7.3.6 EMI Susceptibility and Input Filtering

Operational amplifiers vary with regard to the susceptibility of the device to electromagnetic interference (EMI). If conducted EMI enters the op amp, the DC offset observed at the amplifier output may shift from the nominal value while EMI is present. This shift is a result of signal rectification associated with the internal semiconductor junctions. While all op amp pin functions may be affected by EMI, the signal input pins are likely to be the most susceptible. The OPA2313-Q1 device incorporates an internal input low-pass filter that reduces the amplifiers response to EMI. Both common-mode and differential mode filtering are provided by this filter. The filter is designed for a common-mode cutoff frequency of approximately 35 MHz (–3 dB), with a rolloff of 20 dB per decade.

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. The EMI rejection ratio (EMIRR) metric allows op amps to be directly compared by the EMI immunity. Figure 33 illustrates the results of this testing on the OPA2313-Q1 device. Detailed information may be found in EMI Rejection Ratio of Operational Amplifiers, available for download from www.ti.com.