Data Sheet
AD811
Rev. F | Page 13 of 20
Driving Capacitive Loads
The feedback and gain resistor values i
n Table 3 result in very
flat closed-loop responses in applications where the load
capacitances are below 10 pF. Capacitances greater than this
result in increased peaking and overshoot, although not
necessarily in a sustained oscillation.
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 38. Using a 1.5 k
feedback resistor, the output ripple is less than 0.5 dB when
driving 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.
Figure 37. Recommended Connection for Driving a Large Capacitive Load
Figure 38. Performance Comparison of Two Methods
for Driving a Capacitive Load
Figure 39. Recommended Value of Series Resistor vs.
the Amount of Capacitive Load
Figure 39 shows recommended resistor values for different load
capacitances. Refer again to
Figure 38 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.
Applications that 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/s. However, because of the 100 mA output
current of th
e AD811, a slew rate of 200 V/s is achievable
when driving the same 500 pF capacitor, as shown in
Figure 40.Figure 40. Output Waveform of
an AD811 Driving a 500 pF Load.
Gain = +2, RFB = 649 , RS = 15 , RS = 10 k
AD811
+
–
7
6
4
2
3
RS (OPTIONAL)
CL
RL
VOUT
–VS
+VS
RFB
RG
RT
VIN
0.1
F
0.1
F
00866-E-038
–6
–3
0
3
6
9
12
GAIN
(dB)
FREQUENCY (MHz)
1
10
100
00866-E-039
VS = ±15V
CL = 100pF
RL = 10k
GAIN = +2
RFB = 1.5k
RS = 0
RFB = 649
RS = 30
0
10
20
30
40
50
60
70
80
90
100
VALUE
OF
R
S
(
)
LOAD CAPACITANCE (pF)
10
100
1000
00866-E-040
GAIN = +2
VS = ±15V
RS VALUE SPECIFIED
IS FOR FLATTEST
FREQUENCY RESPONSE
00866-E-041
10
90
100
0%
5V
2V
100ns
VIN
VOUT