NI 9403 Input Impedance

[Balze]

Hello everybody,

I'm trying to use a 9403 as digital Input to test a device.

This device uses 46k4Ohm Pull-ups with 3,3V

I asumed the input impedance of the Input is very large.

Now i figured out, that this seems not to be the case.

Could anyone please tell me what the input impedance of this module is?

I searched all available documents of the 9403 but to no avail.

Thanks in advance!

Best regards

Balze

[Norbert_B]

The 9403 is a digital IO module for TTL. TTL usually does not state impedances as it is common sense that sources are low impedant, definitely way less than 46kOhm.

The issue you encounter is that the voltage drop on the 9403 does not exceed the required 2V (or recommended 2.4V) to detect a high level. Also note that high source impedances can also lead to signal distortion caused by the resistance coupled with (parasitic) capacities making edges blurry.

I recommend to include an impedance converter in your circuit.