Datasheet

AD8571/AD8572/AD8574 Data Sheet
Rev. F | Page 20 of 28
CAPACITIVE LOAD DRIVE
The AD8571/AD8572/AD8574 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.
The RC snubber network shown in Figure 59 can be used to
reduce the capacitive load ringing and overshoot.
5V
V
OUT
V
IN
200mV p-p
AD8571/
AD8572/
AD8574
C
L
4.7nF
Cx
0.47µF
Rx
60Ω
+
–
01104-059
Figure 59. Snubber Network Configuration for Driving Capacitive Loads
Although the snubber network 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 60
shows the output of an AD8571/AD8572/AD8574 driving a
1 nF capacitor with and without a snubber network.
10μs
100mV
V
S
= 5V
C
L
= 4.7nF
WITHOUT
SNUBBER
WITH
SNUBBER
01104-060
Figure 60. 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
L
includes stray capacitances and can 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
L
(nF) Rx (Ω) Cx
1 200 1 nF
4.7 60 0.47 µF
10 20 10 µF
POWER-UP BEHAVIOR
At power-up, the AD8571/AD8572/AD8574 settle to a valid
output within 5 μs. Figure 61 shows an oscilloscope photo of the
output of the amplifier along with the power supply voltage.
Figure 62 shows the test circuit. With the amplifier configured
for unity gain, the device takes approximately 5 µs to settle to its
final output voltage, hundreds of microseconds faster than
many other autocorrection amplifiers.
5µs
1V
V
OUT
V+
0V
0V
BOTTOM TRACE = 2V/DIV
TOP TRACE = 1V/DIV
01104-061
Figure 61. AD8571/AD8572/AD8574 Output Behavior at Power-Up
100kΩ
100kΩ
V
SY
= 0V TO 5V
V
OUT
01104-062
AD8571/
AD8572/
AD8574
Figure 62. AD8571/AD8572/AD8574 Test Circuit for Power-Up Time