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PCM1851A: Test circuit for speaker output

rifo
Expert 1105 points

Part Number: PCM1851A

Hello

I am trying to design a circuit for testing an audio card. The idea is to take the audio jack (from DUT) and plug it into the test circuit instead of  a 2x10 Watt Speakers

I am planning to use a PCM1851A from TI for digital conversion and then take the FFT of the incoming audio.

The audio circuit amplifer works at 12V and sends differential analog signal for both left / right speakers  (Left_p, Left_n,  Right_p, Right_n)

I need a way to scale down the differential audio signal to an acceptable range and change it to a single ended signal before feeding it to the PCM1851A.

Can you please give me some pointers for this operation?

thank you very much

rifo

  • 0
    TI__Guru 57800 points

    Hi, Rifo,

    In general, the use of differential signals is preferred to have better noise immunity, we have newer Audio ADCs with differential input support that can be used instead of the PCM1851, like the PCM1861. Depending on the test you want to perform, you might take only one of the legs of the differential pair as the single-ended source for the ADC. One thing to note is that if the amplifier is a class-D, then you need to make sure that a LC filter is used to remove all PWM carrier content. 

    Best Regards,

      -Diego Meléndez López
       Audio Applications Engineer

  • 0 in reply to Diego Melendez
    Expert 1105 points

    Thanks a lot for your pointer,

    Is there an application note that I can follow for the LC filter, also, do you think that I should use a resistor network to scale down the signal? 

    PCM1861 datasheet says that 2.1V RMS input levels are supported. If I am not wrong then with this, I can't feed in the 2 x 10W signal.

  • 0 in reply to rifo
    TI__Guru 57800 points

    Hi,Rifo,

    About the LC filter app note, we have actually a pretty good document, please refer to SLOA119B app note. For the signal conditioning, if you application allows it, a resistor network can be used to scale down the signal. Just make sure the maximum output of the amplifier is below the maximum voltage spec of the input pins of the ADC. 

    Best Regards,

      -Diego Meléndez López
        Audio Applications Engineer

  • 0 in reply to Diego Melendez
    Expert 1105 points

    Hello Diego,

    I am trying to design the resistor network that should scale the incoming signal.

    I found the below post from TI analog wire blog and planned to use the "three resistors and a voltage source"
    approach.

    https://e2e.ti.com/blogs_/b/analogwire/archive/2016/09/20/three-ways-to-scale-an-analog-input-signal

    In order to determine the scaled voltage levels, I was going through the datasheet and the below forum post

    https://e2e.ti.com/support/data_converters/audio_converters/f/64/t/490550

    and came to the conclusion that input differential signals should be scaled to  -1.32V  & 4.62V range

    I will design the resistor network according to these numbers. 

    Can you please also comment on this approach?

    thanks

    rifo

  • 0 in reply to rifo
    TI__Guru 57800 points

    Hi, Rifo,

    Regarding the voltage level scaling, I think the approach Ron mentioned should be fine for your application. The input scaling mentioned in the e2e link you provided is correct for each input pin of the PCM1861.

    Best Regards,

      -Diego Meléndez López
       Audio Applications Engineer