An obvious and straightforward location for filtering is at the output of the INA20x-Q1 series; however, this location negates the advantage of the low output impedance of the internal buffer. The only other option for filtering is at the input pins of the INA20x-Q1, but the internal 5-kΩ + 30% input impedance complicates input filtering, as illustrated in Figure 9-2. Use the lowest possible resistor values to minimize both the initial shift in gain and effects of tolerance. Equation 1 gives the effect on initial gain:

Equation 1.

Figure 9-2 Input Filter

To calculate the total effect on gain error, replace the 5-kΩ term with 5 kΩ – 30%, (or 3.5 kΩ) or 5 kΩ + 30% (or 6.5 kΩ). One can also be insert the tolerance extremes of R_FILT into the equation. If using a pair of 100-Ω 1% resistors on the inputs, the initial gain error is 1.96%. Worst-case tolerance conditions always occur at the lower excursion of the internal 5-kΩ resistor (3.5-kΩ), and the higher excursion of R_FILT + 3% in this case.

Note that one must then combine the specified accuracy of the INA20x-Q1 in addition to these tolerances. Although this discussion treated worst-case accuracy conditions by combining the extremes of the resistor values, it is appropriate to use geometric-mean or root-sum-square calculations to total the effects of accuracy variations.