If the peak amplitude of the external voltage is expected to vary, the series resistor must be selected to keep the ZCD current source and sink below the design maximum range of ± 600 μA and above a reasonable minimum range. A general rule of thumb is that the maximum peak voltage can be no more than six times the minimum peak voltage. To ensure that the maximum current does not exceed ± 600 μA and the minimum is at least ± 100 μA, compute the series resistance as shown in Figure 1. The compensating pull-up for this series resistance can be determined with the equations shown in Figure 2 because the pull-up value is independent from the peak voltage.

Figure 1. Series R for V range