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PGA2320: Huge White noise with PGA2320 when the volume is set to zero

MEHRAN ERZA
Prodigy 10 points
Part Number: PGA2320

Dear All, 

I designed a new Audio Pre-amplifier PCB board in  which I use PGA2320 in order to control the volume by a Microcontroller. To test the sound performance, after analyzing the THD+N, frequency response, Slew rate etc ... At the end I use a power amplifier which is connected to a loudspeaker to check the sound performance. 

There is huge hiss noise (White noise) when I set the volume to zero (When there is no sound and PGA2320 input is shorted to Analog GND) ... At the beginning I was thinking it comes from power amplifier so I changed the power amp by a Ultra low noise very expensive class AB amp (35000 usd price) at the noise is still huge. 

I was totally confused and totally redesign the preamplifier board by totally separating the DGND and AGND ground plan exactly as it is recommended by datasheet but the noise is still there. I changed the voltages regulators by the lowest noises possible in the market but nothing changed. I also saw in the datasheet that input impedance should be low but in my case the input voltage comes from a non inverter operational amplifier with a feedback gain of 6dB so by  default it is a pure voltage source with a low impedance at its output. In term of layout I totally follower the datasheet. 

Do you have any recommendation ? Any idea ? Is it normal to get this high level of noise? 

Best regards 

  • 0
    TI__Genius 9435 points

    Hi Mehran,

    I see you noticed the noise in your signal chain after connecting a power amplifier at the output of your PGA2320 and a non-inverting gain stage at the input, then setting the PGA2320 volume to zero.

    When you say you set the volume to zero, does this mean you are setting one of the channels to 0x00? Or are you setting both of the channels to 0x00? Or are you using the MUTE pin?

    What were the results of your THD+N analysis for your PGA2320 pre-amplifier board? Did you notice abnormally high noise when testing the PGA2320 stage by itself?

    Were you able to analyze the THD+N of your gain stage that is driving the PGA2320?

    Can you share a schematic? How are you connecting the three stages of your signal chain together?

    Regards,

    Zach

  • 0 in reply to Zach Olson
    Prodigy 10 points

    Dear Zach,

     

    Thanks for your reply. Following please find my comments below :

     

    1. Related to your question about setting the volume to Zero : at this point, I used 0x00 for both channels. In the datasheet page 9, Topic Gain setting, it is mentioned : “For N = 0: Mute Condition. The input multiplexer is connected to analog ground (AGNDR or AGNDL).” So I believe when I set both channels to 0x00, both inputs are connected to AGND.
    2. THD+N results was good roughly around 0.002% in High and Low frequency. I also made a FFT to visualize noise ground and the noise ground was also acceptable and flat around -115 to -120dBV (We used Audio Precision 2722 for these measurements).
    3. The way that I found the problem comes from PGA2320 was following : I removed physically the PGA2320 from PCB and then short circuiting its input – output, and testing again and the noise was definitively removed (reduced )
    4. As the design is confidential is it possible to give me your email for sending the schematics?

    Many thanks and best regards

    Mehran 

     

     

     

     

     

     

  • 0 in reply to MEHRAN ERZA
    TI__Genius 9435 points

    Hi Mehran,

    Thanks for the additional information. Yes, you may send me the schematic over private message.

    I sent you a friend request on E2E, this will allow us to communicate off of the public forum.

    Thanks,

    Zach