Application Notes

GROUND SENSING AND RAIL TO RAIL

The input common-mode voltage range of the AD854xA series extends 200mV beyond

the supply rails. This is achieved with a complementary input stage—an N-channel input

differential pair in parallel with a P-channel differential pair. For normal operation, inputs

should be limited to this range. The absolute maximum input voltage is 400mV beyond the

supplies. Inputs greater than the input common-mode range but less than the maximum input

voltage, while not valid, will not cause any damage to the op-amp. Unlike some other op-

amps, if input current is limited, the inputs may go beyond the supplies without phase

inversion, as shown in Figure 2. Since the input common-mode range extends from (VS− −

0.2V) to (VS+ + 0.2V), the AD854xA op-amps can easily perform ‘true ground’ sensing.

Figure 2 No Phase Inversion with Inputs Greater Than the Power-Supply Voltage

A topology of class AB output stage with common-source transistors is used to achieve

rail-to-rail output. For light resistive loads (e.g.100kΩ), the output voltage can typically swing

to within 5mV from the supply rails. With moderate resistive loads (e.g.10kΩ), the output can

typically swing to within 10mV from the supply rails and maintain high open-loop gain.

The maximum output current is a function of total supply voltage. As the supply voltage

to the amplifier increases, the output current capability also increases. Attention must be paid

to keep the junction temperature of the IC below 150℃ when the output is in continuous

short-circuit. The output of the amplifier has reverse-biased ESD diodes connected to each

supply. The output should not be forced more than 0.5V beyond either supply, otherwise

current will flow through these diodes.

CAPACITIVE LOAD AND STABILITY

The AD854xA can directly drive 1nF in unity-gain without oscillation. The unity-gain

follower is the most sensitive configuration to capacitive loading. Direct capacitive loading

reduces the phase margin of amplifiers and this results in ringing or capacitive loading

reduces the phase margin of amplifiers and this results in ringing or even oscillation.

Applications that require greater capacitive drive capability should use an isolation resistor

1.1MHz, 85µA, CMOS, RRIO Operational Amplifiers

UMW

AD8541,AD8542,AD8544

R

www.umw-ic.com

9

UTD Semiconductor Co.,Limited