7.3.5 Common-Mode Voltage Range

The OPAx990-Q1 is a 40-V, true rail-to-rail input operational amplifier with an input common-mode range that

extends 200 mV beyond either supply rail. This wide range is achieved with paralleled complementary N-channel

and P-channel differential input pairs, as shown in Figure 7-8. The N-channel pair is active for input voltages

close to the positive rail, typically (V+) – 1 V to 100 mV above the positive supply. The P-channel pair is active

for inputs from 100 mV below the negative supply to approximately (V+) – 2 V. There is a small transition region,

typically (V+) – 2 V to (V+) – 1 V in which both input pairs are on. This transition region can vary modestly with

process variation, and within this region PSRR, CMRR, offset voltage, offset drift, noise, and THD performance

may be degraded compared to operation outside this region.

Figure 6-5 shows this transition region for a typical device in terms of input voltage offset in more detail.

For more information on common-mode voltage range and PMOS/NMOS pair interaction, see Op Amps With

Complementary-Pair Input Stages application note.

NMOS

PMOS

PMOS

NMOS

V-

V+

IN+

IN-

Figure 7-8. Rail-to-Rail Input Stage

7.3.6 Phase Reversal Protection

The OPAx990-Q1 family has internal phase-reversal protection. Many op amps exhibit a phase reversal when

the input is driven beyond its linear common-mode range. This condition is most often encountered in non-

inverting circuits when the input is driven beyond the specified common-mode voltage range, causing the output

to reverse into the opposite rail. The OPAx990-Q1 is a rail-to-rail input op amp; therefore, the common-mode

range can extend up to the rails. Input signals beyond the rails do not cause phase reversal; instead, the output

limits into the appropriate rail. This performance is shown in Figure 7-9. For more information on phase reversal,

see Op Amps With Complementary-Pair Input Stages application note.

Time (20µs/Div)

C016

Input

Output

Figure 7-9. No Phase Reversal

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OPA4990-Q1

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