OPA145 Datasheet, PDF (24/40 Pages) Texas Instruments – High-Precision, Low-Noise, Rail-to-Rail Output, 5.5-MHz JFET Operational Amplifiers

OPA145, OPA2145, OPA4145
SBOS427 – JUNE 2017
www.ti.com
Feature Description (continued)
8.3.8 EMI Rejection
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 easier 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 result in adverse effects,
as the amplifier would not have sufficient loop gain to correct for signals with spectral content outside its
bandwidth. Conducted or radiated EMI on inputs, power supply, or output may result in unexpected DC
offsets, transient voltages, or other unknown behavior. Be sure to properly shield and isolate sensitive analog
nodes from noisy radio signals and digital clocks and interfaces. Figure 43 shows the effect of conducted
EMI to the power supplies on the input offset voltage of OPA145.
The EMIRR IN+ of the OPA145 is plotted versus frequency as shown in Figure 42. If available, any dual and
quad op amp device versions have nearly similar EMIRR IN+ performance. The OPA145 unity-gain
bandwidth is 5.5 MHz. EMIRR performance below this frequency denotes interfering signals that fall within
the op amp bandwidth.
See EMI Rejection Ratio of Operational Amplifiers, available for download from www.ti.com.
140
120
100
80
60
40
20
0
10M
100M
1000M
Frequency (Hz)
C004
Figure 42. OPA145 EMIRR IN+
50
V± Supply
0
±50
±100
V+ Supply
±150
±200
100
1k
10k
100k
1M
Frequency (Hz)
C006
Figure 43. OPA145 EMI-Induced Input Offset
Voltage (Power Supplies)
24
Submit Documentation Feedback
Copyright © 2017, Texas Instruments Incorporated
Product Folder Links: OPA145 OPA2145 OPA4145