7 Replies Latest reply on Apr 1, 2020 3:01 AM by JoNe_4268451

    How to outputting internal 1.024V Vref inPSoC 5LP

    JoNe_4268451

      Hello,

      I'm trying to get a constant value of voltage of 1.024V from PSoC 5 LP, I've used  the component "Vref" connected to an "opamp" block with an analog output but it doesn't work, instead of, I obtain 970mV. I don't know what I'm doing wrong.

      Anotación 2020-03-26 181550.png

      Vref is configured as follow:

      conf1.png

      conf2.png

      The opamp is a "follower" and is settled "low power".

      I would like obtain this voltage because I'm practising with ADC_SAR component and I need 1.024V in one output to regulate by a potentiometer.  Is there any other way to get this voltage in PSoC 5 or PSoC 4?

       

      Thank you,

      Jose Antonio.

        • 1. Re: How to outputting internal 1.024V Vref inPSoC 5LP
          BoTa_264741

          Antonio,

          What is the load impedance on this Opamp? (How much is the current draw from this Opamp). Try using one of the Opamap direct output, e.g. P3_6 or P3_7 (see picture). All other outputs have 300-500 oHm serial resistance.

          KIT-059_06_B_opamps.png

           

          /odissey1

           

          P.S. See KIT-059 and PSoC Annotation Library for other options of PSoC5 Prototyping Kit.

          KIT-059: annotation component for CY8CKIT-059 PSoC5LP Prototyping Kit

          PSoC Annotation Library v1.0

          1 of 1 people found this helpful
          • 2. Re: How to outputting internal 1.024V Vref inPSoC 5LP
            JoNe_4268451

            Hi,

            I don't connect any load to the output analog pin except the oscilloscope probe, which has a high impedance input (1MOhm, 85pF-120pF). The analog output is configured as "high impedance analog":

            3.png

            The datasheet says that the maximum current that the opamp provides is 25 mA.

            I've selected pin P3_6 and P3_7 as output, but the result doesn't change, I only get 947mV.

             

            Thank you for the Annotation Library, it looks really useful.

            1 of 1 people found this helpful
            • 3. Re: How to outputting internal 1.024V Vref inPSoC 5LP
              BoTa_264741

              Antonio,

              I tested it with o-scope and voltmeter, both show 1.024V. Output pin P3_0 on KIT-059.

              /odissey1

              VREF 1024_Opamp_Pin30_01.jpg

              1 of 1 people found this helpful
              • 4. Re: How to outputting internal 1.024V Vref inPSoC 5LP
                LePo_1062026

                Antonio,

                 

                Oscilloscopes are great for what they are intended.  They are great for measuring signal as it is displaced in time.

                 

                However, most digital scopes don't brag that they are only 8-bit ADCs with maybe 5% accuracy and that's across the entire full-scale vertical of the screen.

                 

                If you're lucky to have 10 divisions vertical on your scope and your setting is 1V/division with GND centered vertically at division 5 then your top to bottom range is -127 to 127 ADC counts @ ADC resolution of 10*1V/256 =  0.04V/ADC count.

                 

                Your measurement, not taking into account the accuracy of calibration, can be off by +/-0.04V.  Additionally scope ADC inputs are notorious for picking up noise from various sources.  For example, in one project I had nearly 0.3Vpp of noise in measuring a circuit.  (Note: make sure the scope probe is properly grounded to your system).  Once I turned off the fluorescent light I had on, the noise drop to below 0.05Vpp.  The scope probe and grounding wire was acting as a loop antenna picking up the fluorescent light ballast noise.

                 

                /odissey1 used a meter with significant averaging and at least 3 1/2 digits of resolution.

                 

                Len

                1 of 1 people found this helpful
                • 5. Re: How to outputting internal 1.024V Vref inPSoC 5LP
                  JoNe_4268451

                  Hello, I have tested again and I have discovered some things...

                  When I measure with the multimeter I get 1.018V, I think the multimeter may need to be calibrated.

                  multimeter.jpg

                  odissey1, why did you connect a resistance between the pin 3_0 and 3_4? And what is the funcionality of the capacitor?

                  Len, I think you're so right, because when I connect the probe of the oscilloscope, I get a really bad result:

                  oscilloscope.jpg

                  My oscilloscope works through USB, I don't know is this affects. I thought the probe wouldn't disturb due to its high impedance, but it looks like the probe alterate all circuit when it's connected.

                  The subject that you talk about the fluorescent light ballast noise is really interesting, I have had similar problems with that in other projects.

                  I have read the documentation of my oscilloscope but it doesn't show the number of bits of ADC neither I can get the resolution, I only know that I have several scaling leves to select in the software, but it does not have to do with the accuracy:

                  oscilloscope.jpg

                  I really apreciate your help, sorry for my english.

                  Thank you for your time.

                   

                  Jose Antonio.

                  • 6. Re: How to outputting internal 1.024V Vref inPSoC 5LP
                    BoTa_264741

                    Antonio,

                    The resistor  (47k) and cap are leftovers from the previous project. I was lasy to remove them. My scope (Rigol 1054) is surely 8-bit, it measures 1.03V averaged value. I suspect that your scope has 50oHm input, so when Opamp pushes 20mA, it displays

                    ~20mA x 50oHm = ~1V .

                    I suggest to change Opamp to medium/low power to test this idea.

                    /odissey1

                    1 of 1 people found this helpful
                    • 7. Re: How to outputting internal 1.024V Vref inPSoC 5LP
                      JoNe_4268451

                      Hi,

                      I have changed the Opamp to medium/low power without result, I thought that was impossible my scope had 50oHm input, the probe has 1/10MOhm. Anyway, something is wrong with my oscilloscope, I have installed another software to manage it, and surprisingly the value now is higher 1.05V:

                      IMG_20200401_112959.jpg

                      If I disconnect only the probe, the result doesn't change, only change when I disconnect the ground of the oscilloscope, so I tested changing the connection of the ground to another pin of PSoC, and by this way, the multimeter shows 1.017V:

                      IMG_20200401_113016.jpg

                      I also tried to measure with another PSoC:

                      IMG_20200401_112929.jpg  kit4measure.png

                      Whe all is disconnect except PSoC 4, this one get a value of ~1.024V, but when I connect the ground of oscilloscope too, the value changes to 1.017-1.020V, so the problem comes apparently from the ground of my oscilloscope, I will have to measure only with another PSoC if I want get correct values

                       

                      Jose Antonio.