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PCM1803A - full scale voltag

Kevin Groeneveld
Prodigy 100 points
Other Parts Discussed in Thread: PCM1803A, PCM1803

The feature list on the PCM1803A pages states:

Single-Ended Voltage Input: 3 Vp-p

The data sheet for the PCM1803A states:

Analog input voltage, full-scale (–0 dB) 3 Vp-p

It also states:

The differential voltage between VREF2 and AGND sets the analog input full-scale range.

There is no mention in the data sheet of what the VREF2 voltage should be.  If I measure the VREF2 pin it is 5V.  This implies the full scale voltage is 5V?

However, I am seeing with a signal of about 1.5Vpp the ADC is already outputting a full scale digital signal.  If I exceed this voltage the waveform starts to clip.

The data sheet for the PCM1803 eval board http://www.ti.com/lit/ug/sleu061/sleu061.pdf (which may not be the same as the PCM1803A, but in general they are very similar) implies the maximum input is only 1Vrms (approximately 1.41Vp-p). It states, "Analog input up to 1Vrms is connected to CN103 and CN104."  This would agree with what I have found experimentally.

So what is the full scale voltage?

Is it actually supposed to be 3Vp-p?

If yes, what are some reasons the signal seems to clip for me at about half that level?

I am operating the ADC in I2S slave mode if that makes any difference.  Sample rate doesn't seem to make any difference.

Thanks,

Kevin

  • 0
    TI__Mastermind 27850 points
    Hi Kevin,

    It is a little confusing to go back and forth between the data sheet talking in Vpp and the users guide talking about Vrms.

    To clarify:
    The Analog voltage rail is 5V and Vref is tied to Vcc. As it says in the data sheet on page 5, the Full scale voltage is 0.6*Vcc, which 0.6*5V is 3 V, and this is the 3 Vpp.

    When the users guide states that the input is 1 Vrms, this would be 2.82 Vpp or 1.41 Vpeak, Vrms = Vpeak/sprt(2).

    Furthermore, this input should be centered around 2.5 V or sent through an AC coupling capacitor. As the ADC will have a range from 1.5 V to 4.5 V if a signal is passed to the input without an AC coupling cap, then 1.5 V from ground will produce a negative full scale output.

    Justin
  • 0
    Prodigy 100 points
    I just realized where I went wrong. I was doing all my Vp-p/RMS conversions wrong.

    3Vp-p is only about 1Vrms, not 2Vrms. I was converting to/from RMS assuming 3V peak centered around zero, not 3V peak-to-peak.

    So when I said I was using a 1.5Vpp signal it was actually a 3Vp-p signal. All the test equipment I was using displayed in RMS.
  • 0 in reply to Justin Bohr
    Prodigy 100 points
    Hi Justin,

    Thanks for your the response. It literally appeared just as I posted a follow up explaining where I was going wrong before with my RMS/Vp-p conversions. What a coincidence...

    Kevin