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PCM1802: Noisy VREF1.

nigel gordon
Prodigy 80 points
Part Number: PCM1802

The lack of PSR data for this device is a hindrance. The datasheet just says that VCC should be clean. What does that mean? A battery?

I have a clean VREF2 (-106dBu) but my VREF1 (midrail) is much noisier (-85dBu) for some reason, causing out of spec ADC performance . The datasheet suggests VREF1 is derived from VREF2 and I do have all of the recommended decoupling so I am not sure I understand the reason.
My VCC rail is noisier @ -55dBu so I am assuming this is affecting my VREF1? Not sure why it should though if my VREF2 is supplied is clean,

Has anyone managed to achieve the SNR performance of 105dB? No eval board avalaible.

  • 0
    TI__Mastermind 23050 points

    Hi Nigel,

    When you have a noisy supply it is recommended to place a 1kOhm resistor between VCC and VREF2. Can you confirm if you have done this? If you're really trying to get 105dB performance it is always best to use a clean supply for the device and most LDOs should be able to supply a clean enough rail.

    Best,

    Zak

  • 0 in reply to Zak Kaye
    Prodigy 80 points

    I tried the 1K but it didnt work. Moreover I created a completely noise free VREF2 and that didnt work either. VREF1 is still noisy. My next move will be to create a 'cleaner' VCC but the lack of information regarding Power supply rejection doesn't tell me how 'clean' it has to be..

  • 0 in reply to nigel gordon
    TI__Mastermind 23050 points

    Hey Nigel,

    I'm sorry for the trouble here, I would test this for you if I could but unfortunately we don't have any good boards for this part at the moment. We are actually in the process of creating a new EVM for this part, but it will be at least a few weeks before we get the boards.

    That said, if you've measured the noise level on your supply and the corresponding noise level on the output of the ADC, you have a measurement of PSRR and can use that as a reference for how much supply noise you need to cut.

    Best,

    Zak