OPA1678 Datasheet, PDF (16/41 Pages) Texas Instruments – Low-Distortion Audio Operational Amplifiers

OPA1678, OPA1679
SBOS855 – FEBRUARY 2017
www.ti.com
Feature Description (continued)
When the operational amplifier connects into a circuit (see Figure 32), the ESD protection components are
intended to remain inactive and do not become involved in the application circuit operation. However,
circumstances may arise where an applied voltage exceeds the operating voltage range of a given pin. If this
condition occurs, there is a risk that some internal ESD protection circuits can turn on and conduct current. Any
such current flow occurs through steering-diode paths and rarely involves the absorption device.
Figure 32 shows a specific example where the input voltage (V_IN) exceeds the positive supply voltage (V+) by
500 mV or more. Much of what happens in the circuit depends on the supply characteristics. If V+ can sink the
current, one of the upper input steering diodes conducts and directs current to V+. Excessively high current
levels can flow with increasingly higher V_IN. As a result, the data sheet specifications recommend that
applications limit the input current to 10 mA.
If the supply is not capable of sinking the current, V_IN 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 below 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 32. 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 provided 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 33. If available, any dual and quad
operational amplifier device versions have nearly 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.
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