Voltage divider for digital input pin

[s.h._tech]

Hello everyone,

I have a PXI-6220 measurement card and use the digital I/O channels as switches for logic level operations. Now I also want to measure some logic states over another DI/O port.

The status signal is 36V. My idea was to build a voltage divider so that Uout~4.5V (digital trigger between 2.5-5.2V). For the design I would also need the sourcing current, but I am unable to find it in the datasheet. Usually input sources are around 10-25mA. Below is my setup and calculation.

Can I do it like this? Where can I find the max current?

[K C]

Hi,

Sourcing current is something else than the input current of a digital input. 

10-25 mA for a digital input is quite high I think.

The specification says that the input 50k typical and 20k minimum. Top of page 5

If you use small resistors the top becomes a bit hot (look out for smoke detectors !)

I would take 10 times the value. 

Kees

[s.h._tech]

I also found on page 7

[s.h._tech]

Sorry about that, my post apparently did not came through.

I found on page 7

the I_IH value. Is this the sourcing current when the pin is HIGH?

I calculated it with 250uA which results in 

R1 = 124k

R2 = 20k

This is in the range of your suggestion.

Should I design the divider with the 20k-50k OR with the 250uA current?

[crossrulz · Accepted solution]

Instead of a voltage divider, I would probably use a resistor and a zener diode.  The zener limits the voltage going into the digital input and the resistor limits the current.

[K C]

Hi,

Reading specification / datasheets is not always easy.

The 250µA is based on the minimum resistance of 20k.

If you use a 124K resistor you could end up with a minimum input voltage of 2.4V That is too close to the threshold.

Use resistors of 100k and 22k (22k is more general) The input voltage 4.8V -  3.4V depending on the input resistance (20k - 50k)

Kees

[s.h._tech]

@crossrulz: Is this setup (R-Zener-GND) against sudden voltage spikes?

I looked at a basic Zener circuit for voltage limitations. A quick search brought me this Zener diode with a forward current of 250mA. This would be quite high for the digital input, with a current limiting resistor of R=150k I would get

I_out = 36V/150k ~240uA

Is this design valid?

[crossrulz]

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@s.h._tech wrote:

@crossrulz: Is this setup (R-Zener-GND) against sudden voltage spikes?

I looked at a basic Zener circuit for voltage limitations. A quick search brought me this Zener diode with a forward current of 250mA. This would be quite high for the digital input, with a current limiting resistor of R=150k I would get

I_out = 36V/150k ~240uA

Is this design valid?

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1. The zener needs the diode to be set up to be in reverse in order to get your zener voltage.  So your Annode connects to ground and the Cathode to your digital input.  See here for more details.

2. The zener is in parallel with your digital input, so the zener current should not affect the digital input, except to limit the voltage the digital input will see.

[s.h._tech]

I made a schematic of my current setup. I did not test it yet with my NI-Equipment. With a multimeter I measured 5V across the Zener.

I was unable to measure the current (may be too low for my Multimeter).

[crossrulz]

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@s.h._tech wrote:

I was unable to measure the current (may be too low for my Multimeter).

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Doing the math...250uA.  I think you are fine there.