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Cheap High Quality ADC

March Audio

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#1
Want to do some audio recording or some measurements?

A cheap but high quality solution is to use a Texas Instruments PCM4222EVM board. My experience is that this boards performance and sound quality beats internal/external sound cards costing many times more.

The downside? Well there is a bit of DIY involved, but its very simple and even the most inexperienced with electronics should be able to manage it. Also, sample rate selection is on DIP switches. However I dont see this as an issue - just set and leave them to record at higher rates and downsample in software later if necessary.

The board is based, not surprisingly on the TI PCM422 ADC. http://www.ti.com/product/PCM4222

It includes fully differential input buffer (XLR in), dual AES/SPDIF and I2S outputs. http://www.ti.com/tool/pcm4222evm

The board is available from the usual suppliers, the best thing being the price ~ $150 US.

http://www.mouser.com/ProductDetail...M4222EVM/?qs=sGAEpiMZZMtmW1oUCKTs6uC2DXSEnixQ

http://uk.farnell.com/texas-instrum...pn=true&categoryId=&searchRef=SearchLookAhead

http://www.digikey.com.au/product-search/en?keywords=pcm4222evm

Here is a piccy

PCM4222evm small.jpg


To get the board working all you need are 3 power supplies +-15volts and 5volts (0.5 amp should suffice). If you dont have a SPDIF input on your PC soundcard the simplest and cheapest option to get data into your PC is the MiniDSP ministreamer SPDIF to USB card @ $35.

https://www.minidsp.com/products/usb-audio-interface/ministreamer

However this will only accept sample rates up to 96kHz. I have also used the Yellowtec PUC2 lite AES to USB converter up to 192 kHz.

http://www.yellowtec.com/en/products-lp/puc2-lp/puc2-lite.html



To be continued......
 
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amirm

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#2
Thanks a bunch. Every time I look at getting this ADC, I run into a stumbling block of finding a good, cheap AES input to go with it. Didn't know minidsp had such. The Yellow is around $500, yes?

I will go ahead and order the board and minidsp for now.
 

Opus111

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#3
How are pin1s on those XLRs interconnected? Looks like that board might exhibit common-mode noise problems if TI haven't followed industry best practice.

I've downloaded the EVM pdf now from TI and had a look at pin1s. They have an on-board link which permits them to go to the groundplane. This looks to be a mistake - the link should connect them to the GND at the power input via a dedicated track so there's no common-impedance coupling. Best make sure the pin1 grounding link isn't present.
 
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March Audio

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#4
How are pin1s on those XLRs interconnected? Looks like that board might exhibit common-mode noise problems if TI haven't followed industry best practice.

I've downloaded the EVM pdf now from TI and had a look at pin1s. They have an on-board link which permits them to go to the groundplane. This looks to be a mistake - the link should connect them to the GND at the power input via a dedicated track so there's no common-impedance coupling. Best make sure the pin1 grounding link isn't present.
Yes you can see the jumper to the left of the XLR socket. TBH the only noise problem I have encountered whilst using this board was when an Uptone Regen was used. Amir also encountered this specific problem using different kit, so its not the fault of the TI. Otherwise its very quiet.

The only thing I may experiment with is ADC reference caps to further reduce the low frequency noise.

TI noise floor.png
 
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March Audio

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#5
Thanks a bunch. Every time I look at getting this ADC, I run into a stumbling block of finding a good, cheap AES input to go with it. Didn't know minidsp had such. The Yellow is around $500, yes?

I will go ahead and order the board and minidsp for now.
Yes the Yellowtec is not a cheap solution.
 

Opus111

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#6
The noise problems probably won't show up in the audio band - reason being common-mode noise is generally capacitively coupled via interwinding capacitance in mains transformers. Hence only HF stuff gets through. Or/and its SMPSU noise which is always ultrasonic.

I am looking at the OPA1632 input stage and may have suggestions on that too, given a little time.

Thanks for putting up the noise floor - answers the question about your MDAC recording nicely.
 

Opus111

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#8
Yeah you guessed it - IMD between the ultrasonics and the in-band stuff leading to a rise in the general noise floor. But in the absence of anything in-band, no noticeable problems with the noise floor.

Common-mode chokes certainly help - but they're best on the primary side where the currents are lower. That way you get a higher inductance (=higher impedance) in the same physical volume.
 

Mivera

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Opus111

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It might well be nice but definitely a 1% spec for output ripple and noise means you'd need another filter (or linear regulator) beyond it.
 

Mivera

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#11
It might well be nice but definitely a 1% spec for output ripple and noise means you'd need another filter (or linear regulator) beyond it.
You don't think the ones on the board would be enough?
 

Opus111

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#13
From looking at the schematic, the main supplies to the OPA1632 input buffers don't have any regulation. They're probably the most critical.
 

Mivera

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Mivera

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March Audio

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Mivera

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Cool! Didnt know about that one. Definite upgrade.
I talked him into building that last May. He said thanks for the idea and he would work on it. Then finally a couple months ago it was ready. It's a very powerful chip and the only USB interface out there that uses it so far. It's capable of DSD 1024. But it needs to be programmed to get full capabilities out of it. He only just got basic firmware going so far.
 

Mivera

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amirm

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