AD8531/AD8532/AD8534

Rev. F | Page 13 of 20

5V

5Ω

1µF

100mV p-p

AD8532

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

Input Overvoltage Protection 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.

Figure 40. Snubber Network Compensates for Capacitive Loads

starting value is 100 Ω. This value is reduced until the small signal

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.

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

device to ground. This network operates in parallel with the

actual value of the resistor and capacitor is best determined

empirically.

Table 5. Snubber Networks for Large Capacitive Loads

0.47 nF

300 Ω, 0.1 μF

4.7 nF

30 Ω, 1 μF

47 nF

5 Ω, 1 μF

10

0%

100

47nF LOAD

ONLY

SNUBBER

IN CIRCUIT

90

10µs

50mV

50mV

CAPACITIVE LOAD (nF)

3.5

4.0

3.0

2.5

2.0

1.5

1.0

0.5

0

0.01

0.1

1

10

100

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