8 Replies Latest reply on Jun 9, 2018 7:12 PM by omji_289661

    ADC Sigma-Delta Range and AC signals..

      Dear community,


      I've been working with ADC sigma delta in resolution of 18 bits. I need to rest 2 signals (differential signals), like 2-5mV ac signals. Despite they're very small I need to put them over an offset, in this case of 1.024V available from psoc, 'cos I want to preserve low level of noise as  suggestions observed in AN84783 page 11 : Input ± Vref to have the lowest noise and I can't bypass the buffer 'cos I have observed offset falls to 0.980V but even when I change to rail to rail or level shifter there are no changes; offset still falls.


      In page 9 of reference document of ADC Delta-Sigma 3.30 the range with Input ± Vref  with my Vref=1.024V on offset also should be: -1.024±1.024= 0V to 2.048. With all of these considerations I have the next questions:


      1.- Why in example of CE95271 with the same condition as I, in page 2 says that if positive enter of ADC is 0 the converted value is -1.024? or if is 1.024 the value is 0? so the range doesn't go from 0V to 2.048 as page 9 of ADC Delta-Sigma 3.30? Please clarify this I'm very confused.


      2.-. I have read there's a example of how to manage negative voltages on psoc, blog that I couldn't find unfortunately, but is related on applying offset, in this case 1.024 as AGND. Could you tell me where I can find this:  http://www.cypress.com/blog/psoc-hacker-blog/measuring-negative-voltage-using-psoc1????


      3.- Is it possible to interface directly these signals without offset moving input reference as low as Input ± 0.0625 * Vref?


      4.- Do you suggest is better implement an instrumental amplifier with pga's and opamps instead of using ADC Delta-sigma? or in worst case an external INA?


      I hope you can help me with your promptly response, thanks in advance.