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If the supply is not capable of sinking the current, VIN can begin sourcing current to the operational amplifier and

then take over as the source of positive supply voltage. The danger in this case is that the voltage can rise to

levels that exceed the operational amplifier absolute maximum ratings.

Another common question involves what happens to the amplifier if an input signal is applied to the input when

the power supplies (V+ or V–) are at 0 V. Again, this question depends on the supply characteristic when at 0

V, or at a level less than the input signal amplitude. If the supplies appear as high impedance, then the input

source supplies the operational amplifier current through the current-steering diodes. This state is not a normal

bias condition; most likely, the amplifier does not operate normally. If the supplies are low impedance, then the

current through the steering diodes can become quite high. The current level depends on the ability of the input

source to deliver current, and any resistance in the input path.

If there is any uncertainty about the ability of the supply to absorb this current, add external Zener diodes to the

supply pins; see Figure 7-2. Select the Zener voltage so that the diode does not turn on during normal operation.

However, the Zener voltage must be low enough so that the Zener diode conducts if the supply pin begins to rise

above the safe-operating, supply-voltage level.

7.3.3 EMI Rejection Ratio (EMIRR)

The electromagnetic interference (EMI) rejection ratio, or EMIRR, describes the EMI immunity of operational

amplifiers. An adverse effect that is common to many operational amplifiers is a change in the offset voltage

as a result of RF signal rectification. An operational amplifier that is more efficient at rejecting this change in

offset as a result of EMI has a higher EMIRR and is quantified by a decibel value. Measuring EMIRR can be

performed in many ways, but this document provides the EMIRR IN+, which specifically describes the EMIRR

performance when the RF signal is applied to the noninverting input pin of the operational amplifier. In general,

only the noninverting input is tested for EMIRR for the following three reasons:

• Operational amplifier input pins are known to be the most sensitive to EMI, and typically rectify RF signals

better than the supply or output pins.

• The noninverting and inverting operational amplifier inputs have symmetrical physical layouts and exhibit

nearly matching EMIRR performance.

• EMIRR is easier to measure on noninverting pins than on other pins because the noninverting input pin can

be isolated on a printed-circuit-board (PCB). This isolation allows the RF signal to be applied directly to the

noninverting input pin with no complex interactions from other components or connecting PCB traces.

A more formal discussion of the EMIRR IN+ definition and test method is shown in the EMI Rejection Ratio of

Operational Amplifiers application report, available for download at www.ti.com.

The EMIRR IN+ of the OPA167x is plotted versus frequency in Figure 7-3. The dual and quad operational

amplifier device versions have approximately identical EMIRR IN+ performance. The OPA167x unity-gain

bandwidth is 16 MHz. EMIRR performance below this frequency denotes interfering signals that fall within the

operational amplifier bandwidth.

100

1000

10000

Frequency (MHz)

C001

Figure 7-3. OPA167x EMIRR vs Frequency

OPA1677, OPA1678, OPA1679

SBOS855D – JANUARY 2017 – REVISED DECEMBER 2021

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