Datasheet

AD8571/AD8572/AD8574
Rev. E | Page 19 of 24
CAPACITIVE LOAD DRIVE
The AD857x 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 AD857x driving a 1 nF capacitor with
and without a snubber network.
10μs
100mV
V
S
= 5V
C
L
= 4.7nF
WITHOUT
S
NUBBER
WITH
S
NUBBER
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 AD857x settles to a valid output within 5 s.
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. AD857x Output Behavior at Power-Up
100kΩ
100kΩ
V
SY
= 0V TO 5
V
V
OUT
0
1104-062
AD8571/
AD8572/
AD8574
Figure 62. AD857x Test Circuit for Power-Up Time