OPA191: EMI Rejection

Kato-san,

The OPA191 input EMI filter is a first-order, RC differential/common-mode that comes about from the added series resistance and junction related capacitances in the input circuits. Datasheet Figure 60 shows a simplified circuit diagram showing some of the internal components. The junction capacitances of the input differential input pair, and those of the ESD protections diodes combine to provide the filter capacitances.

The EMIRR performance is seen in the Figure 53 graph you have shown above. There isn't anything that can be done with regard to "internal circuit design techniques in detail to improve EMI immunity if possible." The EMIRR performance is determined by the input filter and the inherent rejection of the Op amp design that follows the input filter.

In order to achieve higher EMIRR from a circuit that incorporates the OPA191 it is necessary to add external EMI suppression before the OP amp pins. Depending on the EMI mode, differential or common-mode, different EMI suppression circuits would be needed. Some suppression methods involve RC or LC input filters, X2Y capacitors, ferrite beads, common-mode transformers, etc. There is a wealth of information on line regarding the subject of EMI and methods for dealing with it.

Regards, Thomas

Precision Amplifiers Applications Engineering