4 Replies Latest reply on Jul 6, 2010 9:42 AM by robert.seczkowski

    delta sigma instrument. amp

    robert.seczkowski

      I'm trying to use del sig adc plus 2 pga's as an instrumentation amplifier.(acc to AN60319)

         

      ADC datasheet says, GetResult16() returns signed value; GetResult32() (not known?!!!! - I presume unsigned)

         

      In single input ADC mode, the -Input should be tied to VSS. How it is in differetial mode?

         

      ADC normally cannot measure negative values.

         

      What is the input range (PGA gain 1; ADC gain 1,differential input,internall vref=1.024V)  for PGA+ and PGA- pins?

         

      Example:

         

      PGA+ = 0V; PGA - = 1mV or PGA+=1mV, PGA-=0V,diff mode,vref=1.024V what will be the output counts(res 20bits);?

        • 1. Re: delta sigma instrument. amp
          robert.seczkowski

          In other words:

             

          Counts = Vd*gain/Vref * 2^(n-1) (an60319 page 2) Vd-differetial voltage= Vpositive - Vnegative, n-resolution

             

          What counts will be if Vd < 0,Vd = 0,Vd>0?

          • 2. Re: delta sigma instrument. amp
            content.librarian

             Hi Robert,

               

            The ADC by itself can provide a gain of 32 in the component now. It can provide a maximum gain of 128 in the future version(Beta 5) of PSoC creator. So, just using the ADC will be sufficient for gains less than 128. For gains greater than 128, please refer to the connection shown in this KB

               

            http://www.cypress.com/?id=4&rID=40623

               

            Let me know if your design requires more gain or some other necessity prevents the use of this design

               

            Regards,
            Praveen

            • 3. Re: delta sigma instrument. amp
              robert.seczkowski

              It's not the issue of gain but the ADC output.

                 

              I guess ADC Delsig gives always 2's complement outputs

                 

              ADC_input_plus = 0V

                 

              ADC_input_minus = 1mV

                 

              then if Vref=1.024V,gain 1,res=20bit

                 

              (((0V-1mV)*1)/(1.024+1mV))* 2^(20-1)=511 or 523777

                 

                  Vdiff       K       Inp Range    Max counts

                 

              Now if

                 

              ADC_input_plus = 999mV

                 

              ADC_input_minus = 1V

                 

              then if Vref=1.024V,gain 1,res=20bit

                 

              (((999mV-1000mV)*1)/(2000mV))* 2^(20-1)=262 or 524026

                 

                           Vdiff          K       Inp Range    Max counts

                 

               

                 

              Why same difference give different results?

                 

              Because measurement range changes with -Input DC value   (acc. to DelSig manual)

              • 4. Re: delta sigma instrument. amp
                robert.seczkowski

                2000mV should be replaced with 2024mV to be honest (-Input +- Vref)

                   

                In other words:

                   

                Is there a method to measure voltages around 0 like +- 2mV?

                   

                regards

                   

                robert