Hi Bryan,
We do not spec an input impedance for this module because the inputs use a current-limiting architecture, so the input impedence is dynamic based on the voltage. You could use the <7mA max input current spec in conjunction with your expected source voltage to calculate a pseudo minimum input impedence. Keep in mind that this is dynamic and the 7mA value is more of a guideline since it is dependent on the circuit you have attached to the input of the NI 9421.
Regards,
Andrew W
National Instruments