PSU oscillating

apolon34 · 21st June 2005, 11:44 AM

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%20d...lim%20meca.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 · 23rd June 2005, 09:30 PM

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 · 23rd June 2005, 09:51 PM

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 · 24th June 2005, 03:20 PM

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

21st June 2005, 11:44 AM	#1
apolon34 diyAudio Member Join Date: Jan 2004 Location: rouen	PSU oscillating 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%20d...lim%20meca.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.

23rd June 2005, 09:51 PM	#3
Mr Evil diyAudio Member Join Date: Aug 2004 Location: Behind you	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. __________________ https://mrevil.asvachin.com/

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23rd June 2005, 09:30 PM	#2
apolon34 diyAudio Member Join Date: Jan 2004 Location: rouen	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.

24th June 2005, 03:20 PM	#4
apolon34 diyAudio Member Join Date: Jan 2004 Location: rouen	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