10 Replies Latest reply on Apr 23, 2014 4:06 PM by DaKn_263916

    ADC REFERENCE

       Hi,

         

      I'm trying to use an ADC sigma delta which converts an analog signal with amplitude between 0-5 Volts. I don't understand how to set the ADC reference : in fact when I select the input range VSS-VDDA the reference automatically is set to 1.25 V , that is not the right value for me because in this way i would convert a signal with an amplitude greater than the reference. How can i do?Can someone  help me?

        • 1. Re: ADC REFERENCE
          DaKn_263916

          When you use Vssa to Vdd the internal Vref 1.024 V is not being used, the conversion

             

          is ratiometric to the Vdd supply.

             

           

             

           

          • 2. Re: ADC REFERENCE
            DaKn_263916

            Graphic did not post, attached.

               

             

               

            Also look at config graphic that shows you what the conversion range will be.

               

             

               

            Regards, Dana.

            • 3. Re: ADC REFERENCE

               Thanks for the answer,

                 

              I don't understand in which sense the reference is ratiometric?you mean that the input between 0-5 V is automatically ratioed to 4  in the way to have a signal between 0 and 1.25 V at the input of ADC? I question this because When i set vssa to vdd the reference showed in the component is 1.25V  that is VDD/4 in my case.

              • 4. Re: ADC REFERENCE
                DaKn_263916

                Ratiometric -

                   

                    

                   

                          http://lpvo.fe.uni-lj.si/fileadmin/files/Izobrazevanje/OME/sbaa110_Understanding_Ratiometric_Conversions.pdf

                   

                 

                   

                Regards, Dana.

                • 5. Re: ADC REFERENCE

                  Thanks for the answer . If i understood correctly from the document , once the input signal between vssa and vdd goes to the ADC , it is reduced by a resistior divider in order to fall in the dynamic 0-1.25V and in this way it is possible to use the 1.25 as voltage reference?

                  • 6. Re: ADC REFERENCE

                    Ratiometric in this case means that the input is referenced to the supply voltage. That is especially useful when the input signal is derived from the supply - e.g. by a voltage divider. Then changes to the supply voltage affect both the input voltage and the reference voltage, by the same amount. This increases accurany despite the supply voltage not being all that accurate.

                       

                    If tthis is not what you want, attenuate the input signal using a resistor divider and then use the internal reference (don't forget to use a bypass cap).

                    • 7. Re: ADC REFERENCE
                      DaKn_263916

                      If you are looking for absolute accuracy then use Vref of 1.024 internal, as ratiometric

                         

                      is anything but single measurement absolute accurate, as its ref is a f(Vdd). Unless

                         

                      you supply the Vdd of PSOC with a precision reference.

                         


                         

                      In your case I recomend you use Vref of 1.024 and Vssa to 6.144 V (Vssa to 6*Vref)

                         

                      for in put range to get absolute accurate Vssa - 100 mV to Vdda + 100 mV measure-

                         

                      ment range.

                         


                         

                      Regards, Dana.

                      • 8. Re: ADC REFERENCE

                         Thanks Dana,

                           

                        the final thing that i would like to know and i don't understand is that how it is possible to use an ADC reference of 1.024V and a signal that goes from vssa to about 6V? I mean for example If I have a signal of 4V , how the ADC will convert  this value whether  the maximum value it reads is the reference 1.024V?

                        • 9. Re: ADC REFERENCE
                          DaKn_263916

                          I believe it is done by controling charge transfer thru ratioed capacitors in the modulator,

                             

                          but will confirm this by filing a CASE at Cypress.

                             

                           

                             

                          Regards, Dana.

                          • 10. Re: ADC REFERENCE
                            DaKn_263916

                            Here is the results from tech support -

                               

                             

                               

                                

                               

                                      

                               

                            Hello Dana,

                               

                             

                               

                            Yes, we have confirmation that the higher input range is achieved by reducing the modulator gain

                               

                            through through reduction of the capacitor ratio.

                               

                             

                               

                            Page 385 of the below linked PSoC3 Technical Reference Manual describes about this gain

                               

                            configuration in the Delta Sigma modulator:

                               

                             

                               

                            <a href="http://www.cypress.com/?docID=46233"> PSoC3 Technical Reference Manual </a>

                               

                             

                            Regards, Dana.