Unstable DC OpAmp Sinusoidal Signal Centering - Page 1
Forum section: electronics / projects

author: The Hammer
author_rank: Newbie
timestamp: July 30, 2018, 11:37:02 am
content: First time poster, be gentle. Been a fan of EEVblog since forever, and have been more learned by Dave and the team than anyone else I can think of. Big shout out!I've designed a circuit that in simulations seems to work for centering signals that might come from analog sensors that aren't generally guaranteed to have a 1/2 VDD "center point". Some sensors come with a voltage reference for center, and some don't (ect, ect, ect). As a result I found a good way to kill some time designing this OpAmp setup. I designed a board based on this design with high precision OpAmps and matched resistor networks. Then I ordered the board and sent out for all the parts.http://tinyurl.com/ya58f4z7Upon completion of my PCB it's output doesn't seem to do more than slam from VDD to GND as the signal passes 1/2 VDD in addition to getting exceptionally warm. I haven't double checked my circuit design on the board, nor done decent probing on the real deal. However, I'm curious what the smart people here might have to say about my design. Is there something wrong with my design? Have I hit one of those instances where the real world doesn't agree with what's written down on paper in terms of practical OpAmp circuit design?

author: Zero999
author_rank: Super Contributor
timestamp: July 30, 2018, 10:42:01 pm
content: I can tell it won't work, just by looking at it.Did you try prototyping it on a breadboard or stripboard? Presumably not.Simulators aren't perfect, especially the one you've used. Let this be a lesson to you, to prototype with real parts, before designing the PCB.Sorry I don't have anything more constructive to say.

author: Audioguru
author_rank: Super Contributor
timestamp: July 30, 2018, 10:56:06 pm
content: Your opamp has no part number and it is not powered. Its 60Hz input voltage is not shown and it has no DC bias voltage. The Falstad sim is a nightmare of dots moving all over the place.

author: IDEngineer
author_rank: Super Contributor
timestamp: July 31, 2018, 02:12:48 am
content: This seems needlessly complex for a circuit that doesn't work.I presume from your comments that the signal in question will always be AC? If so, why not AC couple the input, for starters?Then, when you say "1/2 VDD center point", are you talking about a hard reference at 50% of the supply or a varying centerpoint based on the average of the incoming waveform? Unless your waveform is symmetrical even a perfectly operating circuit that does what you describe will appear to have an offset above or below 1/2 the supply. Is that OK? The design will change based on your answer.Lots of unknowns here, but I'm confident it won't take four opamps if your description is anywhere close to what you actually want.

author: StillTrying
author_rank: Super Contributor
timestamp: July 31, 2018, 01:11:16 pm
content: "its output doesn't seem to do more than slam from VDD to GND as the signal passes 1/2 VDD in addition to getting exceptionally warm."That's what the Falstad sim seems to do for me, both of them! with an impossible to findworkingpoint.I don't get it, why would you build something that doesn't work even in simulation.

author: oPossum
author_rank: Super Contributor
timestamp: July 31, 2018, 01:31:58 pm
content: The simulation has the 60 Hz source set to 1 mV. Change it to 10 mV or higher and you will get square waves out.

author: oPossum
author_rank: Super Contributor
timestamp: July 31, 2018, 01:36:26 pm
content: Change the pot to 50k and the sine wave returns.

author: Zero999
author_rank: Super Contributor
timestamp: July 31, 2018, 03:33:32 pm
content: Quote from: oPossum on July 31, 2018, 01:31:58 pmQuote from: oPossum on July 31, 2018, 01:36:26 pmIs that in the simulator, or real life? Try actually building it.

author: oPossum
author_rank: Super Contributor
timestamp: July 31, 2018, 03:48:31 pm
content: That is all in the sim. If the hardware he built uses a 1M pot, that may be causing the square waves due to the gain of the last stage being way to high.

author: In Vacuo Veritas
author_rank: Frequent Contributor
timestamp: July 31, 2018, 04:10:52 pm
content: "I designed a board based on this design with high precision OpAmps and matched resistor networks. Then I ordered the board and sent out for all the parts."Wow, that'sconfidence. What's wrong with a simple resistive divider?"Have I hit one of those instances where the real world doesn't agree with what's written down on paper in terms of practical OpAmp circuit design?"Yes, definitely. Better contact the manufacturer.

author: Zero999
author_rank: Super Contributor
timestamp: July 31, 2018, 08:51:48 pm
content: Unfortunately, it's very easy to design a crap circuit, which works beautifully in SPICE but fails in real life. See the attached for a classic example of a circuit which will either completely fail or perform extremely poorly in real life, yet works perfectly using a simulator program.