AD8571ARMZ datasheet(19/24 Pages) AD

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AD8571/AD8572/AD8574

Rev. E | Page 19 of 24

CAPACITIVE LOAD DRIVE

The AD857x 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. The RC snubber

network shown in Figure 59 can be used to reduce the capacitive

load ringing and overshoot.

VOUT

VIN

200mV p-p

AD8571/

AD8572/

AD8574

4.7nF

0.47µF

60Ω

–

Figure 59. Snubber Network Configuration for Driving Capacitive Loads

Although the snubber network 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

VS = 5V

CL = 4.7nF

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

because actual CL includes stray capacitances and can 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

CL (nF)

Rx (Ω)

200

1 nF

4.7

0.47 μF

10 μF

POWER-UP BEHAVIOR

At 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. 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

VOUT

V+

BOTTOM TRACE = 2V/DIV

TOP TRACE = 1V/DIV

Figure 61. AD857x Output Behavior at Power-Up

100kΩ

VSY = 0V TO 5V

VOUT

AD8571/

AD8572/

AD8574

Figure 62. AD857x Test Circuit for Power-Up Time

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