I have a GPIO driving a logic inverter which is then connected to the input of an NPN Darlington pair IC (ULN2803). The MCU and inverter are powered by the same 3v3 rail.
I am seeing behaviour which I did not expect at the output of the Darlington pair, even when the output is not connected to anything. I have observed this behaviour in LTSpice (when I approximately modelled the ULN2803 internals) and on my physical circuit (by probing with a CRO). The simulation is shown below.
When the input to the Darlington pair is logic low, the output is (as expected) basically 0V. However, when the input is logic high, the floating output of the Darlington pair appears to read ~630mV.
I'm struggling to understand this observation - any insights would be appreciated (even if it means pointing out some aspect of my stupidity :-) ) Thanks!
Blue Trace = Collector of Q1 & Q2