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Simultaneity between ADC modules

Michael He
Expert 2170 points

So the F28M35x has two ADC modules, each with two sample-and-holds. In simultaneous mode, this allows for simultaneous measurement of two channels, correct?

My question is, can simultaneity also be achieved between the two ADC modules? I.e. can I take 4 simultaneous measurements by using the combined four sample-and-hold circuits? Or can the two ADC modules not be triggered simultaneously?

  • 0
    Guru 20525 points

    Hello!

    Perhaps this thread will answer your question http://e2e.ti.com/support/microcontrollers/c2000/f/171/t/236498.aspx.

    Regards,

    Igor

  • 0
    TI__Guru 60865 points

    Michael,

    You should be able to achieve x4 simultaneous sampling.  All you really have to do is setup both ADCs for simultaneous sampling, then pick the same trigger for both ADCs.  They do share the same set of triggers.  

    Do be aware that the two ADCs have separate gain and offset errors.  Mostly these will track, as the two ADCs will be subjected to the same temperature, voltage, and (rough) process variation, but the correlation between two samples on the same ADC will be somewhat better than the correlation between two different ADCs.

    Let me know if you run into any specific problems setting this up.  

  • 0 in reply to Devin Cottier
    Expert 2170 points

    Thanks Devin and Igor.

    The gain and offset errors can be compensated for in advance with some calibration, right?

  • 0 in reply to Michael He
    TI__Guru 60865 points

    There are offset self-calibration circuits on each ADC for offset.  If you poke around the sample code, I think you can find some code to do the self-calibration.  You will want to do this periodically.

    Gain is a little trickier.  In summary, you want to feed the same DC reference voltage (e.g. 2.5V) to both ADCs, periodically convert it, and then look at what the difference between the two ADCs is.  From this, use the CPU to scale the conversion results to match.  Note that there are some pins which go to both ADCs which you can use for this.  

  • 0 in reply to Devin Cottier
    Prodigy 230 points

    Devin,

     I have a query...I was looking at the datasheets..And from what I understand even if i use the same trigger for two simultaneous samplings , since there is only two sample and hold circuits there would be a slight delay betwwen the sampling points...(( 3+ACQPS) *T-ADCCLK...)?

    So when it is said the four samples are simulateneous it is valid depending on the freq of signal to be smapled...? Or may be i am missing something??

  • 0 in reply to sreejith menon
    TI__Guru 60865 points

    Sreejith,

    Each ADC has dual S+H, but only one converter.  For a given ADC in simultaneous sampling mode, both S+H are sampled at exactly the same time.  Both signals are then held, and converted sequentially through the single ADC core.

    If you are using both ADCs, configured identically, then all 4 S+H sample at exactly the same time, then the first pair of conversions is processed, then the second pair.

    In summary, you CAN get 4 signals sampled at exactly the same time, although half the conversions will be ready first, then the second half shortly thereafter.