SBOS925A December 2020 – January 2021 OPA391
The OPA391 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 OPA391 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 7-2 shows the results of this testing on the OPA391. Table 7-1 lists the EMIRR +IN values for the OPA391 at particular frequencies commonly encountered in real-world applications. Applications listed in Table 7-1 may be centered on or operated near the particular frequency shown. Detailed information can also be found in the EMI Rejection Ratio of Operational Amplifiers application report, 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:
High-frequency signals conducted or radiated to any pin of the operational amplifier may result in adverse effects because the amplifier does 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. Make sure to properly shield and isolate sensitive analog nodes from noisy radio signals, digital clocks, and interfaces.
The EMIRR +IN of the OPA391 is plotted versus frequency as shown in Figure 7-2. The OPA391 unity-gain bandwidth is 1 MHz. EMIRR performance less than this frequency denotes interfering signals that fall within the op amp bandwidth.
|FREQUENCY||APPLICATION AND ALLOCATION||EMIRR IN+|
|400 MHz||Mobile radio, mobile satellite, space operation, weather, radar, ultra-high frequency (UHF) applications||39.1 dB|
|900 MHz||Global system for mobile communications (GSM) applications, radio communication, navigation, GPS (to 1.6 GHz), GSM, aeronautical mobile, UHF applications||46.5 dB|
|1.8 GHz||GSM applications, mobile personal communications, broadband, satellite, L-band (1 GHz to 2 GHz)||61.3 dB|
|2.4 GHz||802.11b, 802.11g, 802.11n, Bluetooth®, mobile personal communications, industrial, scientific and medical (ISM) radio band, amateur radio and satellite, S-band (2 GHz to 4 GHz)||69.8 dB|
|3.6 GHz||Radiolocation, aero communication and navigation, satellite, mobile, S-band||82.5 dB|
|5 GHz||802.11a, 802.11n, aero communication and navigation, mobile communication, space and satellite operation, C-band (4 GHz to 8 GHz)||83.6 dB|