OPA189 Datasheet, PDF (14/34 Pages) Texas Instruments – Precision, 36-V, 14-MHz, MUX-Friendly Low-Noise, Rail-to-Rail Output, Zero-Drift Operational Amplifiers

OPA189, OPA2189, OPA4189
SBOS830 – JUNE 2017
www.ti.com
Feature Description (continued)
8.3.3 Input Bias Current Clock Feedthrough
Zero-drift amplifiers such as the OPAx189 use switching on the inputs to correct for the intrinsic offset and drift of
the amplifier. Charge injection from the integrated switches on the inputs can introduce short transients in the
input bias current of the amplifier. The extremely short duration of these pulses prevents the pulses from
amplifying, however the pulses may be coupled to the output of the amplifier through the feedback network. The
most effective method to prevent transients in the input bias current from producing additional noise at the
amplifier output is to use a low-pass filter such as an RC network.
8.3.4 EMI Rejection
The OPAx189 uses integrated electromagnetic interference (EMI) filtering to reduce the effects of EMI
interference from sources such as wireless communications and densely-populated boards with a mix of analog
signal chain and digital components. EMI immunity can be improved with circuit design techniques; the OPAx189
benefits from these design improvements. Texas Instruments has developed the ability to accurately measure
and quantify the immunity of an operational amplifier over a broad frequency spectrum extending from 10 MHz to
6 GHz. Figure 5 shows the results of this testing on the OPAx189. Table 2 lists the EMIRR IN+ values for the
OPAx189 at particular frequencies commonly encountered in real-world applications. Applications listed in
Table 2 may be centered on or operated near the particular frequency shown. Detailed information can also be
found in EMI Rejection Ratio of Operational Amplifiers (SBOA128), available for download from www.ti.com.
The electromagnetic interference (EMI) rejection ratio, or EMIRR, describes the EMI immunity of operational
amplifiers. An adverse effect that is common to many op amps is a change in the offset voltage as a result of RF
signal rectification. An op amp 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
section provides the EMIRR IN+, which specifically describes the EMIRR performance when the RF signal is
applied to the noninverting input pin of the op amp. In general, only the noninverting input is tested for EMIRR for
the following three reasons:
• Op amp 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 op amp inputs have symmetrical physical layouts and exhibit nearly matching
EMIRR performance
• EMIRR is more simple to measure on noninverting pins than on other pins because the noninverting input
terminal can be isolated on a PCB. This isolation allows the RF signal to be applied directly to the
noninverting input terminal with no complex interactions from other components or connecting PCB traces.
High-frequency signals conducted or radiated to any pin of the operational amplifier may result in adverse
effects, as the amplifier would not have sufficient loop gain to correct for signals with spectral content outside the
bandwidth. Conducted or radiated EMI on inputs, power supply, or output may result in unexpected DC offsets,
transient voltages, or other unknown behavior. Take care to properly shield and isolate sensitive analog nodes
from noisy radio signals and digital clocks and interfaces.
The EMIRR IN+ of the OPAx189 is plotted versus frequency as shown in Figure 5. If available, any dual and
quad op amp device versions have nearly similar EMIRR IN+ performance. The OPAx189 unity-gain bandwidth
is 14 MHz. EMIRR performance below this frequency denotes interfering signals that fall within the op amp
bandwidth.
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