Datasheet

Data Sheet AD8551/AD8552/AD8554
Rev. F | Page 19 of 24
CAPACITIVE LOAD DRIVE
The AD8551/AD8552/AD8554 have excellent capacitive load
driving capabilities and can safely drive up to 10 nF from a
single 5 V supply. Although the device is stable, capacitive
loading limits the bandwidth of the amplifier. Capacitive loads
also increase the amount of overshoot and ringing at the output.
An R-C snubber network, shown in Figure 61, can be used to
compensate the amplifier against capacitive load ringing and
overshoot.
5V
V
IN
200mV p-p
R
X
60Ω
C
X
0.47µF
C
L
4.7nF
V
OUT
AD8551/
AD8552/
AD8554
01101-061
Figure 61. Snubber Network Configuration for Driving Capacitive Loads
Although the snubber does not recover the loss of amplifier
bandwidth from the load capacitance, it does allow the amplifier to
drive larger values of capacitance while maintaining a minimum of
overshoot and ringing. Figure 62 shows the output of an AD8551/
AD8552/AD8554 driving a 1 nF capacitor with and without a
snubber network.
WITH
SNUBBER
WITHOUT
SNUBBER
10µs
100mV
V
SY
= 5V
C
LOAD
= 4.7nF
01101-062
Figure 62. Overshoot and Ringing are Substantially Reduced
Using a Snubber Network
The optimum value for the resistor and capacitor is a function
of the load capacitance and is best determined empirically because
actual C
LOAD
(C
L
) includes stray capacitances and may differ
substantially from the nominal capacitive load. Table 5 shows
some snubber network values that can be used as starting points.
Table 5. Snubber Network Values for Driving Capacitive Loads
C
LOAD
R
X
C
X
1 nF 200 Ω 1 nF
4.7 nF 60 Ω 0.47 μF
10 nF 20 Ω 10 μF
POWER-UP BEHAVIOR
At power-up, the AD8551/AD8552/AD8554 settle to a valid
output within 5 μs. Figure 63 shows an oscilloscope photo of the
output of the amplifier with the power supply voltage, and
Figure 64 shows the test circuit. With the amplifier configured
for unity gain, the device takes approximately 5 μs to settle to its
final output voltage. This turn-on response time is much faster
than most other autocorrection amplifiers, which can take
hundreds of microseconds or longer for their output to settle.
V+
0V
0V
V
OUT
5µs
1V
01101-063
BOTTOM TRACE = 2V/DIV
TOP TRACE = 1V/DIV
Figure 63. AD8551/AD8552/AD8554 Output Behavior on Power-Up
V
OUT
AD8551/
AD8552/
AD8554
V
SY
= 0V TO 5V
100kΩ
100kΩ
01101-064
Figure 64. AD8551/AD8552/AD8554 Test Circuit for Turn-On Time