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PCM1794: Using higher resistor in IUC mutes the PCM1794 DAC

Pavao Lubina
Prodigy 60 points
Part Number: PCM1794
Other Parts Discussed in Thread: PCM1792A, PCM1792

Hello all,

We've been installing PCM1794 DAC in existing CD player (dual mono PCM configuration).

PCMs are configured to use internal DF. Due to the problem described in issue, we tried to overcome it by changing the resistor in the analog part (IUC).

However, PCMs stop giving any output when we increase the resistor value (feedback of IUC OP-amp) over 1kOhm.

  1. Is there any way that we can make the PCM work in the range of up to 5kOhm?
  2. Or at least 3.3k?
  3. Is there any way to amplify the current of PCMs?
  4. Does the Iref pin (i.e. it's corresponding resistor connected to pin 20) have something to do with this?

Thanks,

Pavao

  • 0
    TI__Mastermind 28070 points

    Hello Pavao,

    Iref should not be causing this issue, provided you have it connected to a 10kOhm resistance to ground it should be fine. 

    When you say IUC Op-Amp, are you referring to the first stage trans-impedance amplifier?  

    If so, keep in mind that you have to consider the output current and center current of the PCM1794 combined with your feedback resistor and OpAmp. If you increase the feedback resistance too much, you will exceed the input common mode range of your amplifier. To use 3.3k Ohm feedback you'd need an amplifier which could support a common mode voltage range of (6.2mA)*(5kOhm) = 31 Volts. Section 3.2 of this application report shows how to calculate the correct feedback resistor value. 

    Because of that I don't think you can get the PCM1794 to work with 3.3k or 5k Ohm feedback resistances. The only way to increase the output strength is the traditional method of adding another amplifier, or shifting WDCK as Paul suggested in the thread you linked. Still, your best option is probably to switch to a DAC such as the PCM1792A which supports 20-bit mode. 

    Please let me know if you have any other questions.

    Thanks and Regards,
    Brandon Fisher

  • 0 in reply to Brandon Fisher94
    Prodigy 60 points
    Hello Brandon,

    Thanks for the info regarding Iref resistor (I already had it connected, but I thought maybe tweaking it could influence the output current).

    "When you say IUC Op-Amp, are you referring to the first stage trans-impedance amplifier? "
    Exactly. Apologies, for not being precise.

    Thanks for pointing me to transimpedance amplifier circuit design. This confirms that it is not possible to achieved what we planned, with current design.

    In the beginning, it seemed that PCM1794 satisfies our needs (and most of the time does), but this is some special case...
    Sure, PCM1792 seems now like better solution, but this requires additional redesign. We will probably go for some amplification in buffer stage and consider the possibility to use 1792 in the future designs.

    Thanks for your effort.
    Best regards,
    Pavao Lubina
  • 0 in reply to Brandon Fisher94
    Prodigy 60 points
    Hello,


    I've asked the question, but gave some misinformation.

    We are using PCM1794 in a DF bypass mode - we are using external digital filter, dual mono configuration.

    Due to the fact that PCM1794 doesn't support 20/16 bit signal in DF bypass mode, we are receiving signal of lower amplitude at the end.

    We wanted to tweak the feedback resistor in transimpedance amplifier in order to amplify the signal.

    When we increase the value of resistor over 1k, there is no signal at all on the output.

    How does the PCM "know" which value of resistor is used - is it measuring the output current at the startup or something else?
    Can we trick it by having the resistor of lower value which is after startup disconnected with some switch?

    Since I asked with wrong info in issue above, I am not sure if the answer is still valid.

    PCM1792 sounds good, but it is some effort to deal with controller:

    Can PCM1792 somehow be used to process 20bit signals by some HW-pin configuration?
    If no, is there some existing solution from TI - some controller/dev-board ... which can be easily plugged in and configured to handle PCM1792?


    Thanks for your time,

    Pavao
  • 0 in reply to Pavao Lubina
    TI__Mastermind 28070 points

    Hello Pavao,

    You're correct that implementing the PCM1792 would require effort on the controller side. Unfortunately, there's no way to hardware configure it, and we do not have an evalulation module available for the PCM179x family of devices.

    The PCM1794 is a current output DAC, it does not actually "know" if the load resistance is within certain values (below 1kOhm in this case), rather it just tries to output. It is trying to output 6.2mA of current common mode, which across a 5k Ohm resistance would be 31V. This document does a good job summarizing how this circuit really works.

    Since your op-amp can't likely support -31V common mode, you are not seeing the correct output. A better option is to increase the gain in your final stage, where you go from differential to single-ended output. That's probably the way to do this with the least amount of changes to your design. 

    Best Regards,
    Brandon Fisher

  • 0 in reply to Brandon Fisher94
    Prodigy 60 points
    Hello Brandon,

    Sorry to hear that there is no some evaluation module for PCM1792.
    As you said, we could shift, or amplify the signal in latter stage (PCM1794 workaround) - since we don't have such op-amp.
    So, the current goes 6.2mA +/- 7.8/2mA, which is from 2.3 - 10.1 (where 6.2mA is 0). So when we have 16-bit signal, current goes a little bit around 6.2mA, too little to be detected by the op-amp.

    Thanks for your time and best regards,
    Pavao