PSU oscillating

[apolon34]

Hi,

I've designed a PSU for the 5v part of my philips cd-pro2.

you can find schematic and layout there:
http://belinfabien.free.fr/cd/cd diy/carte alim meca.htm

I use ad797 for op-amp and all caps are os-con SG series.
From the schematic, I replaced R9 by a wire. With R9, I got some spikes at low freq with cd-pro2 plugged. Without, it oscillates at several Mhz

I also removed C12 (100nf) and the oscillation ampliftude decreased to about 0.3vpp

unregulated psu is around 9.5v, output is at 5v.

 

[apolon34]

hi,

it seems that adding a 1k/100uF filter on the opamp psu nearly solved the problem. I still have some 20mVpp of oscillation. The opamp psu seems to be clean so there probably is another thing, but I don't know what.

 

[Mr Evil]

According to the NE5534 datasheet it's only compensated for a minimum gain of 3, but in that circuit it has a gain of 1, plus it has extra transistors inside the feedback loop. So try adding an extra compensation capacitor between pins 1 and 8 of maybe 22pF or so.

EIT: Oops, I see you're using AD797. The datasheet for that one looks like you need to take even more care with compensation.

 

[apolon34]

Today i went to the electronic lab of my job to test the PSU. It seems that my trick to add a rc filter on opamp PSU solved the problem. I don't know why I was seeing oscillation with my scope. I'll look again this evening but I hope the problem's gone.

If not, i'll try to add a serial RC on opamp output

thanks mr evil