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Data Sheet

AD8551/AD8552/AD8554

Rev. F | Page 19 of 24

CAPACITIVE LOAD DRIVE

The AD8551/AD8552/AD8554 have excellent capacitive load

driving capabilities and can safely drive up to 10 nF from a

single 5 V supply. Although the device is stable, capacitive

loading limits the bandwidth of the amplifier. Capacitive loads

also increase the amount of overshoot and ringing at the output.

An R-C snubber network, shown in Figure 61, can be used to

compensate the amplifier against capacitive load ringing and

overshoot.

VIN

200mV p-p

60Ω

0.47µF

4.7nF

VOUT

AD8551/

AD8552/

AD8554

Figure 61. Snubber Network Configuration for Driving Capacitive Loads

Although the snubber does not recover the loss of amplifier

bandwidth from the load capacitance, it does allow the amplifier to

drive larger values of capacitance while maintaining a minimum of

overshoot and ringing. Figure 62 shows the output of an AD8551/

AD8552/AD8554 driving a 1 nF capacitor with and without a

snubber network.

WITH

SNUBBER

WITHOUT

SNUBBER

10µs

100mV

VSY = 5V

CLOAD = 4.7nF

Figure 62. Overshoot and Ringing are Substantially Reduced

Using a Snubber Network

The optimum value for the resistor and capacitor is a function

of the load capacitance and is best determined empirically because

actual CLOAD (CL) includes stray capacitances and may differ

substantially from the nominal capacitive load. Table 5 shows

some snubber network values that can be used as starting points.

Table 5. Snubber Network Values for Driving Capacitive Loads

CLOAD

1 nF

200 Ω

1 nF

4.7 nF

60 Ω

0.47 μF

10 nF

20 Ω

10 μF

POWER-UP BEHAVIOR

At power-up, the AD8551/AD8552/AD8554 settle to a valid

output within 5 μs. Figure 63 shows an oscilloscope photo of the

output of the amplifier with the power supply voltage, and

Figure 64 shows the test circuit. With the amplifier configured

for unity gain, the device takes approximately 5 μs to settle to its

final output voltage. This turn-on response time is much faster

than most other autocorrection amplifiers, which can take

hundreds of microseconds or longer for their output to settle.

V+

VOUT

5µs

BOTTOM TRACE = 2V/DIV

TOP TRACE = 1V/DIV

Figure 63. AD8551/AD8552/AD8554 Output Behavior on Power-Up

VOUT

AD8551/

AD8552/

AD8554

VSY = 0V TO 5V

100kΩ

Figure 64. AD8551/AD8552/AD8554 Test Circuit for Turn-On Time

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