Tell me about ripple. or noise. - Page 1
Forum section: electronics / beginners

author: jaunty
author_rank: Regular Contributor
timestamp: November 10, 2020, 10:03:11 am
content: So i have this unregulated 8v DC rail in a piece of gear. It's measuring 10V ± 1.3V ripple and noise. 10V is well within spec of the devices on that rail and it's almost entirely noise (rather than ripple) i would guess as the PS has already been sorted for the most part. I've never seen any information about what exactly causes that amount (or lesser amounts even) circuit noise apart from bad caps etc. is it bad/shorted P/N junctions in op amps or logic ICs dumping current onto the ground plane? I'm wondering what to watch for as i go through the troubleshooting of other areas of this thing. Also i'm wondering just how much ripple or noise one can have in a circuit before it starts affecting some of the active devices. Normally I try to keep most of my PS rails well below 40mV if at ALL possible.

author: bob91343
author_rank: Super Contributor
timestamp: November 10, 2020, 10:13:16 pm
content: Your explanation is too disjointed to allow me to formulate an intelligent answer.The amount of ripple a circuit can tolerate is unique to each circuit. You mention noise but don't describe it.

author: Electro Fan
author_rank: Super Contributor
timestamp: November 10, 2020, 11:02:20 pm
content: It seems like your questions might fall into a few areas:1. Power Supply ripple which you think you have sorted.2. Possibly noise such as common mode which might be present.3. The levels at which ripple or noise might impact the ability to discern signal integrity.As for some of item 1 and also item 2, it might be worth watching this to confirm you have those well sorted.https://www.eevblog.com/2014/03/21/eevblog-594-how-to-measure-power-supply-ripple-noise/As for item 3, you might look this over:https://www.allaboutcircuits.com/textbook/digital/chpt-3/logic-signal-voltage-levels/https://forum.allaboutcircuits.com/threads/cmos-and-lvttl-voltage-levels.62014/http://www.interfacebus.com/Logic_Family_Noise_Margin.htmlHope something here might help.

author: jaunty
author_rank: Regular Contributor
timestamp: November 11, 2020, 09:27:11 am
content: yes it's definitely neither (1) nor (2) - sorry if i wasn't making that clear enough. the ps is all sorted and behaving well - i'm just finding tons of noise downstream and wondering what likely candidates can cause it ... i'm guessing a buggered op amp or piece of logic

author: jaunty
author_rank: Regular Contributor
timestamp: November 11, 2020, 09:28:26 am
content: i said it's all noise and little ripple. over a volt's worth on a 10V rail. how else would you like me to describe it?

author: bob91343
author_rank: Super Contributor
timestamp: November 11, 2020, 07:54:51 pm
content: If you have noise unrelated to power line, it's caused by a combination of intermittent load and insufficient bypass capacitance.When a power supply is loaded, its output voltage drops due to its internal impedance. When you have filter capacitors, they supply some of the energy that momentarily avoids some of the voltage drop. So you have transient loads that draw too much from the bypass capacitors.On the other hand, if the transient loads are very short in duration, the capacitors can't help because they have too much ESL, equivalent series inductance, and so should be bypassed by capacitors better equipped.How fast is this noise? If it's less than a few microseconds in duration, the second explanation above obtains. If it's low frequency, the first explanation is an answer.