Timeline for Why isn't this opamp working correctly?
Current License: CC BY-SA 3.0
7 events
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| Jun 19, 2016 at 23:01 | comment | added | RustyShackleford | I wonder if there's a good way to model this instability in LTspice ? I have the actual LT1006 model (not a generic opamp). Maybe capacitively-couple in a noise signal, sweeping its frequency ? | |
| Jun 17, 2016 at 17:45 | comment | added | RustyShackleford | I need to try to select a more suitable opamp. I was mainly looking for one allowing single-ended supply. | |
| Jun 17, 2016 at 17:44 | comment | added | RustyShackleford | Thanks for the add'l info, @Olin. One of their apps (on that same page) also has a simple RC low-pass filter on the opamp output. I tried that last night and it seemed to work pretty well. | |
| Jun 17, 2016 at 11:39 | history | edited | Olin Lathrop | CC BY-SA 3.0 |
added 997 characters in body
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| Jun 16, 2016 at 23:48 | comment | added | RustyShackleford | Some strange observations... If I connect the circuit to its application - a series string of LEDs, instead of the 50Ω pot I've been testing with - the discrepancy between opamp inputs goes to 0. This is with all 3 components (mentioned in the previous comment). But if I remove the cap on the opamp output, things go crazy: the opamp output goes to 5+v, and current goes up to 700-800ma (and thus the negative opamp input to 700mv, even though the positive input is at 200mv or so). Some very unstable behavior I need to get a grip on; one thing's for sure: it's very unhappy without some damping. | |
| Jun 16, 2016 at 22:33 | comment | added | RustyShackleford | Yes, even 7mv discrepancy tells me something is wrong. I replaced the wire from R1 to the opamp's negative input with a 1 kΩ resistor, and added a 1000pf cap between the output and negative input of the opamp. But, without that capacitor on the opamp's output, I see up to 20mv or so of discrepancy (between the opamp's inputs); with that cap in, the add'l components (1 kΩ and 1000pf) don't reduce the discrepancy (though of course they may make the circuit more robust, as you suggest). | |
| Jun 16, 2016 at 21:46 | history | answered | Olin Lathrop | CC BY-SA 3.0 |