2 Replies Latest reply on Dec 15, 2010 1:06 PM by gautamd_

    ADC offset using PSoC 5 on CY8CKIT-001

       Hi all,




      I am trying to read the analog value output from a LM35 temperature sensor. I am reading the value received with the ADC, but I constantly get an offset of almost 1mV. This is problematic for me, since I am evaluating the PSoC devices for use in future products.




      I am using a single ended delta sigma ADC with the following settings:


      Conversion mode: Multi sample


      Resolution: 16 bits


      Conversion rate: 10 000 SPS


      Input range: Vssa to Vdda (even though changing the range didn't do anything to my faulty reading)


      Buffer gain: 1


      Buffer mode: Rail to Rail


      Reference: 3.2910V (As measured at Vdda)




      I saw a demo where they got uV precision. Is it possible to get this kind of precision with this dev kit? How can I ensure precision when I produce my own PCBs?




      Your help is appreciated.

        • 1. Re: ADC offset using PSoC 5 on CY8CKIT-001

          Here are my suggestions:


          1. Do calibration and correct the Offset error. Go through this App Note on how to do accurate measurements using ADC. - http://www.cypress.com/?rID=40988


          2. Use the ADC range as 0-6*Vref and keep the VRef to internal 1.024V itself




          Why do you need 10Ksps samples to be taken from a Temperature sensor?







          • 2. Re: ADC offset using PSoC 5 on CY8CKIT-001

            Hi Swys,




            If the input Buffer of a Delta Sigma ADC is enabled, there is a small yet inevitable amount of Offset Error you'll see. This is an inherent behaviour of any op-amp based Buffer which comes along with the Higher Input impedance of Buffer.


            Did you try measuring the output of LM35 after bypassing the Buffer in ADC? If the output impedance of LM35 is small enough, you will see that the resulting output in this configuration has no Offset Error. The Small amount of Gain Error which the Buffer would have introduced will also be eliminated.


            Let us know if this helps.