AD8551/AD8552/AD8554

Rev. C | Page 19 of 24

OUTPUT PHASE REVERSAL

Output phase reversal occurs in some amplifiers when the input

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

voltage moves outside of the common-mode range, the outputs

of these amplifiers suddenly jump in the opposite direction to

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

shutting down and causing a radical shifting of internal

voltages, resulting in the erratic output behavior.

The AD855x amplifiers have been carefully designed to prevent

any output phase reversal, provided both inputs are maintained

within the supply voltages. If there is the potential of one or

both inputs exceeding either supply voltage, place a resistor in

series with the input to limit the current to less than 2 mA to

ensure the output does not reverse its phase.

CAPACITIVE LOAD DRIVE

The AD855x family 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 R-C

snubber network, shown in Figure 61, can be used to

compensate the amplifier against capacitive load ringing and

overshoot.

5V

200mV p-p

60Ω

0.47µF

4.7nF

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 AD855x driving a 1 nF capacitor with and without

a snubber network.

WITH

SNUBBER

WITHOUT

SNUBBER

10µs

100mV

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

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

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 AD855x settles 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+

0V

0V

5µs

1V

BOTTOM TRACE = 2V/DIV

TOP TRACE = 1V/DIV

Figure 63. AD855x Output Behavior on Power-Up

AD8551/

AD8552/

AD8554

100kΩ

100kΩ

Figure 64. AD855x Test Circuit for Turn-On Time