virtual ground and rc filtering - Page 1
Forum section: electronics / beginners

author: shaling
author_rank: Contributor
timestamp: July 02, 2025, 09:23:58 am
content: Hi,I have design prototype electronics for an optical centration setup.The sensor is a PSD chip with 4 taps, like a quadrant photodiode.I have transimpedance amplifiers to measure the photon signal with gain, then difference amplifiers to output the optical run-out (which is dynamic as the optic spins on an air bearing table).I have used virtual ground on the transimpedance amplifiers (to bias anode) and difference amplifiers (for bidirectional output), as I am using a signal supply rail.My question is, do RC filters on the output of both amplifiers need to go to virtual ground, or can they go to supply ground?And if so, as I have lots of virtual ground connections, is there a problem routing these to a large pour on a 4 layer stack?Schematic attached.Thanks.

author: RFDx
author_rank: Regular Contributor
timestamp: July 03, 2025, 12:19:46 pm
content: Quote from: shaling on July 02, 2025, 09:23:58 amThe TIA inverting input is a virtual ground node. There is nothing virtual about the voltage supplied to the non-inverting input. Call it artificial/synthetic ground or mid-rail supply or just Vref like you already did in the gain equation.QuoteGND is fine.QuoteThe rail-splitter/Vref section is not shown but I don't see why you couldn't do that.

author: Terry Bites
author_rank: Super Contributor
timestamp: July 03, 2025, 06:07:23 pm
content: RFDx is right. Ground away to the regular plane.It is actually always better to connect these caps to ground proper.So far a signals are concerned supply rail are all "signal grounds" (some better some worse) thanks to decoupling aka bypassing.Ideally decoupling should appear as an AC short across the rails. I wish....Grounding via the "virutal earth" is not a good strategy as it will ineviatably present a higher impedance path to the capacitor currents.RFDx is right again about the ongoing error in describing rail splitters and references as "virtual grounds". Probably more accurately described as pseudo ground.A virtual ground is a current summing node in a negative feedback amplifier that has the same potential as the non inverting input. Since the potential of the non-inverting node is often at 0V or "ground" the inverting terminal is a "virtual ground". A term that goes way back to tube amplifiers. Yes, NFB was invented before transistors or opamps.

author: Zero999
author_rank: Super Contributor
timestamp: July 03, 2025, 08:43:55 pm
content: Virtual ground is another name for rail splitter.There's no rail splitter in the schematic, so it's impossible to comment.

author: RFDx
author_rank: Regular Contributor
timestamp: July 04, 2025, 02:21:51 am
content: Just because (almost) everyone (even TI) calls rail splitters virtual grounds does not make it right. A new ground reference generated with a rail splitter is as real as it gets.

author: Zero999
author_rank: Super Contributor
timestamp: July 04, 2025, 06:58:41 am
content: Quote from: RFDx on July 04, 2025, 02:21:51 amIt doesn't matter whether it's technically correct. Common usage matters more. Plenty of terms are technically incorrect, yet are widely used and accepted.

author: shaling
author_rank: Contributor
timestamp: July 30, 2025, 05:37:22 pm
content: Thanks all.This will simplify routing the PCB.