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Old 14th February 2009, 12:38 AM   #1
Aiace is offline Aiace  Italy
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Default pcm1794 Iref and cap questions

I'm going tu use PCM1794 but the data sheet lacks of information.

1) What about "Iref". I saw the typical connection diagram use a 10k resistor: why? What do it happen if I use a different value?

2) The typical connection diagram use two 47uF to connect VcomL and VcomR to +5V. What kind of capacitor should I have to use? Is a simple electrolitic enogh? Or it involves analog signal and then it's better to use Os-CON?

3) what about the fan out of "zero" signal? Might it drive a 15mA led?

Thank you
Aiace
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Old 14th February 2009, 01:12 AM   #2
anatech is offline anatech  Canada
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Hi Aiace,
Are you working from the manufacturer's data sheet? Do you have a link to the information you have (original source).

Bu doing a search at this site - search function - I came up with a ton of information. Just search using your chip number.

Looking on the TI web site, we find the data sheet for this part, here. It seems to go over most, if not all of your questions. Then there is the applications section. If you go down this page, you will find more application notes and other ICs used in conjunction with this chip. I see these are about $31 USD at Digikey.

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Old 14th February 2009, 09:03 AM   #3
Aiace is offline Aiace  Italy
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thank you anatech,
I didn't find any clue on the TI site about PCM1794.
I suppose some answers are the same of the PCM1704, that you posted the link before . But none information about Iref...

On the PCM1704 data sheet I found that analog GND and digital GND should be tied as close as possible to the chip, but other people say tha it's better to tie them at the power transformer.
Whats the truth?

Thank you

Aiace
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Old 14th February 2009, 09:36 PM   #4
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Default Re: pcm1794 Iref and cap questions

Quote:
Originally posted by Aiace
I'm going tu use PCM1794 but the data sheet lacks of information.

1) What about "Iref". I saw the typical connection diagram use a 10k resistor: why? What do it happen if I use a different value?

2) The typical connection diagram use two 47uF to connect VcomL and VcomR to +5V. What kind of capacitor should I have to use? Is a simple electrolitic enogh? Or it involves analog signal and then it's better to use Os-CON?

3) what about the fan out of "zero" signal? Might it drive a 15mA led?

Thank you
Aiace

First of all, you really need to do a search for both PCM1794 and PCM1792 under partn number on the front page of www.ti.com. These parts are identical bar for the fact that PCM1794 is easier to use for a hobbyist and that a tinkerer will have more access to internals of the PCM1792 when using a microcontroller. The most important features are available on both, though.

I have seen a ton of information including typical schematics (probably in the datasheets) and even an evaluation board with full schematics etc - I actually have one such board in my house that is not being used so it is available for sale - here is some information about it http://focus.ti.com/analog/docs/tech...ctName=sleu037

You cannot assume any similarity between this chip and the 1704. What is likely though is that the "voltage reference" for both is based on a current reference which is reasonably easy to make compared to a voltage reference on such a chip, and that this current is converted to a voltage over the resistor. If you use a smaller say resistor, you will very likely get a smaller signal output - you could probably implent an "analog" volume control here if you wanted. Be aware that the chip is probably optimized for the very resistance being suggested. A larger value is more likely to damage the chip - assuming a reliable current source in the chip - shourt circuit to ground may not be all that great of an idea either but will probably "work" if you desire zero output.

As for the types of capacitor - the evaluation board (from memory) uses expensive units and a lot of them - I have yet to see commercial designs use mica and Silmic to this extent - even generally very costly units are designed by accountants, you'd be surprised! This baby's innards, however are surprisingly expensive.

VcomL&R are supposed to be DC references hence they specify capacitor values which are very hard to fit if you select polypropylene. I would say that a good quality electrolytic is what was planned - you could bypass if you wanted to. The PCM1704 has a similar pin where electrolytic is also suggested on both datasheet and reference design.

Hope this helps.

Petter
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Old 14th February 2009, 11:22 PM   #5
anatech is offline anatech  Canada
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Hi Aiace,
Often, Ti/BB chips are related in technology. True, you can not assume this is 100% true. I just figured the information you were looking for would be in there. I do remember reading about the current set pin on one data sheet. Like Petter said, I highly recommend you leave it where the manufacturer specified it. The manufacturer will always design a circuit where it operates at it's best.

I do recommend that you read the application sheets, and other related information on that site. They will assume that you have read their application notes when they publish their data sheets.

-Chris
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Old 18th May 2009, 09:08 AM   #6
awpagan is offline awpagan  Australia
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Bit of info on IREF.

Not really the PCM1794, but similar idea "I think"

http://focus.ti.com/lit/an/sbaa146/sbaa146.pdf

page 8

Another question to raising the IREF resistor to lower the current output is.................
Does this effect the performance of the PCM1794?
and if so, by how much?
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Old 18th May 2009, 05:11 PM   #7
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Quote:
Originally posted by awpagan
Bit of info on IREF.

Not really the PCM1794, but similar idea "I think"

http://focus.ti.com/lit/an/sbaa146/sbaa146.pdf

page 8

Another question to raising the IREF resistor to lower the current output is.................
Does this effect the performance of the PCM1794?
and if so, by how much?

I did this by accident = I forgot to solder one joint on the PCB that resulted in the 10k resistor being left out of the circuit.

It sounded fine, but the output current was really low.

Now this chip has a rather high output current, as do all the other chips in the series, adding to this they have the highest dynamic range of all TI's chips. I'm no expect, but I'd hazard a guess at the high output current being related to the performance.
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Old 19th May 2009, 12:14 AM   #8
awpagan is offline awpagan  Australia
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Quote:
Originally posted by 5th element



I did this by accident = I forgot to solder one joint on the PCB that resulted in the 10k resistor being left out of the circuit.

It sounded fine, but the output current was really low.

Now this chip has a rather high output current, as do all the other chips in the series, adding to this they have the highest dynamic range of all TI's chips. I'm no expect, but I'd hazard a guess at the high output current being related to the performance.

It's good to know that it will still work.

The manufacturer stat's are usually at the optimum settings.
That could also include why they used 7.8mA out and hence, the 10k.

I would like to know what is the trade-off, and where in the lower range does the dac become noisy.
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Old 17th February 2010, 08:24 PM   #9
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Hi,
I have question for those who have pcm1792/pcm1794 etc. working properly: I've built DAC using pcm1792 using its application circuit as shown in datasheet, and it's working quite good, but there is a problem with the temperature of PCM1792 and I/V op amps.

PCM1792 and I/V converters are getting hot, too much as for me. It happens even if there's no input signal present (=mute). After few minutes temperature of I/V converters rises up to approx. 60 degrees C or even more. PCM1792 is also hot, but fortunately not as much as op amps.

Is it normal ? Do your circuits also behave like that while working ?
Maybe that's caused by instability of I/V stage op amps ?

Last edited by bajk; 17th February 2010 at 08:45 PM.
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Old 19th February 2010, 08:30 AM   #10
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Quote:
) What about "Iref". I saw the typical connection diagram use a 10k resistor: why? What do it happen if I use a different value?
This resistor sets reference current that are mirrored to I DAC
Quote:
2) The typical connection diagram use two 47uF to connect VcomL and VcomR to +5V. What kind of capacitor should I have to use? Is a simple electrolitic enogh? Or it involves analog signal and then it's better to use Os-CON?
X5R or tantalum
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