Using The TI PCM4222 A/D Evaluation Module

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harty
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Re: Using The TI PCM4222 A/D Evaluation Module

Post by harty » Fri Dec 01, 2017 6:29 pm

TI have informed me that the EVM was never meant to be chassis mounted.

Although the pdf seemed to indicate one could have a shielded and secure environment to test.

" The PCM4222EVM provides a complete environment for evaluating the functionality and performance of the PCM4222
integrated circuit." "J7 J9 = neutrik 3-Pin Male XLR Chassis Connector, Horizontal PC Mount."

Engineer says a de-soldering tool is required to remove the chassis AES before mounting as the wrong footprint has been used to chassis mount.
and will consider updating the PDF in next edition.

So it seems soldering wires underneath to a new neutrik chassis connector might be the best option. rather than risk de-soldering the XLRs.

harty
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Re: Using The TI PCM4222 A/D Evaluation Module

Post by harty » Tue Dec 12, 2017 7:45 am

HI

I managed to get the PCM4222evm working, I had some DC offset issues only if the EVM high pass filter was not turned on.

test 1


computer> RME balanced >> EVM AES >> RME AES/computer = No problem. ( even with the EVM HPF switched off) very low noise

test 2 (same as before but insert an external soundcraft preamp)

computer> RME balanced >> balanced preamp unbalanced out >> EVM AES >> RME AES/computer = high DC offset (but not if the PCM4222EVM has its HPF set to on.

test 3
held held media player

android phone unbalance out >> EVM AES >> RME AES/computer = high DC when the phone was in pause/stand by, but not when playing a track.

With the hpf on the EVM the DC offset went away in all cases.

has anyone else noticed DC offset issues with the HPF off? is this normal with sending unbalanced to the EVM?

it just seems strange test 1 was o.k. but not the others.

thanks for help.

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mediatechnology
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Re: Using The TI PCM4222 A/D Evaluation Module

Post by mediatechnology » Tue Dec 12, 2017 2:20 pm

The balanced inputs of the PCM4222EVM sit at the A/D Vcm voltage because it is not AC-coupled.

You really want some electrolytics in series with tip and ring. There are spots on the board for SMT caps you can solder TH components to.

If the device driving the EVM are AC-coupled but have large value pull-down resistors a hefty DC offset will develop.

If the device driving the EVM are DC-coupled op amp outputs with small 47R-ish build-out resistors there will be a small amount of offset and the op amp will sink the EVM's input current.
Mack is naming names.
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harty
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Re: Using The TI PCM4222 A/D Evaluation Module

Post by harty » Tue Dec 12, 2017 7:56 pm

hi thank you.

I had noticed placing a 100uf electrolytic in series before the EVM , almost all the DC went away.

I wonder, what value of cap would you recommend? would 150uf 25v be safe?

it seems the resistors r1 r2 r6 r5 will need to be removed when installing the capacitors.

I am sure your very busy. so either way, thanks for your help.

harty
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Re: Using The TI PCM4222 A/D Evaluation Module

Post by harty » Tue Dec 12, 2017 9:07 pm

additional to above.

I just noticed you mention the polarity issue. I shall try around 150uf with a 100k resistor soldered to ground too.

" If AC-coupling capacitors are installed do note that the the polarity markings on the schematic and the layout are incorrect.
They should be at least 47 µF or greater with the positive side pointing to the OPA1632.
High value resistors (100KΩ) to ground should also be installed on the connector side of the input capacitors to discharge them.

thanks

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mediatechnology
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Re: Using The TI PCM4222 A/D Evaluation Module

Post by mediatechnology » Wed Dec 13, 2017 6:53 am

Glad to help.

The effective capacitance is 1/2 since there are two in series with a differential input.
150 µF will give you about a 4 Hz cutoff. 25V is plenty. No need for a bipolar since there is already a polarizing voltage.
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harty
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Re: Using The TI PCM4222 A/D Evaluation Module

Post by harty » Wed Dec 13, 2017 9:17 am

Thank you

I shall try a few tests with some through hole caps.

All the best

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mediatechnology
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Re: Using The TI PCM4222 A/D Evaluation Module

Post by mediatechnology » Wed Dec 13, 2017 9:29 am

Figure 52 in the PCM4222 datasheet is an alternative input circuit using conventional op amps you might want to look at. Unfortunately you want that circuit - at least the modulator capacitor - very close to the PCM4222 pins so that makes DIY'ing it difficult.

To raise the PCM input impedance, which is pretty low, you might want to add an outboard buffer ahead of the board's XLR connector. You could then ground the - input of the PCM4222 EVM and connect the + input directly to an op amp output. Both PCM42222 EVM inputs could be DC-coupled and the unbalanced source feeding the EVM would then see a high impedance (10K-ish) load. It's not going to be as quiet but as a practical matter I think it might be more universal.
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harty
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Re: Using The TI PCM4222 A/D Evaluation Module

Post by harty » Wed Dec 13, 2017 12:58 pm

Hi thanks

i will have my preamp inside the same box as the EVM. The preamp is taken out of an old mixing desk channel. So i am simply using its unbalanced insert output to go into the evm.

it would be nice to route this preamp into the EVM with as few components as possible to keep it low noise. I never thought to bypass any of the EVM circuitry.

would it be an idea to build the diagram/figure 52 you mention and then solder its output to the EVM's TP4 TP5 TP6 ( I think they are test points?) or are they positioned too far away from the EVM? I think to try and solder onto the evm pins; I just know I would bridge them. I might be able to scratch the needed traces with sand paper to get to the traces, perhaps?

blimmie, they make it hard to evaluate this chip, it seems.

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mediatechnology
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Re: Using The TI PCM4222 A/D Evaluation Module

Post by mediatechnology » Wed Dec 13, 2017 2:11 pm

I think you could open R3 and R4 and use the test points.

As long as you use the on-board 2.7nF you should be OK though I would try to keep the lead length to the driver short.

You also need Vocm and they don't give you a test point for that.
Mack is naming names.
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