Decoupling dilema - Page 1
Forum section: electronics / beginners

author: paulca
author_rank: Super Contributor
timestamp: April 19, 2018, 02:47:29 pm
content: So I have a noisy power rail for an amplifier circuit which uses a virtual ground.When I place a large decoupling cap on the power rail the noise is removed from the power rail but deposited on the ground. The amplifier inputs are ground referenced and I still have noise output by the amplifier.Similarly if I don't decouple the power rail the power rail noise is present on the virtual ground and is amplified by the amplifier.What seems to work is to not decouple the power rail, but to only decouple the virtual ground. But it sounds fishy to me.How does one deal with this issue, or do I have some other issue?

author: Audioguru
author_rank: Super Contributor
timestamp: April 19, 2018, 03:21:04 pm
content: The virtual ground passes the noise. An amplifier is supposed to have good supply noise rejection.The virtual ground should feed a very low current like only to the (+) input of an opamp. Then the "half the supply voltage reference" can be simply two 100k resistors in series from the supply and a reasonable filter capacitor value like 22uF.Please post the schematic of the amplifier and how it is biased with a virtual ground.

author: madires
author_rank: Super Contributor
timestamp: April 19, 2018, 03:32:22 pm
content: In case you like to see the circuit and read the original thread:https://www.eevblog.com/forum/beginners/circuit-review/

author: paulca
author_rank: Super Contributor
timestamp: April 19, 2018, 03:41:03 pm
content: Quote from: madires on April 19, 2018, 03:32:22 pmThanks.I recreated one channel of the circuit on a breadboard and was able to recreate the noise.I tried lots of things to remove the noise being amplified off the power rail, but the thing that was almost perfectly successful was removing C3 completely.

author: paulca
author_rank: Super Contributor
timestamp: April 19, 2018, 03:43:28 pm
content: Quote from: Audioguru on April 19, 2018, 03:21:04 pmWhat about the feedback divider? Ground or 1/2 Vcc VGround?

author: Cerebus
author_rank: Super Contributor
timestamp: April 19, 2018, 05:03:08 pm
content: Here's what you've got, according to your circuit diagram:Obviously I've just used a TL07x OPA because I don't have a model for the OPA551 handy. I've injected 0.1V of noise at 10 kHz into the power rail, and this is what you get as output with 100 mV input at 1kHz:Here's the circuit reworked with a proper active virtual ground, and bypassing on the voltage divider that virtual ground is derived from:There the second OPA could really be a TL07x or any other suitable OPA, it doesn't need to be a beast like the OPA551. And here's the output under the same conditions:Noise gone! The issue was the impedance of the virtual ground, and adequate filtering on the source of that virtual ground.

author: Kalvin
author_rank: Super Contributor
timestamp: April 19, 2018, 05:12:06 pm
content: If you connect Vin to virtual GNDA you will not have a problem with ground noise. For some circuits you can reference the input to virtual GNDA, but some circuits may be constructed so that you have to use the common ground and then you may have this kind of noise problems. Selecting your ground matters.Edit: Fixed typo: s/AGND/GNDA/.

author: paulca
author_rank: Super Contributor
timestamp: April 19, 2018, 06:12:03 pm
content: Quote from: Cerebus on April 19, 2018, 05:03:08 pmThank you very much. I did try an active virtual ground on breadboard last night, but I didn't consider giving the divider that much resistance. It didn't occur to me to split the divider into 4 and bypass between two of them like you did.QuoteWhen I read stuff about how much current the buffer needs to handle people say it needs to be as beefy as the amp, but when I look at it I can't understand why. The current path for the main amp will mostly be via it's power rails and the feedback which has a 10K in it is the only route to the VGND. So ... surely the virtual ground buffer only needs to supply the feedback loop with current when the amp is sinking it.Surely an NE5532 would do for the buffer amp?QuoteI did try and filter the source with large caps, but each 470uF cap only reduced the noise by about 1/3, when I had 3 of them on the main power rail and I still had noise I knew something else was wrong. LOL

author: Cerebus
author_rank: Super Contributor
timestamp: April 19, 2018, 07:02:37 pm
content: Quote from: paulca on April 19, 2018, 06:12:03 pmThat ought to be three resistors, I just couldn't be bothered to coalesce the bottom two; out of curiosity I'd tried something else that needed the extra resistor first.QuoteThat would be fine. The virtual ground needs a low impedance, it doesn't need to actually source or sink much current. The magic of negative feedback reduces the 10s of ohms of actual output resistance of the op amp into the milliohm region.QuoteThe trick is adding some significant resistance before the filter cap. That trick of splinting the top divider resistor and decoupling there can be seen in almost every discrete class AB power amplifier design ever made to get a quiet source for bias voltage from unregulated power rails.

author: paulca
author_rank: Super Contributor
timestamp: April 20, 2018, 02:32:02 pm
content: Spotted this...https://www.tubecad.com/2018/02/blog0412.htmSeems there are many designs for virtual ground splitting!

author: paulca
author_rank: Super Contributor
timestamp: April 30, 2018, 08:46:56 pm
content: Thanks for the help fixing this up guys.Here is the new version with it's new VGnd Opamp and decoupling caps. Works a treat.