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QUESTION ABOUT THE PCM1801 ANSWER PLEASE

Other Parts Discussed in Thread: PCM1801

HI, 

I'm a student and i'm using the PCM 1801 single ended analog input 16 bit stereo audio ADC.

My analog input signal is a 2,8 V peak to peak waveform which oscillates around the anlogic ground ( the zero volt ).

Can i just send it in the single ended input of the PCM 1801  ?  Or do I have to shift its level ?

On the datasheet it says :

INPUT RANGE 2.8 V p-p

CENTER VOLTAGE 2,1 V

What does that mean ?

Please answer this question , i'm so confused.

Best regards.

Matteo 

  • That does seem a little bit confusing, I agree, and there is also no mention of any specific requirements for the ADC later on in the data sheet.

    The fact the ADC is cap coupled though perhaps indicates that no level shifting is required, if you look at the evaluation module too, you will see that no special biasing circuit has been used prior to the ADC. The data sheet also says this...

    "The internal single-ended to differential voltage converter saves the design, space, and extra parts needed for external circuitry required by many delta-sigma converters."

    If the ADC required a buffer prior the inputs for some biasing of the incoming signal, it would contradict the above statement. I don't think the ADC requires you to level shift anything, it would be nice to know what "CENTER VOLTAGE 2,1 V" actually means though. It implies that the input signal should swing between 0.7 and 3.5 volts.

     

  • Hi thank you a lot for your answer 

    I just received an email back from TI technical support which says " Yes you've to add an 2.1 DC voltage to your input " ? 

    That's funny ! Because if i'd add an 2.1 DC voltage this would be destroyed anyway because the input is cap coupled ! 

    What's the truth ? 

    Please someone reply 

    Best regards

    Matteo

  • Hello Matteo,

    You have to shift the center voltage from 0 to 2.1 V. If you look at the ABS MAX table on page 2 of the data sheet, the Analog input voltage must stay between -0.3 and (Vdd + 0.3).

    -Mike

  • Not really ! 

    Because if i shift the center voltage from 0 to 2.1 V, this would be unuseful because the ADC is CAP COUPLED ! Than every DC component would be destroyed ! 

    My professor has said that the PCM 1801 has its own circuitery to shift the level of the input voltage - than i just have to send my voltage which swing around the anlogic ground to the input and the ADC will shift it to the center voltage by itself ( I CAN'T DO ANYTHING from the outside ! )

    Please answer  ! 

     

  • Matteo,

    Your professor is correct. The internal bandgap reference will center the input signal with the help of the coupling cap. If you were to place an oscilloscope probe between the coupling cap and pin of the PCM1801, you would see the input signal centered about 2.1 volts.

    Just send your signal centered about analog ground through the coupling cap and you should be fine.

    -Mike

  • Thanks everyone 

    Best regards 

    Matteo

  • I've the last question about calculating the LSB.

    The ADC provides by itself to shift my signal around Vref=2.5 V. On the datasheet it says that Input range 2.8V peak to peak and Center voltage Vref= 2.1. 

    Is the LSB = 2.8 V / 2^16        or is it LSB = (2.1) *2 / 2^16 

    How do you calculate the LSB if the datasheet doesn't tell you all of this stuff ? 

    Thank you.

    Matteo

     

  • I think that the first formula is the correct one, because quantization error is related to signal amplitude.