4 Replies Latest reply on Jul 6, 2010 9:42 AM by userc_40597

    delta sigma instrument. amp

      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?




      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

          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

             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




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



            • 3. Re: delta sigma instrument. amp

              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

                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?