| Electronics > Beginners |
| Need help understanding this OpAmp behavior. |
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| Wigo:
Hello Community, Yesterday i was doing a little bit of experienting and i ran into a behavior of an OpAmp that i do not understand. Maybe someone can explain it. I use this circuit. (Second Image) In short i try to compare the output of a piezo to a reference voltage. And the output of the piezo cannot exceed 4,7V. If i set the reference voltage to 5V the OpAmp should not trigger at all. But i get this Output. (First Image, Channel 1 is the Output of the OpAmp, Channel 2 is the Input of the Piezo) So my question why is the comparator triggering on the low negative voltages? Or have i messed up somewhere else? |
| Nerull:
According to TI the input voltage should never go more than 300mV below the V- supply rail, and this may cause damage. https://e2e.ti.com/support/amplifiers/f/14/t/559209 If you look in the datasheet under 'Absolute Maximum Ratings', you will see -0.3V listed as the minimum input voltage, with 0V as the recommended minimum input voltage. This would be relative to the negative rail. |
| Wigo:
Thanks, i totally missed that point. :palm: Good that i have a few of them to test. So the "inverted" output could be from a damaged OpAmp? |
| Rerouter:
Use a second op amp to create a virtual ground, at 2.5V, have the piezo and resistor connected to there, instead of ground, and instead use back to back diodes to clip its amplitude to always be in the op amps common mode range I suppose the last part comes down to what your hoping to accomplish with the circuit, if you want to measure any deviation from a set point, a window comparitor may be then better suited to trip on both positive and negative deflection. Edit: something like this, if you want the window comparator version drawn up just ask |
| StillTrying:
An op amp is not a comparator! Comparators are designed to not do strange things when there's a large voltage difference between the +/- inputs. |
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