To identify EMI-hardened operational amplifiers, a parameter is needed that quantitatively describes the EMI performance of operational amplifiers. A quantitative measure enables the comparison and the ranking of operational amplifiers on their EMI robustness. Therefore the EMI Rejection Ratio (EMIRR) is introduced. This parameter describes the resulting input-referred offset voltage shift of an operational amplifier as a result of an applied RF carrier (interference) with a certain frequency and level. The definition of EMIRR is given by Equation 1:
The offset voltage depends quadratically on the applied RF level, and therefore, the RF level at which the EMIRR is determined should be specified. The standard level for the RF signal is 100 mVP. AN-1698 (SNOA497) addresses the conversion of an EMIRR measured for an other signal level than 100 mVP. The interpretation of the EMIRR parameter is straightforward. When two operational amplifiers have an EMIRR which differ by 20 dB, the resulting error signals when used in identical configurations, differ by 20 dB as well. So, the higher the EMIRR, the more robust the operational amplifier.