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PCM1865: Analog Front-End Circuit for Differential + Single Ended Applications

Jay Shoemaker
Intellectual 670 points
Part Number: PCM1865
Other Parts Discussed in Thread: , PCM5242

Hi, 

I'm designing an ADC board with the PCM1865 that will interface with a DSP. I'm trying to design a high quality card that is somewhat generic so that the ADC can be used for both single ended and differential line-in applications. 

Question 1: How important is the AntiAliasing Filter mentioned in the datasheet? We are using this primarily for "pro-audio" in "live production" style work. (mics (requiring preamp), powered speakers (balanced line-level), cd-players, DVD players, etc). 

Question 1b: Assuming we have a powerful DSP, can it provide the AntiAliasing Filter, or does it have to be done in analog?

Image: PCM1865 Datasheet Page 75.

Question 2: What is the 100k resistor tied to ground, as found in the PCM1865EVM datasheet? It appears that the EVM doesn't have an antialiasing filter at all, so is that my sign that I don't need it?

PCM1865EVM Guide Schematic. Pg 15. 

Jay

  • 0
    TI__Mastermind 30210 points
    Jay,

    1.The Input filter is only important if you expect to see a fair amount of HF hash/noise on the inputs. for example, we have customers using these in CB radio type applications where there is a big high power transceiver near the ADC, it can be hard to completely isolate the inputs, so filtering is really the only option there. For most applications we don't see these as necessary, however the point about current limiting for ESD events is a clear benefit too. For me this seems like a small thing that a designer can add to the schematic to ensure a high performance, robust design, even in non ideal environments.

    2. keep in mind that there IS digital filtering that occurs in the ADC. These passive analog filters prevent unwanted noise from entering the system before it becomes a problem. once it is in the audio band, the problem is how to remove the noise without coloring, or changing the original signal.

    Note that these recommendations are not specific to only the PCM186x devices, we make similar recommendations for many of our ADCs and Codecs.

    best regards,
    -Steve Wilson
  • 0 in reply to Steve-Wilson
    Intellectual 670 points

    Hello Steve,

    Thank you for your fast reply! I was assuming that these recommendations were typical. After years of reading through your datasheets I have seen various recommendations on this subject for both inputs and outputs. The ESD protection is clearly a benefit, and preventing any HF hash/noise is a benefit as well. So maybe you are right, incorporate them into the project! On my PCM5242 output phase I've incorporated Type-B into my project. It is discussed here. This is supposed to allow both single and differential outputs. 

    OUTPUT OPTIONS:

    I could maybe just incorporate Type-B for the inputs as well? This would provide some of that protection we discussed (ESD and High Frequencies) but also allow for single and differential inputs? How does Type-B look for the inputs?  

    INPUT OPTIONS?:

    For Type-A, what's the 100k resistor doing?

    Thank for your insight!

    Jay

  • 0 in reply to Jay Shoemaker
    Intellectual 670 points

    So what I'm suggesting is to use the single ended filter with AntiAlias filtering.... Can you confirm the values of the resistors and capacitors? In the datasheet I see 100 OHMS and 0.01uF. But I also have seen 499 OHMS and 2.2 uF. Other than this, my PCB is ready to go to manufacturing with a schematic similar to below. Thanks for the help! :-)

  • 0 in reply to Jay Shoemaker
    Intellectual 670 points

    What's better?

    .01 uF + 100 OHM?

    Or 2.2 uF + 499 OHM

    PS - Originally I had it wrong. The INPUT 3+ and INPUT_3- is coming from the source. The VINL4-VIN4M & VINL3-VIN4P is going to PCM1865. This image above *should* be correct wiring. 

    Just need to confirm the values. Thanks for the heads up!

  • 0 in reply to Jay Shoemaker
    TI__Mastermind 30210 points
    Hello Jay,

    Are you sure you're seeing 500R and 2.2uF? are you sure its not 0.0022uF (2.2nF)? 2.2uf and 500R would be filtering your audio band pretty significantly, which I'm sure you want to avoid.

    I would stick with our recommendation of 100 + 0.01uf.

    best regards,
    -Steve Wilson