AD8531/AD8532/AD8534
Rev. F | Page 13 of 20
5V
RS
5
CS
1F
VOUT
VIN
100mV p-p
AD8532
01
09
9-
0
40
CL
47nF
OUTPUT PHASE REVERSAL
Some operational amplifiers designed for single-supply operation
exhibit an output voltage phase reversal when their inputs are
driven beyond their useful common-mode range. The AD8531/
AD8532/AD8534 are free from reasonable input voltage range
restrictions, provided that input voltages no greater than the
supply voltage rails are applied. Although the output of the
device does not change phase, large currents can flow through
internal junctions to the supply rails, which was described in the
currents can easily destroy the amplifier. The technique
should therefore be applied in those applications where the
possibility of input voltages exceeding the supply voltages exists.
Figure 40. Snubber Network Compensates for Capacitive Loads
The first step is to determine the value of the resistor, RS. A good
starting value is 100 Ω. This value is reduced until the small signal
transient response is optimized. Next, CS is determined; 10 μF is a
good starting point. This value is reduced to the smallest value
for acceptable performance (typically, 1 μF). For the case of a
47 nF load capacitor on the AD8531/AD8532/AD8534, the
optimal snubber network is 5 Ω in series with 1 μF. The benefit
is immediately apparent, as seen in
Figure 41. The top trace was
taken with a 47 nF load, and the bottom trace was taken with
the 5 Ω in series with a 1 μF snubber network in place. The
amount of overshoot and ringing is dramatically reduced.
Table 5illustrates a few sample snubber networks for large load
capacitors.
CAPACITIVE LOAD DRIVE
The AD8531/AD8532/AD8534 exhibit excellent capacitive load
driving capabilities. They can drive up to 10 nF directly, as
the device is stable, a capacitive load does not come without a
penalty in bandwidth. As shown in
Figure 39, the bandwidth is
reduced to less than 1 MHz for loads greater than 10 nF. A snubber
network on the output does not increase the bandwidth, but it
does significantly reduce the amount of overshoot for a given
capacitive load. A snubber consists of a series RC network (RS,
CS), as shown in Figure 40, connected from the output of the device to ground. This network operates in parallel with the
load capacitor, CL, to provide phase lag compensation. The
actual value of the resistor and capacitor is best determined
empirically.
Table 5. Snubber Networks for Large Capacitive Loads
Load Capacitance (CL)
Snubber Network (RS, CS)
0.47 nF
300 Ω, 0.1 μF
4.7 nF
30 Ω, 1 μF
47 nF
5 Ω, 1 μF
0
10
99-
041
10
0%
100
47nF LOAD
ONLY
SNUBBER
IN CIRCUIT
90
10s
50mV
CAPACITIVE LOAD (nF)
BANDW
IDI
T
H
(
M
Hz
)
3.5
4.0
3.0
2.5
2.0
1.5
1.0
0.5
0
0.01
0.1
1
10
100
01
099
-03
9
VS = ±2.5V
RL = 1k
TA = 25°C
Figure 41. Overshoot and Ringing Are Reduced by Adding a Snubber
Network in Parallel with the 47 nF Load
Figure 39. Unity-Gain Bandwidth vs. Capacitive Load