Application Notes

Ground Sensing And Rail To Rail

The input common-mode voltage range of the AD863xA series extends 300mV

beyond the supply rails. This is achieved with a complementary input stage—a

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 500mV 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− − 0.1V) to (VS+ + 0.1V), the

AD863x 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. See the Typical Characteristic curve, Output Voltage

Swing as a function of Output Current, for more information.

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

480

μA, 6.5MHz, RRIO CMOS Operational Amplifiers

UMW

AD8631,AD8632,AD8634

R

www.umw-ic.com

8

UTD Semiconductor Co.,Limited