NI 9403 max input current

[CaveJohnson]

I am looking at interfacing a NI 9403 module with sinking inputs. I believe this will work but am unsure of the max sinking current of the NI 9403. In the specs it lists 2 mA per channel as the max output current, but doesn't mention a max input current. I believe it will sink current, I just don't want to break anything trying it, or have longevity issues. 

For reference the sinking inputs I'm trying to connect to list 5 mA typical at 4V. 

Thank you for any insight!

[ZYOng]

According to NI-9403 Specifications

Input Current (0 V ≤ Vin ≤ 4.5 V)	±250 µA maximum
Module output current[1]	64 mA maximum

[rolfk]

You are mixing and matching names here that are not likely what you really mean.

This module can be both a digital input or output. When used as digital input there is a +- 250 uA maximum input current. This is the load that the pin has on an external signal when it is configured as input. It guarantees that it will not load the external voltage source with more than 250uA. Basically not sourcing more than 250uA when the output is low logic, and not sinking more when it is active high.

If you operate it as an output it does not specify a maximum current but rather that if you want to have a logic high above 4.4 V, your load should not draw more than 2mA. Similarly if you want the low output level to stay below 0.26V you can not sink more than 2mA. If you sink more than 2mA, the output will climb accordingly to a higher voltage, so 5mA will likely cause the output signal to not go lower than maybe 0.5V. There is no specified maximum current limit for sinking and sourcing for an individual pin, but the current from all pins together can not exceed 64 mA. Theoretically that would mean you can draw 64mA from one pin if you don't have any of the other pins work as output, but I doubt you would get a meaningful logic level at that current.

5mA sinking current is probably safe although you have to account for the higher output voltage above GND. At 5mA per output you can however only use up to 12 outputs.

More important than the current in this case is the kind of load you try to switch. If it is not purely ohmic, the switching on can cause much higher currents for capacitive loads and switching off of inductive loads generates huge voltages that almost certainly will destroy the transistors in the output driver if not protected by fast free running diodes.

[crossrulz]

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@CaveJohnson wrote:

For reference the sinking inputs I'm trying to connect to list 5 mA typical at 4V.

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If you are dealing with loads at all (ie requiring more than 1 or 2 mA), I will knee jerk to using a buffer of some type. Something like a TI SN74HCT541DWR might do the job for you.