What is this trap for young players I'm in? Earth Ground and noise - Page 1
Forum section: electronics / beginners

author: Galaxyrise
author_rank: Frequent Contributor
timestamp: August 01, 2016, 02:37:26 am
content: I've seen and heard plenty about when oscilloscope ground can cause shorts, but not much about how it interacts with a circuit that is isolated from earth ground. I foolishly assumed that I could probe a floating circuit freely! It all started when I put earth ground on an opamp that was powered from a mains transformer, and the opamp to oscillated. So I switched to a battery, and the oscillations went away. But it still matters where ground is connected and I don't understand why.I reduced it down to a fairly trivial circuit: 9V battery -> 10k resistor -> diode. This is sitting in a cookie tin. If I put the scope probe across the diode with the ground on the negative side, my scope is at its noise floor. But if I flip the probe around then I get a bit of noise (about 300uVpp) and bit of RF. If I attach the cookie tin to earth ground, then the RF goes away, but the extra bit of noise is still there when probed "backwards".Why does it matter where earth ground goes in this circuit?PS Differential probes should arrive next week

author: kg4arn
author_rank: Unknown
timestamp: August 01, 2016, 09:27:56 am
content: Can you send some screen shots of the scope output?

author: tszaboo
author_rank: Super Contributor
timestamp: August 01, 2016, 09:57:52 am
content: Please dont simplify the circuit. The diode will act differently than an opamp. 30pF on an opamp input can upset the circuit to create some noise and what not, but without knowing exactly what are you doing, we cannot answer your question.

author: Galaxyrise
author_rank: Frequent Contributor
timestamp: August 01, 2016, 02:10:18 pm
content: Quote from: NANDBlog on August 01, 2016, 09:57:52 amThe battery+resistor+diode is exactly what I am doing. The opamp investigation led me here, but right now I just have the battery, resistor, and diode in the circuit; and that is what I am probing and still not understanding.I will endeavor to get some pictures of the circuit and the scope results tonight.

author: StillTrying
author_rank: Super Contributor
timestamp: August 01, 2016, 02:36:46 pm
content: Everything contains electrical noise (and capacitance) relative to ground, as you can see if you touch just the scope probe's input.In the 1st diagram you provide a low impedance path to ground which shorts out most of the noise, in the 2nd ground is connected through a 10K resistor, so some noise is still there.

author: David Hess
author_rank: Super Contributor
timestamp: August 02, 2016, 12:50:39 am
content: My guess is that connecting the oscilloscope ground to the diode couples ambient RF into the cookie tin which the diode rectifies producing the result you see. The scope probe is high impedance so it does not couple as much into the cookie tin.If you added a couple of feedthrough capacitors to the cookie tin, then the noise should go away.When probing sensitive nodes like virtual summing junctions, you can sometimes tie a small resistor in series with the probe or multimeter to further isolate the probe capacitance from the circuit. Better is of course to avoid probing this junctions.

author: Galaxyrise
author_rank: Frequent Contributor
timestamp: August 02, 2016, 05:13:38 am
content: Quote from: kg4arn on August 01, 2016, 09:27:56 amChannel 1 is AC coupled, 20MHz filter enabled, x1. Acquisition normal. I think all the other relevant settings are displayed.I didn't take a picture of the third setup, but the green wire you see in the second picture has been plugged in at the top, where the resistor and diode meet.

author: Galaxyrise
author_rank: Frequent Contributor
timestamp: August 02, 2016, 06:06:04 am
content: Quote from: David Hess on August 02, 2016, 12:50:39 amAh... perhaps the battery leads are most of the pickup. So like StillTrying was suggesting, a noise source in the circuit relative to ground: If only the battery is a noise source with respect to ground, then I can simulate the behavior I'm observing.Thanks a lot for the ideas! I'm going to order some feed-through capacitors and see if I can actually get the circuit to be quiet under measurement, but I feel I have a much better grasp over what could be happening.Best regardsGalaxyrise

author: EPTech
author_rank: Regular Contributor
timestamp: August 02, 2016, 08:35:12 am
content: Hi there,Noise is abundant. Try switching off LED lights and gas discharge lamps with electronic ballasts, in essence anything with switching supplies in them, you will be amazed. I always do this when making measurements on sensitive circuits. In general I leave them on because it is also a kind of immunity test, for example to test the tightness of your tin. Feed through capacitors will definitely help.A nice tip I got is to throw away the switch mode LED driver in my work space lighting and use a linear supply with current sourcing capabilities. A linear lab power supply will also work.As to why the noise is greater at one point than the other, I am not sure but I guess since the circuit is floating, the longer negative wire will pick up more noise than the shorter diode.

author: qno
author_rank: Frequent Contributor
timestamp: August 02, 2016, 01:30:10 pm
content: There is a ground differential you are measuring.Scope has its own ground.Your setup uses a tin can capacitively coupled to the breadboard and more important the battery (larger area) that is capacitively coupled to the surrounding.When you put the scope ground on the ground to the board ground the the noise picked up by the tin can is related to scope ground and thus cancelled. If you put the tip on the can (capacitively coupled |) you pickup all the noise your can is picking up from the environment.If you remove the tin can from under the breadboard it should be the same.

author: StillTrying
author_rank: Super Contributor
timestamp: August 02, 2016, 01:53:38 pm
content: Quote from: Galaxyrise on August 02, 2016, 06:06:04 amExternal noise sources being injected into the circuit.I did wonder if you could simulate the effect by injecting some high impedance noise into the circuit to see the effect, when I saw you'd had a go, I had a go!Edit;"I get a bit of noise (about 300uVpp) and bit of RF."I just threw those values in the circuit and Scope2 shows 340uVpp.