The common-mode rejection ability of an instrumentation amplifier degrades with increasing frequency often resulting in a major portion of the “advertised” bandwidth becoming unusable due to common-mode noise errors. An optimally designed passive input stage filter can provide significant improvement in AC common-mode rejection. Calculation of correct component values and selection of appropriate capacitors holds the key to the solution. A spreadsheet that gives graphic demonstration of the analysis and performs the necessary calculations to achieve this improvement is also included.