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PCM3168a input noise

Other Parts Discussed in Thread: PCM3168A, PCM3168, OPA1632, OPA134

I'm trying to track down noise on the input side of a PCM3168a I'm using.  The noise I hear is swirly in that there's a pitch that moves around in frequency.


I'm using the ADC in single-ended mode with the reference buffer shown in Figure 61 of the datasheet using these values: R1=10k, R2=1k, R3=47, C1=470p, C2=22n.

It's about a 5" run from the analog circuitry to the converter and I've placed the buffer along with the C2 caps at the analog side.  Any ideas on what I can try to get this noise out?


Thanks

  • Hello,

    I will look into this for you and see if I can replicate the noise issue.

     It looks like the photo you tried to attach did not upload properly. Can you try to upload it again?

    Regards,

    Matt

     

  • Hi Matt,

    The noise is still there but I managed to bring it down a bit by moving the 22n caps to the converter side.  I also added 100n caps across the VCOM and VREF lines which weren't there before.

    I've attached a recording of the noise.2235.Exm Noise.m4a

    Thanks,

    Shahin

  • Hi Shahin,

    Noise can be tricky to solve but I would first consider a few things.

    Is the noise file you sent on the output of the buffer? How was it recorded? If you recorded it using the PCM3168, if you can disconnect the output of the buffer to the input of the PCM3168, does it go away?

    We should be sure that the buffer is causing the noise you are hearing not the PCM part.

    Assuming it is the buffer the layout seems to be playing a major role due to a change in noise when you move the caps to the converter side as mentioned.

    First, 5” is a rather long run to the PCM3168. This long trace will have parasitic inductance which will allow for all sorts of high frequency noise to couple in. I would recommend moving the buffer closer if possible. Using a full differential driver would give better results for long signal runs due to common mode rejection. I would recommend switching to a full differential driver since this will reduce the necessary discrete components. I would consider the OPA1632.

    Second, the capacitors you use for filtering and decoupling should be surface mount with very small traces. They should be X5R dielectric or better for best noise performance. You also need to make sure that you are not reducing the actual operating capacitance due to the voltage coefficient of the capacitor. A just a couple of volts of DC offset on the cap can reduce the capacitance by half! I would check your datasheets and measure the DC across the capacitors. As a rule of thumb you should always double the voltage rating of the cap for the expected voltage for high performance audio applications.

    Third, make sure to adequately filter the voltage rails for the input buffer and the PCM3168 with separate decoupling caps for each part placed right at the rail supply pins of the parts. Use large value 10uf caps in parallel with smaller 0.1uf caps or similar arrangement.

    Fourth, are you buffering the common mode voltage reference from the PCM part with an op-amp? This is recommended.

     

    Here is a typical application with the OPA1632 driver. Note the additional OPA134 to buffer the Vcom reference as well as voltage rail decoupling caps and fully differential output to reduce noise. This diagram is from the OPA1632 datasheet HERE.

    Regards,

    Matt

  • Hi Matt,


    The audio was recorded at the speakers in my system but I've narrowed the noise down to the ADC stage.  I've disconnected the input buffer and I have a digital volume control as the first stage of my DSP processing.  Turning down the volume gets rid of the noise.


    I'm going to try and implement the differential buffer and see if there's an improvement.  I also didn't think to buffer VCOM.  I'll do that too and let you know how it goes.  Thanks!

    Shahin