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Continuous time Comparator for voltage signals

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geozog86

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Hello everyone! I want to build a continuous time comparator (one input seeing a threshold and the other an integrated current on a capacitor, so a rising voltage), without external clock. So the basic comparator used in ADC is useless, cause it needs the fast clock to reset it and make it take a quick desicion with the positive feedback. I just used gain stages (opamp without compensation if you want) but depending on the gain, sometimes the "output" changes state BEFORE the rising voltage crosses the threshold! What am i doing wrong? Can you guide me? Thank you very much
 

Take your comparator, tie both inputs to to your reference threshold, and run a DC analysis to see if any transistors are in the incorrect operating mode, or if the currents are badly matched.
 

thx for that, i'll try that, but i'm facing problems even when using an ideal "limiting diff ampl" from cadence platform. I mean i give it outputs at 1.8 and 0 (my supplies) and a huge gain (of 10000 for example) and i still see it starting to change slightly before the crossing of the threshold!!! To be honest, i notice that when i'm sweeping temperature. And i don't know if i'm missing some basic idea for how the comparator should behave! :)
 

You obviously either had a mismatch somewhere, or your threshold reference was corrupted by some other circuit operations.
 

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