1 2 Previous Next 22 Replies Latest reply on Jan 23, 2015 3:17 PM by du.lo

    14 bit ADC for Load Cell

    du.lo

       I'm working on a project to read a load cell. I've fed the load cell sig+/- through the INA125 with an offset to output 0.454V-5V to an analog pin of a PSoC1 CY8C28433. 

         

      I've tried using the DelSigPlus and ADCINC with 14bit of resolutions. If my math is right, I should be able to read 5/(2^14) = 0.3mV/div 

         

      The resolution I care is more in the region of 2.44mV/div 

         

      However, I've run into a problem somewhere. The hardware portion seems to working correctly. Every 1 lb change on the load cell translates to about 2.44mV change at the INA125 output. The PSoC however is not detecting this change at all. To make sure the code was "working" I applied a very large weight (~135lbs) and saw the ADC output a change of 551 counts (difference). The math doesn't seem to match though. Is there a limitation that I am missing? Perhaps I can't sample with 0.3mV/div of resolution?

         

      Thank you for your help.

        • 1. Re: 14 bit ADC for Load Cell
          du.lo

           To clarify a bit, I've tried using DelSigPlus once and ADCINC the other time, both had the same symptoms. I've also tried 12bit with similar results.

          • 2. Re: 14 bit ADC for Load Cell
            user_14586677

            Post an archive of your project so we can see all settings -

               

             

               

                

               

                      

               

            “File”                                                           Designer

               

            “Archive Project”

               

             

               

            Regards, Dana.

            • 3. Re: 14 bit ADC for Load Cell
              du.lo

               Thanks Dana!

                 

              One more clarification. I see a 2mV change on the oscilloscope, but the ADC did not seem to detect it based on the returned value.

              • 4. Re: 14 bit ADC for Load Cell
                user_14586677

                The first thing I see are you have read modify writes to ports with

                   

                mixed I/O. You need to use shadow registers, see ap note for discussion.

                   

                 

                   

                    

                   

                         

                   

                http://www.cypress.com/?rID=2900     AN2094 - PSoC® 1 - Getting Started with GPIO

                   

                 

                   

                Keep in mind this constraint, from PGA datasheet -

                   

                The input and output voltage ranges of the amplifier do not extend to the power supplies (i.e., they are not "rail-to-rail" opamps). The allowed input range is a combination of input limit, output limit, power supply voltage, analog ground value, and selected gain. This is illustrated in the DC and AC Electrical Characteristics section.

                   

                 

                   

                You can eliminate the PGA and its CM problems, as well as its offsets, drift,

                   

                by connecting + ADCinc input directly to a pin. P2_1 in this case. Otherwise

                   

                set OpAmp bias in global properties to high power.

                   

                 

                   

                You have the column clock for ADCinc set at 1 Mhz, but optimal from datasheet

                   

                is 2 Mhz.

                   

                 

                   

                Your CPU clk is set for Sysclk/8, unless you are trying to save power consider

                   

                setting it to Sysclk/1.

                   

                 

                   

                If the INA125 is running off split supplies keep in mind the CM input range for

                   

                PSOC 1 is analog ground / Vss, you cannot input - signals into PSOC. So you

                   

                have to offset the INA125 output.

                   

                 

                   

                Regards, Dana.

                • 5. Re: 14 bit ADC for Load Cell
                  du.lo

                   Thanks Dana!

                     

                   

                     

                  I just tried the opamp high power option and the clock changes recommendations. The sensitivity is better, but still not enough. I'm going to try switching out the PGA and give the other option you suggested a try.

                  • 6. Re: 14 bit ADC for Load Cell
                    du.lo

                     Hello Dana,

                       

                    I changed to read P2_1 and got rid of PGA. Unfortunaly same results. I added weights until the ADC reported a +1 division change and that was a difference of 69mV. For lack of better describing terms, the resolution I got from this simple incomplete test is that I'm getting 69mV/div instead of the calculated .3mV/div for the 14bit ADC.

                       

                     

                       

                    I think there's still some setting I'm getting wrong...

                       

                     

                       

                    Thank you for your continued help.

                    • 7. Re: 14 bit ADC for Load Cell
                      user_14586677

                      Your reference is Vdd, a divider off that. Point is are you averaging readings

                         

                      to take out noise ? A good measure is to use your DSO on infinite persistence

                         

                      and look at Vdd rail. You will probably see  at least a couple hundred mV which

                         

                      you need to reduce.

                         

                       

                         

                          

                         

                                http://www.cypress.com/?rID=2843     AN2096

                         

                          

                         

                                http://www.cypress.com/?rID=2641     AN2239

                         

                       

                         

                      Best bulk caps for this purpose is Polymer Tantalum, their ESR curves and order

                         

                      of magnitude better than traditional Tanatalums, And of course a ceramic disk in

                         

                      parallel. 01 uF or better.

                         

                       

                         

                      Do you have a link for the load cell datasheet ?

                         

                       

                         

                      Regards, Dana.

                      • 8. Re: 14 bit ADC for Load Cell
                        du.lo

                         I will look to improve VCC noise. 

                           

                        I haven't been averaging or doing any filtering. I'm actually hoping to see the noise in the ADC reading, but do not see any. The readings are consistently one value until I try to apply at least 69mV worth of weight, in which case it goes up, but once I remove the weight, the ADC goes back to the initial value. Can't see any noise in the ADC readings.

                           

                        Load Cell: SBO-2K (https://www.transducertechniques.com/sbo-load-cell.aspx)

                        • 9. Re: 14 bit ADC for Load Cell
                          du.lo

                           I improved the VCC a bit, but maybe is not there yet. I have the VCC ripple down to 60mV peak to peak. I'll look into getting a quieter supply.

                             

                          Another bit of information about what I just observed. This is rough estimates and not precisely measured at all.

                             

                          Placing 30lbs on the LC, the ADC reports a +1 change, placing 31lbs has a +2 change, 32lbs, gives +3 change... 

                             

                          Thank you for your help.

                          • 10. Re: 14 bit ADC for Load Cell
                            user_14586677

                            First a lkittle troublshooting -

                               

                             

                               

                            1) Connect ADCinc input to a pot connected to VDD. Using a voltmeter

                               

                            set the pot to different positions and compare what you read on voltmeter to

                               

                            what PSOC is producing.

                               

                             

                               

                            Remember ADCINC_iClearFlagGetData() returns a signed integer, Vdd/2 input =

                               

                            0x0000.

                               

                             

                               

                            Regards, Dana.

                            • 11. Re: 14 bit ADC for Load Cell
                              du.lo

                               Would it still return a signed integer when the setting has been changed to unsigned in the configuration GUI?

                              • 12. Re: 14 bit ADC for Load Cell
                                user_14586677

                                Looking at .lst file the ASM does not appear to test for global

                                   

                                flag/setting, however I am not an ASM program capable person

                                   

                                on this processor. The pot test should quickly answer that question.

                                   

                                 

                                   

                                Regards, Dana.

                                • 13. Re: 14 bit ADC for Load Cell
                                  du.lo

                                   I put a 10k pot and checked the ranges and they seem to behave like they should. Changing the pot to yield 18mV change gave the ADC a change of 2 division. The math doesn't seem to be correct, but this might be because the signed /unsigned setting is not correct.

                                     

                                  ADC count of 0x1243 for 583mV reading on the oscope. Mathematically 0x1243 should yield 4675*(5/(2^14))= 1.426V but that might be something else I can deal with after calibration and filtering.

                                     

                                  The symptoms seems very strange though, with the pot the ADC values seem to change linearly with change in voltages, however with the INA125 output it does not seem to be that way.

                                  • 14. Re: 14 bit ADC for Load Cell
                                    user_14586677

                                    Mathematically 0x1243 should yield 4675*(5/(2^14))= 1.426V

                                       

                                     

                                       

                                    If its signed then it would be 4675*(2.5/2^13) + 2.5 = 3.93 V

                                       

                                     

                                       

                                    Don't forget to pay attention to the Common Mode Range limitations

                                       

                                    of the IA125, from datasheet -

                                       

                                     

                                       

                                    INPUT COMMON-MODE RANGE

                                       


                                    The input common-mode range of the INA125 is shown in
                                    the typical performance curves. The common-mode range is
                                    limited on the negative side by the output voltage swing of
                                    A2, an internal circuit node that cannot be measured on an
                                    external pin. The output voltage of A2 can be expressed as:
                                    V02 = 1.3VIN – (VIN – VIN) (10kΩ/RG)
                                    (voltages referred to IAREF terminal, pin 5)

                                       

                                    The internal op amp A2 is identical to A1. Its output swing
                                    is limited to approximately 0.8V from the positive supply
                                    and 0.25V from the negative supply. When the input com
                                    mon-mode range is exceeded (A2’s output is saturated), A1
                                    can still be in linear operation, responding to changes in the
                                    non-inverting input voltage. The output voltage, however,
                                    will be invalid.

                                       

                                    Regards, Dana.

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