11 Replies Latest reply on Jun 4, 2015 6:22 PM by user_373995786

    PSoC 4 – Low ADC input resistance.

    user_373995786

       Hello,

         

      The input resistance for the SAR ADC in the PSoC 4 is 2.2KOhms, is this correct?     

       

         

      Just for comparisons the following are the input resistance of PSoC 4, PSoC 5 and Atmel atmega328p:     

       

         

       

         

      PSoC 5 ADC Input resistance:     

       

         

      20-bit Delta-sigma ADC DC Specifications     

       

         

       

         

      Rin_Buff ADC input resistance Input buffer used 10 – – MOhms     

       

         

      Rin_ADC16 ADC input resistance Input buffer bypassed, 16-bit,     

       

         

      Range = ±1.024 V – 74 – kOhms     

       

         

      Rin_ADC12 ADC input resistance Input buffer bypassed, 12 bit,     

       

         

      Range = ±1.024 V – 148 – kOhms     

       

         

       

         

      SAR ADC:     

       

         

      RIN Input resistance – 180 – kΩ     

       

         

       

         

       

         

      Atmel Atmega328P:     

       

         

      Rain -  Analog Input Resistance 100 MOhms     

       

         

       

         

      Thank you,     

       

         

      Best regards,     

       

         

      João Nuno Carvalho     

       

        • 1. Re: PSoC 4 – Low ADC input resistance.
          user_78878863

          According to the data sheet yes. If you need a higher input resistance, use an OpAmp as buffer.

          • 2. Re: PSoC 4 – Low ADC input resistance.
            user_373995786

             But if one uses one of the two opamps in the 4200 the SAR ADC machinery of the multiple channels can’t be used.

               

             

               

            Best regards,

               

            João Nuno Carvalho

               

                 

             

            • 3. Re: PSoC 4 – Low ADC input resistance.
              user_14586677

              The SAR reference is +/- 1%. If you use the internal Vbg reference

                 

              and add the buffer, its error, Voffset, will add to the signal chain error.

                 

              So consider doing a cal cycle, like grounding buffer input, to get the

                 

              offset and correct readings if you are trying to get close to the Vbg

                 

              error. If you use external .1% Vref you can get near 12 bit accuracy

                 

              so becomes even more importent to do a cal cycle to remove buffer

                 

              offset. Buffer offset is speced at 1 mV high power mode.

                 

               

                 

              Or use PSOC 5LP which has a Vref error of +/- .1%

                 

               

                 

              Regards, Dana.

              • 4. Re: PSoC 4 – Low ADC input resistance.
                Mark Hastings

                The input resistance is not as simple as just X ohms.  It is really a function of the source resistance, the internal switch resistance (~1K) and the sample capacitor ( ~ 10pF).    You can easily increase the effective input resistance by increasing the acquisition time.  If the ADC clock is running at 18 MHz (Max speed) the default acquisition time is about 195ns, or 4 clocks.  If you look at the "Channels" tab of the ADC configuration dialog, you can increase you aquisition time to over 1000 clock periods.  This provides more time for the internal 8 pF cap to charge with your sensor's source resistance, effectively increasing you input resistance. 

                   

                Here is a simple equation to calculate how much time you need for a 12-bit ADC to settle.

                   

                Tacq >= 9 * (Rsrc + Rint) * Csh

                   

                where:

                   

                Tacq => Acquisition time

                   

                Rsc => Your circuits source resistance

                   

                Rint => Internal source resistance (Use about 1K)

                   

                Csh => Internal sample cap (Use 10 pF)

                   

                So if you source resistance is about 10k

                   

                tacq = 9 * (1000 + 10,000) * 10pF  = 990ns or ~= 1us

                   

                Adjust the Acquisition time until your aquisition time is at lease 1 us.

                   

                 

                   

                Mark

                • 5. Re: PSoC 4 – Low ADC input resistance.
                  user_14586677

                  Here is a handy settling time calculator that may be useful -

                     

                   

                     

                      

                     

                            http://designtools.analog.com/dt/settle/settle.html

                     

                   

                     

                  That would be a nice tool to have in Creator.........hint..........hint.......

                     

                   

                     

                  Regards, Dana.

                  • 6. Re: PSoC 4 – Low ADC input resistance.
                    user_373995786

                     Thank you all for the explanations,

                       

                    I was thinking that the Rin of the ADC like a DC resistance, I will read more carefully the documentation of the SAR ADC and read the chapters on ADC’s of the AoE.     

                     

                       

                    With the configuration that meh explained the equivalent resistance will go up.     

                     

                       

                    Is there a voltage reference chip model that is more commonly used with PSoC 4 or PSoC 5 SAR’s, and PSoC 5 Sigmal Delta?     

                     

                       

                     

                       

                    Thank you,     

                     

                       

                     

                       

                    Best regards,    

                     

                       

                    João Nuno Carvalho    

                    • 7. Re: PSoC 4 – Low ADC input resistance.
                      user_14586677

                      The 5LP internal ref is +/- .1%.

                         

                       

                         

                      What are you trying to achieve -

                         

                       

                         

                      1) Resolution

                         

                      2) Absolute accuracy or relative accuracy, and what is the accuracy target

                         

                      3) Sample Rate

                         

                      4) Measurement range in V

                         

                       

                         

                      Regards, Dana.

                      • 8. Re: PSoC 4 – Low ADC input resistance.
                        user_373995786

                         Hello,

                           

                        Currently I am in a sponge mode trying to understand and collect has much knowledge has I can because I’m studding the PSoC 4 family and in the future I also want to understand the PSoC 5 family (when 059-KIT comes out).     

                         

                           

                        In terms of immediate project needs (next week), I want to make and ADC that’s connected to a piezo and that the intensity of light is a function of the presence of very small vibration in the top of a table (touching the table ligthly) or in the floor.     

                         

                           

                        But this is only one project in a challenge that I ‘m making in which three friends have to make each, a project in their spare time in the space of 2 weeks for the next three months, and that will be 18 projects. The scope and theme of those projects can be whatever each of us desire.     

                         

                           

                        Parallel to that I’m trying to help other group of friends that normally use Arduino to use PSoC 4 049-KIT boards, in this regard and in that group my function is to teach them and to answer there question regarding PSoC 4 because in that group I’m the one that has a little bit more experience with the chip.     

                         

                           

                        So regarding the main question of the thread, I was trying to understand sensitivity (Rin ADC) while connecting directly (without opamp), I now understand the Rin equivalent concept (but I will study a little more).     

                         

                           

                        Regarding the voltage reference, it’s simply a follow up question so that I can fully utilize (with accuracy) the 10bits in PSoC 4 ADC and the 20bits in PSoC 5 ADC when the 059-KIT comes out.     

                         

                           

                        To answer your question I’m trying to understand am trying to understand the options that I have in those 4 paths:     

                         

                           

                        1) Resolution     

                         

                           

                        2) Absolute accuracy or relative accuracy, and what is the accuracy target     

                         

                           

                        3) Sample Rate     

                         

                           

                        4) Measurement range in V     

                         

                           

                         

                           

                        Thank you,     

                         

                           

                        Best regards,     

                         

                           

                        João Nuno Carvalho        

                         

                        • 9. Re: PSoC 4 – Low ADC input resistance.
                          user_367854703

                          meh, so where's the 2K2 "Input resistance" in your calculations?
                          Or is it really "Input series resistance", and you are using 1K as typical (as 2K2 is datasheeted as worst case)?
                          Looking at the datasheet I interpreted "Input resistance " as "DC input resistance" meaning 2K2 when looking at the input to GND (or Vss), but I'm not an EE... should this parameter be renamed "Input series resistance"?

                          • 10. Re: PSoC 4 – Low ADC input resistance.
                            user_14586677

                            1) Resolution

                               

                             

                               

                            The delta V / bit of the A/D. If I have a 10 bit A/D, 5V Vref, then resolution is

                               

                            5V / 1024 = ~ 4.88 mV / bit or count of the A/D

                               


                               

                            2) Absolute accuracy or relative accuracy, and what is the accuracy target

                               

                             

                               

                            Absolute, if Vx = 5 V and you measure it using a perfect Vref for the A.D what is the

                               

                            accuracy of the measurement ? Thats absolute accuracy.

                               

                             

                               

                            But say the Vref is something like Vdd, which is usually +/- 5 or 10 %. Then the

                               

                            absolute measurement accuracy is now +/- 5 or 10 % because it is relative to

                               

                            the Vref.

                               

                             

                               

                            3) Sample Rate

                               

                             

                               

                            How many A/D conversions per unit time.

                               

                             

                               

                            4) Measurement range in V

                               

                             

                               

                            Trying to measure 0 - 1 V or 0 to 5V or.......

                               

                             

                               

                            www.dropbox.com/s/aq8pyz7d9oc8r5t/analog_digital_conversion.zip

                               

                             

                               

                            Regards, Dana.

                            • 11. Re: PSoC 4 – Low ADC input resistance.
                              user_373995786

                               Thank you very much Dana,

                                 

                              Best regards,

                                 

                              João Nuno Carvalho