AD8571/AD8572/AD8574

–14–

REV. 0

Input Overvoltage Protection

Although the AD857x is a rail-to-rail input amplifier, care should

be taken to ensure that the potential difference between the inputs

does not exceed 5 V. Under normal operating conditions, the

amplifier will correct its output to ensure the two inputs are at

the same voltage. However, if the device is configured as a com-

parator, or is under some unusual operating condition, the input

voltages may be forced to different potentials. This could cause

excessive current to flow through internal diodes in the AD857x

used to protect the input stage against overvoltage.

If either input exceeds either supply rail by more than 0.3 V, large

amounts of current will begin to flow through the ESD protection

diodes in the amplifier. These diodes are connected between the

inputs and each supply rail to protect the input transistors against

an electrostatic discharge event and are normally reverse-biased.

However, if the input voltage exceeds the supply voltage, these

ESD diodes will become forward-biased. Without current-limiting,

excessive amounts of current could flow through these diodes

causing permanent damage to the device. If inputs are subject to

overvoltage, appropriate series resistors should be inserted to limit

the diode current to less than 2 mA maximum.

Output Phase Reversal

Output phase reversal occurs in some amplifiers when the input

common-mode voltage range is exceeded. As common-mode

voltage is moved outside of the common-mode range, the outputs

of these amplifiers will suddenly jump in the opposite direction to

the supply rail. This is the result of the differential input pair shut-

ting down, causing a radical shifting of internal voltages which

results in the erratic output behavior.

The AD857x amplifier has been carefully designed to prevent

any output phase reversal, provided both inputs are maintained

within the supply voltages. If one or both inputs could exceed

either supply voltage, a resistor should be placed in series with

the input to limit the current to less than 2 mA. This will ensure

the output will not reverse its phase.

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 will limit the

bandwidth of the amplifier. Capacitive loads will also increase

the amount of overshoot and ringing at the output. An R-C

snubber network, Figure 52, can be used to compensate the

amplifier against capacitive load ringing and overshoot.

5V

60

200mV p-p

AD857x

4.7nF

0.47 F

Figure 52. Snubber Network Configuration for Driving

Capacitive Loads

Although the snubber will not recover the loss of amplifier band-

width from the load capacitance, it will allow the amplifier to drive

larger values of capacitance while maintaining a minimum of over-

shoot and ringing. Figure 53 shows the output of an AD857x

driving a 1 nF capacitor with and without a snubber network.

10 s

100mV

WITH

SNUBBER

WITHOUT

SNUBBER

Figure 53. 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 actual

from the nominal capacitive load. Table I shows some snubber

network values that can be used as starting points.

Table I. Snubber Network Values for Driving Capacitive Loads

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 will settle to a valid output within 5

µs.

Figure 54a shows an oscilloscope photo of the output of the ampli-

fier along with the power supply voltage, and Figure 54b 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.

5 s

1V

V+

0V

0V

BOTTOM TRACE = 2V/DIV

TOP TRACE = 1V/DIV

Figure 54a. AD857x Output Behavior on Power-Up