Hello,
I did design a product that include a analog and a digital section. It's a form of digital sound recorder. Many design restriction are in consideration that I will not describe here. Just to say, that I must work with off the shelves low cost parts and it must run a long time on batteries.
The product is operational, but I did try to add some improvement and I run in some strange behavior that you may give me help.
Until now, both section (A&D) was running on the same switching supply. The model I was using was not very good and it switching frequency was low enough to give us trace of it in the analog signal. So I replace it by a higher frequency switcher with good result. So trying to push it, I decide to use two supply's, one for each section. The analog section only drawn
25ma while the digital section drawn 80ma, So we talk very small supply using National LM2672 switcher. The two supply option, give me the option to power down the analog section while not use.It work, but I found a 4khz tone in my recording. I can not figure out where it come from except that if I stop the switcher supply for the analog section and use the digital power all over, the tone disappear. I have good scopes & tone generators, but not much more sophisticate equipments. I try to see the tone on my analog signal without success, I can see it on the scope, so it is very low level, but it is audible when played back.
That 4khz tone can not be the switcher frequency that run around 260khz, but I suspect it can be a interference between both supply, maybe on the A/D chip that use both supply. Anyway, I can not find a simple way to get rid of it.
Any suggestions may be welcome to correct my problem. If I can not find any solution, I will have to go back to the one supply solution and find a way to clean the digital +5vdc to be use for the analog section.
Bye
Jacques