## Solution

The input impedance for the 9401 is not a discrete value since it is variable, based on the components of the front end of the module. Since the NI 9401 is a DIO TTL (transistor-transistor logic) signal, impedance typically doesn’t affect these types of applications. What is more important when using TTL modules is the amount of current needed to drive the NI 9401.

Below is the equation to calculate the amount of current you would need to drive the input of the 9401:

I_input = [Vcc * f_in * (C_in_9401 + C_cable) + I_input_9401]

Where:

I_input = current to drive the NI 9401

Vcc = logic device on your side driving the 9401

f_in = the frequency of the logic device driving the 9401

C_in_9401 = the input capacitance

C_cable = the capacitance of the cable

I_input_9401 = the input current of the 9401

As specified in the user manual:

The input current (0 V ≤ Vin ≤ 4.5 V) is ±250 µA typ (I_input_9401 in the equation)

The input capacitance is 30 pF typ (C_in_9401 in the equation)