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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.

schematic

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.

enter image description here

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):

enter image description here

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.

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  • \$\begingroup\$ Measure your power supply - you'll probably see it is also changing. How much current do you think flows from the comparator through the transistor's BE junction with no resistance to limit it ... ? \$\endgroup\$ – brhans Jun 1 '17 at 16:11
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    \$\begingroup\$ @brhans I think that's just the way he drew it here..that transistor is internal. BEsides... if that were the case the droop would be on the output when low. \$\endgroup\$ – Trevor_G Jun 1 '17 at 16:17
  • \$\begingroup\$ Thanks for your suggestion, but I did measure the power supply and it doesn't change (noticeably). The transistor is symbolizing the open collector output that is integrated into the comparator. \$\endgroup\$ – Felix S Jun 1 '17 at 16:23
  • \$\begingroup\$ DId you leave the balance /strobe pins open? \$\endgroup\$ – Trevor_G Jun 1 '17 at 16:26
  • \$\begingroup\$ yes I did. I just found the problem. Sorry for wasting everyone's time! \$\endgroup\$ – Felix S Jun 1 '17 at 16:27
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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?

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    \$\begingroup\$ no leave it here...someone else might have the same issue. \$\endgroup\$ – Trevor_G Jun 1 '17 at 16:33
  • \$\begingroup\$ It's a good question. Don't delete. \$\endgroup\$ – Blair Fonville Jun 1 '17 at 17:19
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    \$\begingroup\$ Go ahead and mark as accepted. \$\endgroup\$ – JRE Jun 1 '17 at 17:26

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