Post by thread:PIC causing noise (?) on power supply

Now that I received my 100MHz Tektronix scope from Ebay (let's ignore the brokerage fee that got tacked on) I can finally get better insight as to the operation of things I'm currently working on without having to run around the world looking for a vacant scope. One thing I noticed is that when I'm running a 4MHz 16F876 (Yeah, same original project I initially was asking about here, how to do A/D with it) from a 7805, and then I also power a filter (yeah, that formerly loaded down output thing that now works) from the same regulator, the filter output shows interference from the PIC circuit. When I power just the filter, it's fine, but as soon as I throw the pic on there, the sinewave starts doing the wave itself...the filter output wave is wavering so there must be a somewhat simple thing I can do to snub this effect... The 0.1 on the op amps didn't do anything...so I was out of ideas.

-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 >One thing I noticed is that when I'm running a 4MHz 16F876 (Yeah, same >original project I initially was asking about here, how to do A/D with it) >from a 7805, and then I also power a filter (yeah, that formerly loaded down >output thing that now works) from the same regulator, the filter output >shows interference from the PIC circuit. 1: bypass the pic properly. 0.01uF has lowest impedance around 4 MHz, you may need 0.0033 for third harmonic supression, and a 0.1 or even a 1uF for slower events at higher currents. 2: pic's power and ground should route back to the power supply separate of anything else. (I separate the micro's supply track, and route power through an EMI filter ($0.25) to the bypass cap located at the uP ground pin, and there on to the VCC pin. It's a compromise, but it works. Ground is flooded.) 3: Put your scope probe on ground. See noise? Got current through ground point, and a multiple ground point problem. (See Jim Williams's book on analog design) For measurements in noisy environments, the scope earth ground may have to be lifted. This poses a safety problem, in that the case of the scope can now be raised to lethal voltages, but the NEC can't change the laws of physics. Which scope did you get exactly? -----BEGIN PGP SIGNATURE----- Version: PGPfreeware 6.5.2 for non-commercial use <http://www.pgp.com> iQA/AwUBOMzJ2IFlGDz1l6VWEQIr5ACeMOgS/OFqcHJS9yXbhwV+QOSidJsAn2LB eTXjQULqSc5QrxbwTpwrVkvr =nS1+ -----END PGP SIGNATURE-----