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TMS320C28343 - ADC

Thalles Alberto Mariano Pinto
Prodigy 50 points
Other Parts Discussed in Thread: ADS7865, TMS320C28343, CONTROLSUITE

I’m using on my graduate work a TMS320C28343. This board has 2 external ADC (ADS7865) and I’m using the example Bellow:
<CONTROLSUITE_INSTALL_DIR>\device_support\c2834x\v112\DSP2834x_examples_ccsv4\adc_on_xintf
Now I’m trying to change this code to work with sinusoidal input. Because when I try to use the graph to see the data only the positive values appear.
Somebody know what i have to change on code and hardware to see a complete sinusoidal with the graph?

Thanks for helping.

  • 0
    TI__Mastermind 22025 points

    Hi Thalles,

    I suspect you are using a function generator to make a sin wave that is centered around ground.  Am I correct?

    If this is the case, you are seeing exactly what you should see when you graph the ADC conversions.  The ADC's references are connected to 3.3V and Ground which means it can convert analog values between these two voltages.  When your signal goes negative the TL7762 devices in the schematic clamp the sinusoid to the ground rail, preventing it from damaging the ADC.

    I would highly recommend you read through the entire ADS7865 datasheet so you fully understand how this ADC operates.

    Regards,

    Trey

  • 0 in reply to Trey German289
    Prodigy 50 points

    That's exactly what i'm doing. I'm doing to simulate the signal that i'll have to use.

    But even if I use the negative channel I won't see negative signals?

    is The only way offset the signal? 

  • 0 in reply to Thalles Alberto Mariano Pinto
    TI__Mastermind 22025 points

    Thalles,

    I'm afraid that not quite how it works. 

    The + and - input are subtracted (inside the ADC) and then sampled and converted to form the result.  The - input is not for negative voltages.

    You will need to convert your signal into a differential signal and then you should be able to drive it into the ADC.  Look at page 4 of this App Note:

    http://www.ti.com.cn/cn/lit/an/sloa099/sloa099.pdf

    It describes how to do exactly what you want.  You can even use the REFout signal from the ADC to drive the Vcm of the opamp you use.  I would also recommend you do a lot more reading about ADC, Opamps, and linear circuits in general.  A quick google search is your friend!

    Regards,

    Trey