Texas Instruments | OPA322, OPA2322, OPA4322 EMIR Immunity Performance | Application notes | Texas Instruments OPA322, OPA2322, OPA4322 EMIR Immunity Performance Application notes

Texas Instruments OPA322, OPA2322, OPA4322 EMIR Immunity Performance Application notes
Technical Brief
SBOT005 – November 2012
OPA2322 EMI Immunity Performance
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 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 report 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
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 printed circuit board (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.
A more formal discussion of the EMIRR IN+ definition and test method is provided in application
report SBOA128, EMI Rejection Ratio of Operational Amplifiers, available for download at
The EMIRR IN+ of the OPA2322 is plotted versus frequency as shown in Figure 1. If available, any
dual and quad op amp device versions have nearly similar EMIRR IN+ performance. The OPA2322
unity-gain bandwidth is 100 MHz. EMIRR performance below this frequency denotes interfering
signals that fall within the op amp bandwidth.
PRF = -10 dBm
VSUPPLY = ±2.5 V
VCM = 0 V
Frequency (MHz)
Figure 1. OPA2322 EMIRR IN+ vs Frequency
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SBOT005 – November 2012
OPA2322 EMI Immunity Performance
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OPA2322 EMI Immunity Performance
Table 1 shows the EMIRR IN+ values for the OPA2322 at particular frequencies commonly
encountered in real-world applications. Applications listed in Table 1 may be centered on or operated
near the particular frequency shown. This information may be of special interest to designers working
with these types of applications, or working in other fields likely to encounter RF interference from
broad sources, such as the industrial, scientific, and medical (ISM) radio band.
Table 1. OPA2322 EMIRR IN+ for Frequencies of Interest
400 MHz
Mobile radio, mobile satellite/space operation, weather, radar, UHF
29.8 dB
900 MHz
GSM, radio com/nav./GPS (to 1.6 GHz), ISM, aeronautical mobile, UHF
39.4 dB
1.8 GHz
GSM, mobile personal comm. broadband, satellite, L-band
50.1 dB
2.4 GHz
802.11b/g/n, Bluetooth™, mobile personal comm., ISM, amateur radio/satellite, S-band
51.9 dB
3.6 GHz
Radiolocation, aero comm./nav., satellite, mobile, S-band
59.6 dB
5.0 GHz
802.11a/n, aero comm./nav., mobile comm., space/satellite operation, C-band
69.5 dB
EMIRR IN+ Test Configuration
Figure 2 shows the circuit configuration for testing the EMIRR IN+. An RF source is connected to the
op amp noninverting input terminal using a transmission line. The op amp is configured in a unity gain
buffer topology with the output connected to a low-pass filter (LPF) and a digital multimeter (DMM).
Note that a large impedance mismatch at the op amp input causes a voltage reflection; however, this
effect is characterized and accounted for when determining the EMIRR IN+. The resulting dc offset
voltage is sampled and measured by the multimeter. The LPF isolates the multimeter from residual
RF signals that may interfere with multimeter accuracy. Refer to SBOA128 for more details.
50 Ω
RF Source
DC Bias: 0 V
Modulation: None (CW)
Freq Sweep: 201pt Log
Ambient Temperature: +25ºC
Averaging Multimeter
Not shown:
0.1-µF and 10-µF supply decoupling
Figure 2. EMIRR IN+ Test Configuration Schematic
Chris Hall and Thomas Kuehl, “EMI Rejection Ratio of Operational Amplifiers,” application report
SBOA128, Texas Instruments, August 2011.
Gerrit de Wagt and Arie van Staveren, “A Specification for EMI Hardened Operational Amplifiers,”
application report SNOA497A, Texas Instruments, January 2010.
OPA2322 EMI Immunity Performance
SBOT005 – November 2012
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