How to measure ripple in relative grounds at different points in system?  - Page 1
Forum section: electronics / projects

author: MrtSrt
author_rank: Newbie
timestamp: December 30, 2022, 04:38:16 pm
content: Caveats: I am a long-time lurker, and I just joined to ask this question. I am sorry of this has already been asked and answered before, but I have searched extensively, and I have yet to see a good answer.I am into modular Eurorack synthesizers. The proper grounding of these synthesizers is a large and contentious issue among Eurorack enthusiasts (if interested, you can see modwiggler.com, our forum, for debates about this).Larger Eurorack synthesizers have decentralized power systems, where different parts of the synthesizer are powered by independent power supplies. A common problem is that if these power supplies don't establish a tight ground connection between them, fluctuations in their relative grounds become audible. Unfortunately, the electrical standard was originally developed for smaller systems (by the German firm, Dopefer), and this standard is not good for establishing tight ground connections in larger systems. In practice, the power and ground are distributed over small-diameter ribbon cables and unshielded mini-jack cables, often over distances of several meters.I am experimenting with different ways of improving the power distribution and the ground connections in such a system. To measure the effects of these changes, I need to measure the relative fluctuations in the ground potentials at two different points in the system. I am struggling with the best way to do this.The ripple / noise in the ground potentials arise both from the different power supplies themselves (which are typically switching supplies), and from the different modules in the synthesizer, some are analogue and some are digital, which send a wide range of signal and clock noise into their respective grounds. I would say the ground ripple is of the order of tens up to maybe a hundred mV, up to about 1MHz.I am an amateur, and I have access to a standard Keysight DSOX1202G two-channel oscilloscope and a Keysight 34461A multimeter.My thinking is that I need to use the oscilloscope with a differential probe, since using a grounded probe will obviously affect the measurements.Most differential probes, however, seem to be designed for higher voltages, and I worry that they are not particularly precise in the mV range. (All voltages in the system are within 12V of the ground potential, so large common-mode voltage is not a problem)I could use two channels of the oscilloscope and subtract them, but I would lose precision, and I am not sure how well this works in practice in the mV range.I believe I am really looking for a differential probe for low-voltage signals, but these don’t seem to exist (at least not for less than thousands of dollars)At this point, I am leaning towards building a small differential instrument amplifier, maybe using something like the INA128/129 (Texas Instruments), as a front-end for the oscilloscope, and use that.But there must be a standard way to approach this kind of measurement problem. I just cannot seem to find it described anywhere.Any help and advice would be greatly appreciated.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: December 30, 2022, 06:48:48 pm
content: I do a lot of audio stuff, though don't know anything about synthesizers. My weapon of choice is an old Tektronix 545B with a 1A7A high gain differential plug-in. More modern is a 7xxx mainframe with a 7A22 plug-in. One can also get the Tek ADA400A, though you'll need a power supply. IMO, too expensive and you're better off just building a differential preamp in a box, powered by a wall wart.All that said, I like the Richard P. Feynman method. "He fixes radios by thinking." You need to consider the various loops and where noise might be injected. On a similar note, see what I did to fix an old Sansui receiver-http://www.conradhoffman.com/Sansui_881_mods.htm

author: MrtSrt
author_rank: Newbie
timestamp: December 30, 2022, 07:13:40 pm
content: Quote from: Conrad Hoffman on December 30, 2022, 06:48:48 pmThanks, that helps a lot. It sounds like a building a small differential instrument amplifier is the way to go, which should be pretty straightforward with the components available today (INA128/129). I only need it for short periods, when doing these measurements, so a bare-bones version should suffice (and I have an E3630A power supply that can power it when needed, so that makes it even easier).That's the plan then!

author: Terry Bites
author_rank: Super Contributor
timestamp: December 30, 2022, 09:00:06 pm
content: I feel your pain- I got to deep in to modular synths back in the day. I didnt find many problems on the audio side but CV artifacts were very noticable.I eneded up making a central power and ground hub where all AC, DC 0V and mains earths converged.To do this I broke the local rack 0Vs from mains Earth connections. Then I used loads of sleeved coax braid to lead back the 0Vs to a copper grounding block.And it worked, nearly all nasties went away. I think there was some RF crap going on as well- lighting people- tsssk, pah!Make a diff probe with a small audio transformer. It needs no power and provides all the isolation you need here. Tweak the resistor for best 50Hz response.I vaguely remember using a TY-250. Costs less than a decent INA too. Any line or mic transformer will do.

author: MrtSrt
author_rank: Newbie
timestamp: December 30, 2022, 10:24:59 pm
content: Quote from: Terry Bites on December 30, 2022, 09:00:06 pmUsing a transformer, that's an interesting idea. I need to look into this. Thanks a lot.Btw., it sounds like I am heading in the same direction that you did. I have bought loads of 10AWG stranded wire and a couple of copper blocks to distribute 0V throughout the system (disconnected from mains earth, cough). But I have separate cases, and I cannot consolidate it into a single central power hub. And I would like to measure how well it works.

author: jonpaul
author_rank: Super Contributor
timestamp: December 31, 2022, 04:27:29 am
content: Bonjour, the entire Euro Rack system design was poorly implimented for grounding, power supply and noise, from the start.To find the issues, use "Occams razor" and eliminate some connections and power supplies one by one.Try to get a SINGLE power supply, and reduce ground loops with a star single point grounding system.To see the noise differentially, many digital scopes have sensitive vertical and averaging, use matched probes, and function Ch1-Ch2, to make a simple differential measurementWe use Yokogawa DL7440 and Dl1740In analog scopes, tektronix 7000, plus 7A22, or the hard to,find Tektronix AM501 differential amp are the best.See the classic noise texts, like Ott, Noise Reduction Techniques, and our AES friends Jim Brown and Bill Whitlock's SYNAUD CON site for good debugging and good engineering solutions.Bon ChanceJon