Simple input power filtering solutions? - Page 1
Forum section: electronics / beginners

author: msat
author_rank: Regular Contributor
timestamp: December 16, 2020, 02:48:37 am
content: So I finally fired up KiCad and started working on my 3 channel transimpedance amplifier circuit from my other thread. Since the questions I have now would be quite off-topic, I figured it was more appropriate to start a new thread.Power for my circuit will be coming from a split supply bench PSU and thus is fairly well regulated, but I'm thinking that, especially since it will be connected via maybe 2ft leads, that it would be good practice to do some sort of power conditioning on the amp PCB. I know that such a thing could get complex very quickly, but I'm hoping to go for the low-hanging fruit that has a good payoff to effort ratio. At first I was thinking about those symmetrical C-L-C EMI filters that you see on AC inputs:But I never worked with inductors, and to be honest, they kind of scare me! I'm afraid of unintentionally establishing some LC oscillator. Also, I have decoupling caps on the op amp rails, add in the inductance of the PCB traces and I become even more concerned (perhaps unnecessarily). Since power usage is not of a particular concern, I could substitute the inductors for resistors, though I don't yet know how to determine those values because I also don't know how to determine my load resistance.Similarly, and perhaps with better performance characteristics, I was thinking of putting resistors just upstream of the decoupling caps on the op amp rails, but I have a sneaking suspicion that, without doing something closer to where the power comes in, I'd be letting more junk noise into the circuit.On top of that, I intend on feeding power in via shielded twisted pairs (or triple if I want to add ground in there. I was considering a separate shielded ground conductor). It's quite possible that I'm going way overboard, but I figure that if I can improve noise performance with a little care and a handful of passive components, it would be worth while.Any suggestions on how to approach this would be greatly appreciated!

author: Doctorandus_P
author_rank: Super Contributor
timestamp: December 16, 2020, 02:56:13 am
content: You can buy such filtered completely canned in a nice shielded box.Tadaaa.Probably cheaper too then the separate components.Just don't put in such a 20 year old filter with Rifa caps.Then move on to the next challenge.

author: msat
author_rank: Regular Contributor
timestamp: December 17, 2020, 01:07:53 am
content: I know such pre-packaged EMI filters exist but I don't know if they'd even be a good idea (due to possibly causing oscillations). And if they are a good idea, I'd certainly want more information before I implement them in my project.

author: msat
author_rank: Regular Contributor
timestamp: December 17, 2020, 03:07:48 am
content: I'm leaning towards a symmetrical RC (not CRC) filter right at the power input of the instrument, but I'm confused about the effects on the power rails to the op amp because to my understanding (but I certainly could be wrong), the op amp will appear as a variable resistance load, and thus the voltage seen at its power rails will vary. But I also have my doubts about that being the case because it's not unheard of to use resistors in series with an op amp's power rails. What am I missing here?

author: HB9EVI
author_rank: Frequent Contributor
timestamp: December 17, 2020, 10:53:15 am
content: what should oscillate on pure passive components filter?these kind of filters are common practice almost everywhere nowadaysthink about the level of filtering you need and buy one; periodthere isn't anything magic about them....RC-filters are clearly NOT the way to go in the mains domain

author: msat
author_rank: Regular Contributor
timestamp: December 17, 2020, 11:58:42 pm
content: Just to clarify, the instrument (a 3 channel transimpedance amp) isn't connected to mains. I'm powering it from a split supply DC bench PSU.