OPA1632Ds getting super hot in a PCM1792A DAC I'm building - Page 2 - diyAudio
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Old 19th June 2017, 04:50 AM   #11
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Thanks for the oscillator recommendations!

I got the Bink test CD and made some measurements. It looks mostly clean, but I did manage to catch something weird on one of the channels. The blue/purple are +/- of one channel, and the cyan/yellow are the +/- of the other channel.
Are these tiny spikes on the yellow channel the mess I am looking for?

Unrelated, but my MCK signal looks terrible anything I can/should do about that?
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Old 19th June 2017, 06:40 AM   #12
Shinja is offline Shinja  Japan
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In the I/V stages, OPA1632's Vocm pins looks problematic.The Vocm pin set *output* common voltage.+- input pins are virtual-shorted each other, but not to Vocm.PCM1792A output "sources" (↔sink) 6.2mA at digital silence.So OPA1632's +- input pins potential will be 0V+750Ω*6.2mA=4.65V which exceeds PCM1792A's limitation, or even its supply voltage, it must be clamp protected somehow.
I don't know where your waveform probed, but I can't see why, how your DAC runs in such a condition.The solution is to set Vocm pin -4.65V or sink common 6.2mA with CCS.
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Old 19th June 2017, 06:45 AM   #13
Dimdim is offline Dimdim  Greece
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Originally Posted by eranrund View Post
Unrelated, but my MCK signal looks terrible anything I can/should do about that?
I don't suppose that by any chance your oscilloscope's probe is set to x1?
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Old 19th June 2017, 07:20 AM   #14
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Quote:
Originally Posted by Shinja View Post
In the I/V stages, OPA1632's Vocm pins looks problematic.The Vocm pin set *output* common voltage.+- input pins are virtual-shorted each other, but not to Vocm.PCM1792A output "sources" (↔sink) 6.2mA at digital silence.So OPA1632's +- input pins potential will be 0V+750Ω*6.2mA=4.65V which exceeds PCM1792A's limitation, or even its supply voltage, it must be clamp protected somehow.
I don't know where your waveform probed, but I can't see why, how your DAC runs in such a condition.The solution is to set Vocm pin -4.65V or sink common 6.2mA with CCS.
Why do you think it exceeds it's limitation? It's analog supply voltage is +5V. Waveforms were probed on the PL connectors - so at the end of the signal chain.
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Old 19th June 2017, 07:23 AM   #15
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Quote:
Originally Posted by Dimdim View Post
I don't suppose that by any chance your oscilloscope's probe is set to x1?
Hm, good call. Pretty sure it was set to x1. I'll try again with x10. Thank you!

Edit: Looks a bit cleaner with x10 but still not a nice square wave

Last edited by eranrund; 19th June 2017 at 07:36 AM.
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Old 19th June 2017, 11:27 AM   #16
amb is offline amb  United States
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Don't expect a nice looking square wave if you're measuring tens of MHz with a regular scope's test probe. The ground lead has enough inductance to make the waveform ugly. Read up on RF measurements...
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Old 19th June 2017, 12:11 PM   #17
Shinja is offline Shinja  Japan
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Originally Posted by eranrund View Post
Why do you think it exceeds it's limitation? It's analog supply voltage is +5V. Waveforms were probed on the PL connectors - so at the end of the signal chain.
Oh, I remembered the supply is 3.3V.Then it is inside of rail voltage.Although that, I recommend the node voltage to be 0V or so as nominal condition in the datasheet. When the node voltage is 4.65V , internal "current segments" (CCSs and switches) comprised of Pch MOSFETs are running low Vds and may affect switching characteristics - it may be the cause of the glitches in your waveforms.
I found similar discussions in this thread.Current differential to voltage line level from PCM1794 et al
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Old 19th June 2017, 05:15 PM   #18
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Originally Posted by amb View Post
Don't expect a nice looking square wave if you're measuring tens of MHz with a regular scope's test probe. The ground lead has enough inductance to make the waveform ugly. Read up on RF measurements...
Didn't think 12MHz would be fast enough for that effect... I'll google and read about that. Thanks
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Old 19th June 2017, 05:16 PM   #19
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Quote:
Originally Posted by Shinja View Post
Oh, I remembered the supply is 3.3V.Then it is inside of rail voltage.Although that, I recommend the node voltage to be 0V or so as nominal condition in the datasheet. When the node voltage is 4.65V , internal "current segments" (CCSs and switches) comprised of Pch MOSFETs are running low Vds and may affect switching characteristics - it may be the cause of the glitches in your waveforms.
I found similar discussions in this thread.Current differential to voltage line level from PCM1794 et al
I'll take a look at that thread. Thank you!
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Old 19th June 2017, 08:56 PM   #20
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Shinja - thanks again for pointing out the Vocm issue and the other thread. While I am not sure it contributes anything to the heat issue, it's definitely a problem in the design. I'm wondering where to go from here. I could create a -4.65V reference on Vocm but to my understanding this will result in DC offset I'll need to remove with capacitors. Is that correct?

Would using a CCS be better (and not result in DC offset)?
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