5 Replies Latest reply on Aug 15, 2014 5:04 AM by user_14586677

    Accurate Measurement Using PSoC5LP Delta-Sigma ADC

    user_202566985
              I have a question about Delta-Sigma ADC. We were evaluated for each resolution 20bit, 19bit, 18bit of by DelSig_I2C project use of AN84783, but does not seem to be normally distributed If you look at the histogram of 18bit. Effective resolution's are significantly lower for this, but I can solve what if? Evaluation environment of I is as follows. -Use development board: CY8CKIT-050 · I have been short-circuited input http://jp.cypress.com/?rID=82678   
        • 1. Re: Accurate Measurement Using PSoC5LP Delta-Sigma ADC
          user_1377889

          In the english manual for AN84783 here: www.cypress.com/ you find on page 10 a note concerning noise. Did you check and measure all of that discussed major noise sources and verified that each of them (and the sum) is smaller than your wanted noise level. Remember that 20 bit resolution @ Vref = 1.024V  is equivalent to ca. 1µV/Lsb. Checking noise and ripple is done best using an oscilloscope, AC nd high gain and look for the level of the overshoots.

             

           

             

          Bob

          • 2. Re: Accurate Measurement Using PSoC5LP Delta-Sigma ADC
            user_202566985
                    Thank you for reply. Though it is normal distribution with a resolution of 20bit, it is not a normal distribution it's a resolution of 18bit. By not become a normal distribution even when the resolution of 20bit if there is noise in the input? I think noise is the lowest input of the ADC because it short.   
            • 3. Re: Accurate Measurement Using PSoC5LP Delta-Sigma ADC
              user_1377889

              ...and the shorting wire is not able to catch up some noise? How did you wire the cap for bypassing the Vref??

                 

              We are talking about µV, this is quite less than what you get when you near your hand to a high impendance input. You shorted the inpiut pins. Shorted to what? Ground?? Shorted, but left floating??? The application note clearly states that even the noise margin of a resistor would be too much and that you need a precision voltage source to be able to measure the performance of the ADC correctly.

                 

              And precisely: 1.024V / 218  ~ 4µV/lsb

                 

               

                 

              Bob

              • 4. Re: Accurate Measurement Using PSoC5LP Delta-Sigma ADC
                user_14586677

                Can you post you measured histogram plots for all three ranges ?

                   

                 

                   

                Attached a guide to meauring for the case input shorted. A scope irrelevant,

                   

                and in fact its noise path typically 8 bit digitizer, so even with averaging not

                   

                useful in accurate 20 bit measurments, let alone 16, for accurate signal path

                   

                noise measurements pre ADC inputs for these kinds of resolution.

                   

                 

                   

                Regards, Dana.

                • 5. Re: Accurate Measurement Using PSoC5LP Delta-Sigma ADC
                  user_14586677

                  Assumed that you setup as follows(per ap note, note DC source not ground referenced) -

                     

                   

                     

                  Of course you can make the measurements with 0 Vdc (differential in) by shorting

                     

                  inputs, but use ohmmeter tool to establish short path R, which in turn adds thermal

                     

                  noise at the input. Lastly note that when making rail measurements, you must have

                     

                  the inpuit buffer of ADC bypassed in its properties, otherwise you are outside its CM

                     

                  range.