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PCM1803A - DC Signals

Jamaal_Charles
Genius 3245 points
Other Parts Discussed in Thread: PCM1803A, PCM1803

Hello,

We are considering using the PCM1803A, and will have roughly 3Vp-p DC signals coming in most of the time.  Reading another forum post helps, but doesn't answer everything we need....

http://e2e.ti.com/support/data_converters/audio_converters/f/64/t/403678

If we use the part in BYPASS mode, will the ADC stay locked, or could we experience drift?

If we don't use an AC cap, we have 5Vcc and 3Vdd, what is the range we should expect (i.e. 1V is 000, 4V is FFF)?

Finally, we also have a scenario where we will have a 10kHz AC signal coming in.  We believe that should still work with out the blocking cap in place.  Is that correct?

Thank you!

  • 0
    TI__Mastermind 27900 points
    Hi Jamaal,

    You shouldn't experience drift since you should either be biasing the input with Vref, or have an AC coupling cap on the input.

    Yes the 3 Vpp range will be centered at VCC*0.5, and if VCC is 5V then the full scale input would be 4V.

    Yes, the AC cap will only block DC signals and should have little to no effect on high frequency content.

    Justin
  • 0 in reply to Justin Bohr
    Genius 3245 points

    Thanks Justin,

    So, one clarification on your response.  We don't plan on using an AC coupling cap, because we want to measure DC signals most of the time.  However, there is a scenario where we would be measuring an AC signal without the coupling cap.  We assume that is ok, correct?

    Also, we are assuming that the SCLK can be almost anything (assuming it meets the 8ns min low and high period), and that Table 1 in the Datasheet pg 11 is listing typical audio sample rates (which are meaningless for our application).  Is that correct?

     

  • 0 in reply to Jamaal_Charles
    TI__Mastermind 27900 points
    Hi Jamal,

    Yes, measuring any signal without the AC coupling caps is ok if you have the signal biased correctly to be input into the part, i.e. using the Vref pins to bias the incoming signal in the input stage.

    Yes as long as ratios between clocks are acceptable (SCK to LRCK and SCK to BCK) and you meet the min/max frequency requirements then the part will operate.

    Justin
  • 0 in reply to Justin Bohr
    Genius 3245 points

    Just to clarify, do I apply 5V Vcc to Vref2 (along with decoupling caps), and bring out Vref1 (and decouple it) to use as my ref for signal conditioning my bipolar signals to be 1V to 4V (3Vpp, centered around 2.5Vref)?

     

    On the PCM1803 dev bd, Vref2 is tied to VCC (5V), but all the documentation just talks about decoupling it. 

     

    Thanks

  • 0 in reply to Jamaal_Charles
    TI__Mastermind 27900 points
    Hi Jamal,

    You are correct, just Vref 1 for bias and Vref2 should be connected to VCC with a decoupling cap.

    Justin