AD8531/AD8532/AD8534
5V
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
Input Overvoltage Protection section. Without limit, these fault
currents can easily destroy the amplifier. The technique
V
AD8532
OUT
R
5Ω
S
V
IN
100mV p-p
C
1µF
C
L
S
47nF
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 5
illustrates a few sample snubber networks for large load
capacitors.
recommended in the Input Overvoltage Protection section
should therefore be applied in those applications where the
possibility of input voltages exceeding the supply voltages exists.
CAPACITIVE LOAD DRIVE
The AD8531/AD8532/AD8534 exhibit excellent capacitive load
driving capabilities. They can drive up to 10 nF directly, as
shown in Figure 25 through Figure 28. However, even though
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
4.7 nF
47 nF
300 Ω, 0.1 μF
30 Ω, 1 μF
5 Ω, 1 μF
50mV
100
90
47nF LOAD
ONLY
4.0
V
R
= ±2.5V
= 1kΩ
S
L
3.5
3.0
2.5
2.0
1.5
T
= 25°C
A
SNUBBER
IN CIRCUIT
10
0%
50mV
10µs
1.0
0.5
0
Figure 41. Overshoot and Ringing Are Reduced by Adding a Snubber
Network in Parallel with the 47 nF Load
0.01
0.1
1
10
100
CAPACITIVE LOAD (nF)
Figure 39. Unity-Gain Bandwidth vs. Capacitive Load
Rev. F | Page 13 of 20
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