AD8531/AD8532/AD8534

REV. D

–10–

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

W. 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

the device to ground. This network operates in parallel with the

value of the resistor and capacitor is best determined empirically.

CAPACITIVE LOAD – nF

4

3.5

0

0.01

100

0.1

110

2

1.5

1

0.5

3

2.5

Figure 35. Unity-Gain Bandwidth vs. Capacitive Load

5V

5

100mV p-p

AD8532

47nF

1 F

Figure 36. Snubber Network Compensates for Capacitive

Loads

good starting value is 100

W. This value is reduced until the

mined—10

mF is a good starting point. This value is reduced to

the smallest value for acceptable performance (typically, 1

mF).

For the case of a 47 nF load capacitor on the AD8531/AD8532/

AD8534, the optimal snubber network is a 5

W in series with

1

mF. 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

W—1 mF snubber network in

place. The amount of overshoot and ringing is dramatically

reduced. Table I below illustrates a few sample snubber networks

for large load capacitors:

Table I. Snubber Networks for Large Capacitive Loads

Load Capacitance

Snubber Network

0.47 nF

300

W, 0.1 mF

4.7 nF

30

W, 1 mF

47 nF

5

W, 1 mF

47nF LOAD

ONLY

SNUBBER

IN CIRCUIT

100

90

10

0%

50mV

10 s

50mV

Figure 37. Overshoot and Ringing Is Reduced by Adding

a Snubber Network in Parallel with the 47 nF Load