I just built up a circuit with a LM311P comparator with open collector and emitter (I don't think that's relevant). The negative input is a voltage I create with a voltage divider and the positive input is the signal I want compared.
simulate this circuit – Schematic created using CircuitLab
However it seems like the comparator is heavily loading the voltage divider because the voltage at the negative input looks like this (yellow trace), blue trace is the output signal.
It's fluctuating by almost 2V. And this is after I already made the voltage divider smaller. The fluctuation was even worse with a combined resistance of about 20kOhms. I know I can stabilize it with a big capacitor but I first want to understand where this is coming from. Here is a screenshot from the datasheet of LM311P (http://www.produktinfo.conrad.com/datenblaetter/175000-199999/175994-da-01-en-IC_LM_311P_DIP8_TID.pdf):
The current through the voltage divider is 2.1 mA which is 7000 times as big as the worst case bias current of 300nA.
Am I missing something here? Any help understanding why this is happening would be very appreciated.
Best Answer
The problem was that the input signal went below zero. It can go a little negative without problems but at around a diode forward drop it starts loading down the inputs. Sorry for wasting everyone's time. Should I delete this question?