• State TI Thinks Resolved
  • Locked Locked
  • Replies 3 replies
  • Answers 1 answer
  • Subscribers 30 subscribers
  • Views 1860 views
  • Users 0 members are here
Support feedback
Options
Options
Similar topics

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

PCM1704 DAC - Is 768kHz the maximum sample rate allowed?

David Fischer2
Prodigy 20 points
Other Parts Discussed in Thread: DAC11001A

Hello,

I want to implement a type of PCM direct stream audio using 1 MHz 20 bit SAR ADC and PCM1704 for playback. Due to the excessive data rate, I plan to losslessly compress the raw data from the SAR ADC, but that doesn't relate to my question. It should certainly be possible to run both the SAR ADC and PCM1704 at 768kHz. However, higher sampling rates lessen the requirements of both the analogue anti-aliasing filter on the ADC and the analogue reconstruction filter on the DAC. So it would be nice to run the PCM1704 at 960kHz (20 x 48kHz) or 970.2kHz (22 x 44.1kHz). 

From what I can find in the PCM1704 datasheet, the 25MHz limit on the bit clock for serial audio data appears to be the limiting factor. In other words, 781.25kHz sample rate using a 32 bit data frame, but 1.042MHz should be possible using packed 24 bit data, or 1.25MHz with packed 20 bit data. Using packed data frames is a bit tricky with modern MCUs, but it does appear to allow a faster sample rate?

My question, will this work? The PCM1704 has a settling time of 200ns, so theory the output should still be okay? But there are may be other internal components inside the PCM1704 that cannot handle a sample rate above 768kHz? 

It is quite an expensive IC, and as it is no longer manufactured, so is getting difficult to replace if I burn it out.

I would really appreciate it if somebody knows the answer.

Thanks,

David Fischer.

  • 0
    TI__Guru 53573 points

    Hi David,

    This is a very old device so there is not a ton of information for these other running modes.  The interface seems valid for greater than 768ksps, but that does not mean that the internal operation of the DAC can run that fast.  It is possible that it will work but performance my suffer.  768ksps is a tested condition so you should  be safe at that speed.

    If you are interested in an R2R type DAC that might be good for audio, you could look at the DAC11001A.  This is a new 20-bit R2R device from our team and I am working on an audio reference designs with it as well.  While this DAC was not created with audio in mind, with the correct configurations it can produce very good dynamic performance.

    Thanks,
    Paul

  • 0 in reply to Paul_Frost
    Prodigy 20 points

    Hi Paul,

    Thanks, for getting back to me. I think I will probably give it a go. I can only use the PCM1704 for prototyping anyway, since it's not in production.

    And, I didn't know that TI had a 20 bit R2R DAC. I don't necessarily have to use a R2R DAC, but I want a true PCM DAC with zeroth order hold output. Unfortunately, I think the benefits of using a true PCM DAC for audio are pretty much negated unless I use oversampled PCM at every stage in the audio chain. The idea is that I can maintain both the frequency and time domain characteristics of the signal. By removing the need for a digital interpolation filter there should be no pre-ringing or post-ringing on impulses. Of course, I don't know if these effects will be audible, but it will be a faithful reproduction of the audio signal. 

    Has your company every considered using the switched capacitor DACs used in SAR ADCs as general purpose multi-bit DACs? One way would be to have two matched capacitor DACs and simply switch between their respective outputs at high speed 5 - 10 MHz. The constant frequency of the switching would be easy to filter out, and the settling and glitch is hidden as well as the problem of capacitive loss affecting the output. Anyway, just an idea, as you can tell I would really like some more options for true multi-bit PCM DACs.

    Kind regards,

    David.

     

     

  • 0 in reply to David Fischer2
    TI__Guru 53573 points

    Hi David,

    The switched cap is an interesting idea.  I'll bug our product definers next time I see them. 

    You are correct about the benefits and drawbacks of the over-sample/interpolation with an R2R DAC. Generally with devices like the DAC11001A you see the best performance with a pretty high sample clock (500ksps).

    Thanks,
    Paul