Noise on power supply to opamp and stability - Page 1
Forum section: electronics / beginners

author: arthurn
author_rank: Newbie
timestamp: November 08, 2021, 02:03:34 am
content: Hi, first time poster here.I am hoping someone can point me in the right direction. I am trying to build a case that an instrument at work is effected by a noisy power supply (solar PWM reg/battery set-up). I'm not allowed to pull the instrument apart to really get into the specifics but I do know that instrument uses opamps to convert the incoming signal into something the micro-controller can understand and does not have active power regulation between them and the supply - this is all I can see on the board without taking the instrument apart further. The instrument was designed in the late 90's and appears to have been intended to operate solely off stand-alone batteries only.My measurements indicate to me that the solar regulator is the culprit. I'm hoping someone can help by pointing me in the direction of some resources about general opamp function with noisy power supply (preferably not too technical as I need to make the case to layman in the field of electronics) or tell me if I'm barking up the wrong tree.Thanks.

author: Jwillis
author_rank: Super Contributor
timestamp: November 08, 2021, 02:40:52 am
content: What your experiencing is EMI or Electro Magnetic Interference which is either propagating through the air or more likely on the power lines . This is due to the high frequency PWM or Pulse Width Modulation being used inside the inverter which sounds like it has very limited EMI suppression built in . In some cases the inverter may even have poor earth grounding .Here's some info that may help.ELECTRO MAGNETIC INTERFERENCE IN PWM INVERTERandDifferential and CommonMode Noise

author: Manul
author_rank: Super Contributor
timestamp: November 08, 2021, 02:51:39 am
content: Hi,So why is it not enough to show, that instrument's performance is degraded when it is used from that solar power supply vs batteries? A plain fact is much stronger then generalized, theorectical explanations / speculations.Power with batteries, compare performance (better in some objective values/numbers), write everything on paper and it will be a strong case.Generally, yes, devices build for battery power may have little filtering, circuit design might have poor PSRR, because with batteries it does not matter. Noisy supply could be a huge problem for such devices. Also common mode noise. If you run it from switch mode power supply you can expect all kinds of interference. That is quite typical.

author: arthurn
author_rank: Newbie
timestamp: November 08, 2021, 03:07:48 am
content: Quote from: Manul on November 08, 2021, 02:51:39 amHistory, basically. I have the direct evidence but want to back it up with the theory. I wont go into the issues we have but without the evidence and theory very little will get done.

author: basinstreetdesign
author_rank: Frequent Contributor
timestamp: November 08, 2021, 03:21:53 am
content: Quote from: arthurn on November 08, 2021, 03:07:48 amI tend to agree with Manul. This a classic case of the Monkey Principle. You need to make them feel the effects and the pain of the failure of this instrument to do its job properly. When it is their problem, then it will get fixed. Theoretical arguments will have little effect if they are not suffering from the situation.When the Monkey is on their back you will see action!

author: arthurn
author_rank: Newbie
timestamp: November 08, 2021, 03:44:41 am
content: Quote from: basinstreetdesign on November 08, 2021, 03:21:53 amDo you think I'd be going to the trouble if it was that easy?

author: Jwillis
author_rank: Super Contributor
timestamp: November 08, 2021, 04:20:19 am
content: https://tangentsoft.net/audio/opamp-ps.htmlThe only way you can convince people that a problem exists is by practical experiment . Run your instrument on the noisy supply and show your results . Then move the instrument or supply it with clean power and show the results .If they are still not convinced then that's their problem . Some times it's impossible to convince some people of an existing problem until it starts costing them money . Some will even do exactly the opposite of logic just to save face. Not much you can do in that case .