6 Replies Latest reply on Jul 12, 2012 5:28 AM by naresh.vemishetty

    PGA

    naresh.vemishetty

       hello sir/mam,

         

                     I went into sample project of Programmable Gain Amplifier(PGA)  in PSoC creator, when i give input as

         

                      Freq:500 hz,,input voltage=.2v,,Gain =1,,Medium power,Offset=0,,Vref=internal vss  the output  obtained is not exact as input as i shown in Pic..

         

                     But when i  Increase the Offset to 1v its giving the correct output . 

         

                       Later i increased the gain to "2" and for every offset value(0,1,2,3) the output voltage i am getting is equal to the input voltage.....plz help me in getting  the correct output....

        • 1. Re: PGA
          user_14586677

          Input CM range for analog is Vssa to Vdda, so you need to effect a DC offset

             

          into the PGA. One approach just using resistors -

             

           

             


          http://electronicdesign.com/analog/use-excel-calculate-d-level-shifter-resistor-values

             

           

             

          Attached is excel file to calculate R values.

             

           

             

          Regards, Dana.

          • 2. Re: PGA
            naresh.vemishetty

            i am not getting the output Sir .......can i get some more solutions for my previous question.....plz......

            • 3. Re: PGA
              user_1377889

              When you upload the complete project here, we all can have a look at and probably see what's going amiss.

                 

              To do so: Build -> Clean Project

                 

              File -> Create workspace Bundle(minimal)

                 

              and then upload the resulting ZIP here.

                 

               

                 

              Bob 

              • 4. Re: PGA
                gautam.das.g

                Hi naru,

                   

                 

                   

                Providing the right offset is important in application like this. Else, the output will be clipped.

                   

                The operating voltage of the device should be greater than the input/ output of the PGA. Are you operating at 3.3V or at 5V?

                   

                As Bob has already mentioned, please upload your project here. It  becomes easier to figure out the root cause.

                   

                 

                   

                 

                   

                Thanks,

                   

                Gautam

                • 5. Re: PGA
                  user_14586677

                  The input CM range of the PGA is

                     


                  Vdda >= Vin  >= Vssa

                     


                  The output PGA CM range is

                     


                  (Vdda - .3V) >= Vin X G >= (Vssa + .3V), the .3 is loaded spec, see datasheet for load.

                     

                   

                     

                  Example, Vssa = 0, Vdda = 3.3, You want a 2.5V Pk-Pk swing on output, your input =
                  .2V, ground referenced. So to start with your input violates input CM range because it
                  swings .1 V below ground on negative peak (assuming sinewave), we need to offset the
                  input or AC couple it to PGA to meet input CM range. To DC offset, hence avoid a coupling
                  cap, use the prior post R divider to Vdda. To AC couple bias with an R divider from Vdda
                  to PGAin to Vssa, which biases input such that peak output does not exceed PGA output CM
                  range.

                     

                   

                     

                  So G = 2.5 / .2 = 12.5. Closest G = 8 in configurator. If you picked the next higest G the
                  output of the PGA would be in clipping. Therefore output swing will be 8 x .2 = 1.6V.
                  Nominally the DC component of the input signal should be offset such that ouput swings
                  symmetrically about (Vdda - Vssa) / 2 = 1.65 V. You can do this by placing a resistor
                  divider, on PGA in and AC coupling signal. Or offseting the ground referenced input signal
                  with a DC offset.

                     


                  So Vout = G x ( Voff + Vsig), or Voff = Vout/G - Vsig. For Vsig = 0 (output then at Vdd/2),
                  Voff = Vout/G = Vdd / ( 2 * G) = 3.3 / 16 = ~.21 V. So your divider should set that up as
                  offset.

                     


                  This then satisifies both input and output CM range of the PGA.

                     

                   

                     

                  Regards, Dana.

                  • 6. Re: PGA
                    naresh.vemishetty

                     I got output of PGA ....Thank you for all( SIR) your support .....