Hello everyone! First time posting in this forum . . . hopefully this is the right place!
We have an ASIC that is outputting a 0.5 - 4.5V signal into an ATTiny5 that is reading the ADC voltage and processing it into another signal. One issue I'm facing is that the Vout line of the other ASIC is also the programming pulse line so we have a resistor and 5.1V zener diode protecting it (feeding directly into the ATTiny5). The issue is increasing the voltage on the diode is causing a lot of noise and non linearity on the ADC input. For instance, the current pulled across the resistor is less than 1 uA at 1.5 V, is 50 uA at 3.9V, and about 300 uA at 4.5V! This obviously presents us with a pretty noisy (and non linear) issue.
Does anyone have an alternate approach to this issue? If I continue the route I'm headed, I can probably find a better suited zener for this application (we're currently using a MMBZ5231BW zener, not sure why this was picked as, unfortunately, no one ran the design by me until it was built up . . .) and a better resistor value (currently 1k due to cost/power/size) but I'd like to hear if anyone else has another potential approach to protecting our ATTiny5 from a 13.5V programming pulse.
Unfortunately, the chip is full so no digital algorithms can be used to solve the noise issues so I'm looking for an analog alternative.
Any help you guys can give me would be appreciated! Thanks!