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Old 8th October 2017, 04:07 PM   #21
rsavas is offline rsavas  Canada
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Can you share the schematic(s) for the mono PCM1794A DAC's/IV convertors?

I assume that the SPDIF receiver design is Bryan's? I wonder why external buffers were used instead of using the 8 dedicated SPDIF internal to the WM8805?

No need for a transformer isolated SPDIF input?

Inputs should be terminated in 75 ohm R if using 75 ohm coax.

Phono pre-amp design for those that want one?

4+1 internal analog inputs might be a bit short, I guess it depends on each user and their setup.

What instrument did you use to measure 25uV of noise?

Last edited by rsavas; 8th October 2017 at 04:10 PM.
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Old 8th October 2017, 04:28 PM   #22
jwilhelm is offline jwilhelm  Canada
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We're still working on the DAC I/V converters. Valery's having trouble finding any useful info on the output signal from the DAC so we're prototyping and measuring for our self, but it's slow going.

I designed the SPDIF switch. WM8804/5 becomes stupid in software mode, much better IC in hardware mode.

The coax inputs are Rod Elliot's design. They are 75 ohm impedance. No transformer required. This is a prototype to see if the design will operate with all coax input SPDIF sources. I have a feeling it might not work with weak sources.

4 analog inputs is lots for most people these days. Everything is digital now. Phono preamp is coming.

I used a Rigol DM3058E to measure output noise.
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Old 8th October 2017, 05:39 PM   #23
rsavas is offline rsavas  Canada
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I got WM8804 mixed up with WM8805.
Yeah I guess making a discrete I/V is a lot more involved than just following the app note examples :-) I do wonder how one can tell/hear the difference in performance between discrete and opamp implementations.
I have not looked at this digital audio stuff in a long while. I know Cirrus bought out Wolfson. I thought that they were going to discontinue CS8406,8416, not the case.
I do see that Onsemi is sell the Sanyo part LC89091JA-H at a fraction of the cost of a WM8804, wonder what the differences are, have to look further, no H/W mode only S/W mode, maybe not brain dead though. Lots of work to evaluate each offering.
I am wrong, LC89091JA-H offers a H?W op mode.

Last edited by rsavas; 8th October 2017 at 05:44 PM.
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Old 8th October 2017, 05:48 PM   #24
jwilhelm is offline jwilhelm  Canada
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I built a first prototype for the DAC following the datasheets. Output was very low and the op amps were running far too hot for my liking. We've since figured out why the output was low. They don't tell you enough on the datasheet to be useful. Their suggested 10k resistor for reference current is way off.
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Old 8th October 2017, 06:18 PM   #25
rsavas is offline rsavas  Canada
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Makes you wonder how they can document something that does not work per the datasheet app note.
Did you look at
http://www.ti.com/lit/an/sboa237/sboa237.pdf
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Old 9th October 2017, 01:53 AM   #26
jwilhelm is offline jwilhelm  Canada
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I hadn't seen that application note. Thanks.
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Old 9th October 2017, 02:50 AM   #27
vzaichenko is offline vzaichenko  Russian Federation
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Quote:
Originally Posted by rsavas View Post
Makes you wonder how they can document something that does not work per the datasheet app note.
Did you look at
http://www.ti.com/lit/an/sboa237/sboa237.pdf
Hi Rick!

Thanks a lot for the link - we have found out a number of things, hitting some issues and finding the solutions, collecting information bit by bit from different sources - this paper finall confirms we were moving in the right direction

We were just exchanging our thoughts with Jeff about a few surprises, related to the way our custom I/V converters work, coming to some conclusions, when your link came in to clarify those points.

Now we know for sure how to make the thing working properly

Cheets,
Valery
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Old 9th October 2017, 04:11 AM   #28
anatech is offline anatech  Canada
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Hi Jeff,
So that's what you've been busy at!

I am interested with your project. As always, my bench is your bench if you ever need. One big change is I switched to LED overhead lights, so distortion readings don't have to be done in the dark anymore.

Just call, I'll make the time for you. I agree with your desire to have a better quality of optical encoder - strongly!

-Chris
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Old 9th October 2017, 03:02 PM   #29
rsavas is offline rsavas  Canada
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Hi Valery and Jeff

Glad I can help out in a small way, can never have too much design use information.
TI also has a AEC-Q100 qualified part, the DS for that details their recommended layout guideline as shown on p17 and maybe some other info since they had a second crack at a DS.
http://www.ti.com/lit/ds/symlink/pcm1794a-q1.pdf

I am puzzled as to why the opamps are running so hot in the PCM1794A application ckt?
What was the opamp supply voltages?

The same ckt is used in DEM-PCM1792, DEM-DSD1792, DEM-PCM1794, DEM-DSD1794 EVM User's Guide
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Old 9th October 2017, 03:18 PM   #30
jwilhelm is offline jwilhelm  Canada
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I was running the op amps at +/- 12V. Every board I've seen with this same design has the same hot op amps. They aren't outside safe operating temperature, but I prefer not to have anything generating that much heat if I can avoid it.
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