AD8571/AD8572/AD8574

Rev. B | Page 19 of 24

CAPACITIVE LOAD DRIVE

The AD857x has 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 RC snubber

network, shown in Figure 59, can be used to compensate the

amplifier against capacitive load ringing and overshoot.

5V

200mV p-p

AD8571/

AD8572/

AD8574

4.7nF

Cx

0.47µF

Rx

60Ω

+

–

Figure 59. 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 60 shows the output

of an AD857x driving a 1 nF capacitor with and without a

snubber network.

10μs

100mV

WITHOUT

SNUBBER

WITH

SNUBBER

Figure 60. 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 since

tially 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

Rx

Cx

1 nF

200 Ω

1 nF

4.7 nF

60 Ω

0.47 μF

10 nF

20 Ω

10 μF

POWER-UP BEHAVIOR

On power-up, the AD857x settles to a valid output within

5 μs. Figure 61 shows an oscilloscope photo of the output of the

amplifier along with the power supply voltage, and Figure 62

shows the test circuit. With the amplifier configured for unity

gain, the device takes approximately 5 μs to settle to its final

output voltage, hundreds of microseconds faster than many

other autocorrection amplifiers.

5µs

1V

V+

0V

0V

BOTTOM TRACE = 2V/DIV

TOP TRACE = 1V/DIV

Figure 61. AD857x Output Behavior on Power-Up

100kΩ

100kΩ

AD8571/

AD8572/

AD8574

Figure 62. AD857x Test Circuit for Turn-On Time