method, using a series resistor,
the loss of flatness does not occur.
10
100
LOAD CAPACITANCE – pF
1000
Figure 33. Recommended Value of Series Resistor vs. the
Amount of Capacitive Load
R
FB
Figure 33 shows recommended resistor values for different load
capacitances. Refer again to Figure 32 for an example of the
results of this method. Note that it may be necessary to adjust
the gain setting resistor, RG, to correct for the attenuation which
results due to the divider formed by the series resistor, RS, and
the load resistance.
+V
S
0.1F
R
R
G
R
(OPTIONAL)
S
V
Applications which require driving a large load capacitance at a
high slew rate are often limited by the output current available
from the driving amplifier. For example, an amplifier limited to
25 mA output current cannot drive a 500 pF load at a slew rate
greater than 50 V/µs. However, because of the AD811’s 100 mA
output current, a slew rate of 200 V/µs is achievable when driv-
ing this same 500 pF capacitor (see Figure 34).
OUT
AD811
V
IN
C
R
L
L
T
0.1F
–V
S
2V
100ns
Figure 31. Recommended Connection for Driving a Large
Capacitive Load
100
90
V
IN
12
R
R
= 1.5k⍀
= 0
FB
9
6
S
10
V
OUT
3
G = +2
= ؎15V
0%
R
R
= 649⍀
= 30⍀
V
FB
S
R
C
= 10k⍀
= 100pF
S
L
L
5V
0
Figure 34. Output Waveform of an AD811 Driving a
500 pF Load. Gain = +2, RFB = 649 Ω, RS = 15 Ω,
RS = 10 kΩ
–3
–6
1M
10M
FREQUENCY – Hz
100M
Figure 32. Performance Comparison of Two Methods for
Driving a Capacitive Load
–10–
REV. D
ADI [ ADI ]
