Isolated digital input with switchable pullup resistor

[jars121]

Hi all,

I'm working on a DAQ design, whereby I need to receive X number of low speed (<10kHz) digital signals (across X number of input connector pins). I would like to isolate the signals from the rest of the board.

I have an existing design (shown below) which addresses half of this requirement; the below works where the switched input is switching to an external VDD/Vref, providing a positive voltage to the DAQ input. The resistor divider, diode and Zener are chosen to ensure a safe ~3V input to the Si838x isolator across the expected voltage range and to minimise any reverse polarity issues.



I would also like to accommodate a second use case, whereby the external electronics to which the DAQ is interfacing instead switches the input to ground. In this scenario, I'd enable a pullup (via MCU GPIO) on the input line to an isolated 12V supply, which would provide a 3V3 output from the Si838x isolator when the switch is open, with the signal line pulled to ground when the switch is closed. I.e. the logic on the MCU side would be inverted (note that the diagram above does not show the inverted signal). Here's an initial pass at what this might look like:



The Si838x requires at least 1mA of input current and a voltage of at least 2V to register a logic HIGH. I'd like to be able to safely accommodate input voltage between 6-24V (with protection against spikes up to 80V; perhaps I need (a) TVS diode(s)?). The R3 value of 4.7k will keep the switched ground current to ~2.5mA which should be fine for the source/external electronics.

Does the above seem reasonable? Have I missed, misunderstood or overlooked anything?