Tone (audio) generator : breaking the ground loop - Page 1
Forum section: electronics / projects

author: ratatax
author_rank: Regular Contributor
timestamp: November 16, 2019, 03:23:47 pm
content: Hi,It's me again with my ground problems.I'm designing a sound generator in a grounded/earthed metal enclosure... Everything was fine until I had my first ground issues, when connecting my device to other mains earthed devices.The easy solution would be to get rid of earth, but I can't disconnect it from my chassis since most AC adapters have (harmless but nasty) leakage, like 20V creating a buzz feeling when touching the chassis.So I tried to separate grounds by using an isolated DC/DC converter for my analog/output circuitry. However a problem shows up where the DAC send its signal to the opamp. They both have different ground references (see schematic below).I initially thought the DC blocking capacitor at the DAC output was enough... But I feel it's not. It gets rid of the absolute difference between grounds, but coudn't the DAC be damaged if the potential between the two grounds changes rapidly ?I'm searching for the proper solution to this problem

author: NiHaoMike
author_rank: Super Contributor
timestamp: November 16, 2019, 03:29:26 pm
content: Move the DAC to the other side and use a digital isolator.

author: ratatax
author_rank: Regular Contributor
timestamp: November 16, 2019, 04:02:12 pm
content: Possible but costs quite a lot : needs another isolated DC/DC converter (for +3.3V) and a several digital isolator chips that are quite expensive.Is there a simpler way ?

author: MagicSmoker
author_rank: Super Contributor
timestamp: November 16, 2019, 04:34:36 pm
content: No, your scheme won't work because there is no defined return path for current from the DAC (except for parasitic capacitances).Just replace the RC network with a digital isolator to transmit the DAC information digitally. You won't need an additional isolated power supply or much in the way of expense. Silicon Labs makes a whole range of digital isolators for tasks like this and which don't cost too much - maybe $1-$3.

author: SiliconWizard
author_rank: Super Contributor
timestamp: November 16, 2019, 04:52:14 pm
content: Quote from: ratatax on November 16, 2019, 04:02:12 pmI would also suggest isolating at the digital level.Why would you need another DC/DC converter? Can you not generate the +3.3V voltage from the existing isolated DC/DC converter with just an additional LDO?The digital isolator ICs are not that expensive (define your budget). You can find them as 4 isolated I/Os. If your DAC is SPI, that should do it?

author: magic
author_rank: Super Contributor
timestamp: November 16, 2019, 05:27:00 pm
content: Quote from: ratatax on November 16, 2019, 03:23:47 pmIt's going to be horrible, the output of the DAC will jump up and down following the DAC's ground and this huge 50Hz noise will couple through the capacitor like any other signal. And yes, the opamp will clamp it to its rails and may or may not get blown, depending on how much power leaks from the PSU.Quote from: MagicSmoker on November 16, 2019, 04:34:36 pmOf course there is a return path: through protective earth of the analog load, through mains, the DAC PSU and its Y capacitor, back to the DAC.editNo, scrap all of that. I forgot that you want to connect the DAC to protective earth.Then it's simple: you have replaced a short ground connection from the DAC to the load with a long ground connection from the DAC to the load through mains protective earth wiring. Things will only get worse, but only a little worse

author: ratatax
author_rank: Regular Contributor
timestamp: November 16, 2019, 05:46:04 pm
content: Quote from: SiliconWizard on November 16, 2019, 04:52:14 pmThe opamps are powered with +/-15V, generating 3.3V seems complicated from that with a linear regulator... I don't know. The DAC consumes about 430mW.My DAC is dual SPI, one for data another one for setup, and a reset line, it's at least 7 inputs to isolate.

author: ratatax
author_rank: Regular Contributor
timestamp: November 16, 2019, 05:56:32 pm
content: If i'm not mistaken, there are only two solutions:- Isolate the DAC at its digital inputs- Isolate at the very end (after opamp, before jack connector) with isolation transformers. I'm not familiar with this but it seems in audio gear they use 1:1 ratio transformers. It's probably even more expensive than buying several digital isolators...

author: SiliconWizard
author_rank: Super Contributor
timestamp: November 16, 2019, 06:06:42 pm
content: Quote from: ratatax on November 16, 2019, 05:46:04 pmIt's not complicated, but with just an LDO, it won't be very efficient indeed. I don't know how much current you can get from the +15V for instance... Maybe the additional current that would be drawn by the DAC would exceed what your converter can provide. You could use a DC/DC converter (not isolated) instead of an LDO, to go from +15V to +3.3V. A lot more efficient. Still, your isolated DC/DC converter should be able to provide the additional 430mW, not sure it can.Otherwise, yes, you can consider using an additional isolated DC/DC converter. This for instance: PQME1-S5-S3-M is 3.3V/660mW and 3.91€ per 1 (check for input voltage though - this one may not be a match, but I think you'll find ones that will in similar price range). I don't think it's expensive, but again I don't know what your budget is, so...Quote from: ratatax on November 16, 2019, 05:46:04 pmOK, what's the SPI clock? That would require two of those common digital isolators. We need to check whether they'd be able to handle your SPI clock speed though.The Si digital isolators (quad versions) start at around 2€/per 1, it's not that expensive really. (AD and TI also have similar parts.)You can take a look there:https://www.silabs.com/products/isolation/digital-isolatorsSo all in all: 1 small isolated DC/DC + 2 digital isolators = about 8€ for very low quantities, significantly less for high quantities. Is that expensive for your application? Your call.

author: ratatax
author_rank: Regular Contributor
timestamp: November 16, 2019, 06:20:59 pm
content: Quote from: SiliconWizard on November 16, 2019, 06:06:42 pmIt's a product I'd like to commercialize so it's not negligible, but probably OK i'll check in higher quantities.LCSC has some Analog Devices 4 channel isolators for $1.3 each, seems good !Thank you so much for the advices!