AD8531ART Analog Devices, AD8531ART Datasheet - Page 10

• Manufacturer Part Number: AD8531ART
• Description: Low Cost, 250 ma Output Single-supply Amplifiers
• Manufacturer: Analog Devices

AD8531/AD8532/AD8534
This input current is not inherently damaging to the device as
long as it is limited to 5 mA or less. For the AD8531/AD8532/
AD8534, once the input voltage exceeds the supply by more
than 0.6 V the input current quickly exceeds 5 mA. If this
condition continues to exist, an external series resistor should
be added. The size of the resistor is calculated by dividing the
maximum overvoltage by 5 mA. For example, if the input
voltage could reach 10 V, the external resistor should be (10 V/
5 mA) = 2 k . This resistance should be placed in series with
either or both inputs if they are exposed to an overvoltage con-
dition. For more information on general overvoltage character-
istics of amplifiers refer to the 1993 Seminar Applications Guide,
available from the Analog Devices Literature Center.
Output Phase Reversal
Some operational amplifiers designed for single-supply opera-
tion exhibit an output voltage phase reversal when their inputs
are driven beyond their useful common-mode range. The
AD8531/AD8532/AD8534 is free from reasonable input voltage
range restrictions provided that input voltages no greater than
the supply voltage rails are applied. Although the device’s out-
put will not change phase, large currents can flow through
internal junctions to the supply rails, as was pointed out in the
previous section. Without limit, these fault currents can easily
destroy the amplifier. The technique 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 exhibits excellent capacitive
load driving capabilities. It can drive up to 10 nF directly as
shown in Figures 21 through 24. However, even though the
device is stable, a capacitive load does not come without a
penalty in bandwidth. As shown in Figure 35, the bandwidth is
reduced to under 1 MHz for loads greater than 10 nF. A “snub-
ber” network on the output won’t increase the bandwidth, but
it does significantly reduce the amount of overshoot for a given
capacitive load. A snubber consists of a series R-C network
(R
the device to ground. This network operates in parallel with the
load capacitor, C
value of the resistor and capacitor is best determined empirically.
S
Figure 35. Unity-Gain Bandwidth vs. Capacitive Load
, C
S
), as shown in Figure 36, connected from the output of
3.5
2.5
1.5
0.5
4
3
2
1
0
0.01
L
, to provide phase lag compensation. The actual
0.1
CAPACITIVE LOAD – nF
1
10
V
R
T
A
S
L
=
= 1k
= +25 C
2.5V
100
–10–
Figure 36. Snubber Network Compensates for Capacitive
Loads
The first step is to determine the value of the resistor, R
good starting value is 100 . This value is reduced until the
small-signal transient response is optimized. Next, C
mined—10 F is a good starting point. This value is reduced to
the smallest value for acceptable performance (typically, 1 F).
For the case of a 47 nF load capacitor on the AD8531/AD8532/
AD8534, the optimal snubber network is a 5
1 F. The benefit is immediately apparent as seen in the scope
photo in Figure 37. The top trace was taken with a 47 nF load
and the bottom trace with the 5 —1 F snubber network in
place. The amount of overshoot and ringing is dramatically re-
duced. Table I below illustrates a few sample snubber networks
for large load capacitors:
Figure 37. Overshoot and Ringing Is Reduced by Adding
a Snubber Network in Parallel with the 47 nF Load
Table I. Snubber Networks for Large Capacitive Loads
Load Capacitance
(C
0.47 nF
4.7 nF
47 nF
L
47nF LOAD
100mV p-p
)
IN CIRCUIT
SNUBBER
ONLY
V
IN
100
0%
90
10
AD8532
50mV
50mV
+5V
R
5
C
1 F
Snubber Network
(R
300 , 0.1 F
30 , 1 F
5 , 1 F
S
S
S
, C
S
)
C
47nF
L
in series with
10 s
V
OUT
S
is deter-
S
. A
REV. B