aaron_do wrote on Nov 29th, 2009, 5:04pm:Hi Steve,
thanks for the help. Actually I was also thinking that maybe Ken was right. I was going to hook the power supply board up to a network analyzer and see if there are any high impedance frequencies.
Another thing is that I used electrolytic capacitors for the power supply bypassing. Is this going to affect the oscillation frequency? I was told I should switch to tantalum...
thanks,
Aaron
I can tell you up front that if you hook a network analyzer up to all that wiring on the prober, you will get weird results. What do they mean--who knows? I favor a different approach. I wasn't kidding about the tweezers. Hold the tweezers and lean the same hand on some grounded surface. If you touch the supply pins and get changes in your noise output, this is pretty good evidence that your supply bypassing is suspect (maybe not 100% of the problem but significant).
The frequency where bypass capacitors become ineffective is pretty much where they go self resonant (or resonant with the external lead length on the prober). Electrolytic caps have higher internal inductance than tantalums. We typically use 4-10uF tantalums plus 0.1uF ceramic or film caps in parallel, on each supply to ground. The simple minded idea is that when the big cap quits working due to resonance, the little cap takes over (which has a much higher self-resonance), but this is a simplification. Actually, the resonance on the big cap is still there and can cause problems in some cases. I'd go with tantalum plus 0.1uF in parallel, as close as you can get to the chip (I realize there are probe pins in the way). Keep lead lengths (supply and ground side) as short as possible. You can't get rid of the probe card, but no meandering traces!
If you still have problems, sometimes you can add resistors to damp out resonance. One place is to add a series R-C between the supply pin and ground--like maybe 25 Ohms in series with 0.1uF. You could also try supply to supply if you have split supplies (ground on your board may not have any relation to what the chip is seeing as ground). The other place you could put resistors is in series with probe pins. There are obvious problems with this idea--you're very limited how much resistance you could put in a current carrying line, like the supply. You can put them in series with high impedance pins like inputs, but be careful you don't build a filter and ruin what you're trying to measure.