Noise Handeling With respect to PCB design. - Page 1
Forum section: electronics / general

author: SajjadBro
author_rank: Contributor
timestamp: February 06, 2012, 11:34:57 pm
content: This is my first post. I like how Dave delivers his knowledge.I was wondering that one thing Dave haven't touched yet is the Noise. Recently i designed and etched my PCB board having a 10 bit ADC on chip PIC16f876A. Vcc to pic = 5.12V so that 5.12/1024 = 5 mV exactly. There was noise in my PCB of 8 to 12 mV so my ADC value were not stable. Dave used 12 bit ADC and used 1mV step size in his power supply design. I wonder how he made such low noise circuit.There was another noise to coming from my energy saver bulbs. 6 bulbs are installed at 2.5 feet above my work bench at a distance of 2 feet from each other. A little bit of noise was also being picked from them. i made PCB 3 times and modified my lay out but no good.So i hope you people agree for having Dave a video to handle different type of noises. Thanks.

author: Rerouter
author_rank: Super Contributor
timestamp: February 07, 2012, 01:27:58 am
content: usually simple decoupling, ceramic cap right next to the supply pins, and the output pins, really if you knew what frequency your noise was, you could create a lowpass filter with a cap and inductor, both only effect changing signals, so are used in dc applications to clean up noise

author: ejeffrey
author_rank: Super Contributor
timestamp: February 07, 2012, 09:25:50 am
content: Dave used external ADCs in part because those built into microcontrollers are typically quite high noise. Still, 12 mV is pretty high. Can you explain how you did your measurement? What type of power supply are you using? Can you post a schematic and your layout, or even a photo of your board? Pro tip: you don't need to post a 3000x2000 pixel image, scale it down to something still readable but that fits on a screen if possible. Some people here have problems with that...It is pretty unlikely that your lamps are contributing to the noise. Much more likely it is noise coming from within your PCB and/or power supply. Like Rerouter, I suspect improper bypassing on the power pins of the MCU. My second guess would be inadequate power supply filtering.

author: Psi
author_rank: Super Contributor
timestamp: February 07, 2012, 10:50:13 am
content: Quote from: ejeffrey on February 07, 2012, 09:25:50 amheh, don't underestimate the effects of a bad CFL lamp, some of the designs are awful and do put out quite a bit of EMI.I had one that would randomly turn off my TV.To this day i'm still not sure exactly how it did it. If it was IR noise hitting on the remote control code for "off"Or if the EMI was crashing the processor in the TV.But in any case, i proved the cause was the CFL in the room.

author: SajjadBro
author_rank: Contributor
timestamp: February 07, 2012, 12:33:44 pm
content: Thanks for the replies guys. I am also making a Constant Current Load but up to 10 Amps.I am using IRF3710 x 4 in parallel. And i am using 0.1 Ohms 100W resistor unlike dave as he used 1 Ohms because my current is much higher. The 0.1 Resistor is of golden type almunimum sink resistors. I am using 100 W instead of 10W because i know resistance varies due to heat so it needed to be cool. Rest i am using PIC and LCD to Display all those figures.Well I figured something out.1st problem: i was using a 10 turns POT like dave used but value was 100K. i used a buffer (LM358) and after that a gain of 5.12 then to ADC. I probe the middle leg of the POT and there was noise as in Fig 1. I guess the POT was catching noise and it was transferred via buffer and multiplied by gain.2nd. when i replaced the POT 1K and attached 10nF cap from mid of POT to ground,the noise was gone(Didn't check without the cap). but wait some thing is still left. It was the ringing effect as i observed in many SMP power supplies. I guess it was suppressed by the higher noise when 100K POT was installed. the Magnitude of the first ring goes up to 10mV. The power supply is 12V 2A from a wireless modem, its of good quality. but when i plugged simple liner transformer power supply then it was gone. but still random noise of upto 2mV exist and no more greater then 50Hz.My requirements have been fulfilled.but i think 2mV noise is also too much for 12bit ADC. DO you think it will be gone when i will use 12 bit ADC.My step size is 5mV/bit and LM358 offset is upto 7mV. Could there be a connection between these two?If i cover the inside of power supply with aluminum foil, will that noise be gone?

author: ejeffrey
author_rank: Super Contributor
timestamp: February 07, 2012, 01:07:22 pm
content: Quote from: Psi on February 07, 2012, 10:50:13 amI should have been more precise. I doubt that radiated emissions from the CFL is affecting his circuit at all. He easily could be seeing conducted emissions coupled via the power supply. Unless the CFL is really awful, I would chalk that up to insufficient power supply filtering/rejection, even if the source of the interference is the CFL. I think the fact that they are mounted directly over his workspace is a red herring. The real potential issue would be if they were on the same electrical circuit. In your particular case I would guess that the CFL flickering was confusing the IR receiver, but a glitch on the MCU reset pin is not out of the question.

author: SajjadBro
author_rank: Contributor
timestamp: February 09, 2012, 03:16:40 am
content: i have been reading to separate Analog Gnd from Digital Gnd. by doing so i have managed to reduce noise at the POT side. but still so much noise on the ADC pin or u can say that noise is only in digital section. This noise is being cause by the internal switching i think and i am using PWM at 20KHz and 30% duty cycle. Anyways how can reduce that noise at ADC PIN?

author: nessatse
author_rank: Regular Contributor
timestamp: February 09, 2012, 10:49:49 am
content: Use averaging/oversampling to get rid of the major noise components.I think 2mV (that is about 0.5 LSB) of noise in your case may actually be desirable. It allows you to do oversampling and get 11 or 12 bits of resolution out of your 10bit ADC. I would not worry too much about it.

author: dfnr2
author_rank: Regular Contributor
timestamp: February 09, 2012, 11:39:19 pm
content: It sure could be bad layout. It could also be conducted interference, as suggested. I don't think anyone can guess what's wrong until you post some schematics and pictures. Also, I doubt you'll really gain much by separating analog and digital grounds unless your circuit is pretty special. How many layers is your board? Post a picture of the layout, along with a description of the layer stack--that will give us something to work with.Good luck,Dave

author: Neilm
author_rank: Super Contributor
timestamp: February 10, 2012, 06:49:40 am
content: Quote from: SajjadBro on February 09, 2012, 03:16:40 amA few years ago I had to redesign a load on instruments that had failed a new EMC spec. The instruments were older models that had been designed to meet much more relaxed specifications. (the new EMC spec did not allow the reading to go out of its specification). In every case my first attempt was to make the PCBs 4 layer and connect all the separate 0Vs (Analog, digital, "low noise"). In most cases, this one change fixed all the problems.Neil