I have a situation where a single pin on a board must handle digital inputs and analog outputs. The digital input is actually a comparator (LM239A - Open Collector Output).

The problem is, when serving the role of a digital input, I need to be able to tell the difference between floating, and ground. This requires me to have a pullup on the input.

However, in the case where the pin is acting as an analog output, the DAC will be fighting against that pullup. If the pullup is weak, say 100k, is the DAC able to able to overpower it? For reference, the DAC would be driving an analog voltmeter. The DAC output range is 0-10V and is a DC signal (ie, it's rate of change is pretty much zero).

I drew up a simplified example of the input to maybe help.

Thank you for any advice you can provide!


simulate this circuit – Schematic created using CircuitLab


As the DAC is internal, it will depend on the current drive capability of the device.

The simple answer is that the DAC output drive needs to be significantly greater than the pull-up current.

The worst case is when the DAC tries to drive to 0V, when the pull-up has 280uA of current; to overcome this to within 1%, the DAC would need to be able to drive 28mA when in this state.

Much depends on how much error you can tolerate.

| improve this answer | |
  • \$\begingroup\$ Thank you! Can you elaborate a bit more on why the DAC's drive strength must be 100x the pull up strength? The DAC only has a drive capability of +/- 10 mA. \$\endgroup\$ – Geomancer Feb 15 '17 at 17:26

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.