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Hi All
Just assembled and am testing a Bufallo-III/PlacidHD/Legato/PlacidHD-BP setup.
I've run up against a problem in that there is a 258kHz sawtooth ripple of about 62mV on both AVCC outputs of the AVCC module. The excursions of the ripple are around 3.20 mV to 3.82mV. Both DVCC supplies are clean. The frequencies of the left and right side ripples are not synchronised.
The ripple perculates through the the outputs of all 8 DAC channels (as one would expect, I suppose).
I've unhooked the Legato and it's power supply as well as the data source to no effect. The Placid/HD output is clean and all voltages (with the exception of the two AVCC outputs) are nominal as per the integration guide.
On a side note, with the system all hooked together I was able to drive it with an I2S PCM source and obtain the expected output from the Legato, albeit with the sawtooth ripple superimposed. So I'm pretty sure the basic configuration is OK.
Any ideas anyone?
Cheers
Joseph
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That is unusual. I have never seen this occur myself. One thing you could do that might resolve the issue easily - overcompensate the opamps (it is a dual) with 22pf(22-100pf should be OK) from -IN to OUT. This will slow down the error correction slightly but will make the error amp rock stable. Perhaps there is some EMI that the AVCC module is sympathetic to. The attached PIC shows convenient soldering points in case you wish to try it. Edited by user Saturday, July 28, 2012 6:27:42 PM(UTC)
| Reason: Not specified Russ White attached the following image(s):  AVCC_R2_CC_PCB.jpg (73kb) downloaded 198 time(s).You cannot view/download attachments. Try to login or register.
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 1 user thanked Russ White for this useful post.
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Thanks for the quick reply Russ. I tried fitting 0805 COG caps as per your annotated layout diagram. I found that 100pF got rid of the oscillation completely. 47pF had no effect on it by the way, so I went straight to your suggested maximum value. For the record I've attached a 'scope image of the ripple in case anybody else comes up against this. Everything was unpowered and disconnected except the Bufallo-III and the Placid-HD so I'm not sure about the EMI angle. Whatever: Both AVCC outputs are clean now at 3.6V as is the signal outputs from the Legato so I can move on to the next stage of the project. Thanks for your help. Joseph Joseph attached the following image(s): AVCC_ripple.JPG (141kb) downloaded 72 time(s).You cannot view/download attachments. Try to login or register.
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Thanks for the update.
Glad it worked. I could only guess... I double checked a few stock AVCC modules I have here and they tested clean. Still I believe I will add a spot for extra compensation on the next run of AVCC boards though as I think its probably a good idea in any case.
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Hi Russ, Had the same problem but was solved with 120pF. I just wondered if there isn't other way to get rid of this oscillation? Maybe a snubber on output? I don't have the schematic in order to simulate and find myself what is going on ... Kind regards, Mihai Edited by user Thursday, October 18, 2012 11:47:16 AM(UTC)
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The very best way to solve it with the version you have is to either replace the op-amp or use the extra compensation
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Originally Posted by: Russ White  The very best way to solve it with the version you have is to either replace the op-amp or use the extra compensation What op-amp do you recommend? I believe that putting 120pF in order to reduce loop gain is not a very good solution because will also reduce the speed. Edited by user Friday, October 19, 2012 5:07:40 AM(UTC)
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Well qualified guess I think. Or OPA2211.
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We use the OPA2209 in the new version (soon to be released).
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Any reason for not using the 2211, it seems to have even lower noise (I haven't compared the data sheets yet) Edited by user Friday, October 19, 2012 2:57:43 PM(UTC)
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Originally Posted by: avr300 Any reason for not using the 2211, it seems to have even lower noise (I haven't compared the data sheets yet) It is a very good replacement but much more expensive. I think is a must for Trident designated for MCLK clock ;-)
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Hi Folks,
had the same problem here. 100pF across the op-amp as indicated by Russ solved the problem!
OPA 2211 are on their way...
Cheers,
Max
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