1 2 3 Previous Next 33 Replies Latest reply on Aug 5, 2015 7:01 AM by anna.leshinsky

    VDAC8 output

    anna.leshinsky

      Hi,

         

      What is the output lavel of the VDAC8?

         

      I am inputting ~2V P-P signal to ADC filtering it and output threw VDAC8- the output is 150mV P-P riding on ~50Hz noise.

         

      Is it expectable results?

         

      THANKS 

        • 1. Re: VDAC8 output
          user_14586677

          From datasheet (you may want to buffer it) -

             

          Both output ranges have 255 equal steps. The VDAC8 is implemented by driving the output of the current DAC through resistors and obtaining a voltage output. Because no buffer is used, any DC current drawn from the DAC affects the output level. Therefore, in this mode any load connected to the output should be capacitive.

             

           

             

          The Zout of the VDAC is 4K for the 1V range, 16K for the 4 V range.

             

           

             

          I assume you are NOT connecting a 50 ohm load to it, that would not work whether or not

             

          its buffered.

             

           

             

          Regards, Dana.

          • 2. Re: VDAC8 output
            user_14586677

            You did this step -

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            • 3. Re: VDAC8 output
              anna.leshinsky

              Ok. understood.... and the current is low? because of it I am getting 150mV signal?

                 

              Ok, I will put the buffer .

                 

              Tel me please ....one additional issue:  the total number of the PGA and OPAMP is 4? or there is 4 OPAMP and some additional PGA?

                 

              Thanks

                 

              Anna

              • 4. Re: VDAC8 output
                user_14586677

                Total is 4 OpAmps. Each time you use a PGA it uses one OpAmp.

                   

                 

                   

                Don't forget, in some situations you can mux them, use one in a time slot fashion with different

                   

                configurations. Make it do multiple duties.

                   

                 

                   

                Regards, Dana.

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                • 5. Re: VDAC8 output
                  anna.leshinsky

                  Thanks.

                     

                  I am thinking about muxing but I need all of them on the main path.

                     

                  Do you know any other way to buffer Vref ? This action is using 1 OPAMP for me...

                     

                  Can I use ONLY one filter? If I need another one? I am getting a 50Hz noise on the output.....

                     

                  Regards!

                     

                  Anna

                  • 7. Re: VDAC8 output
                    anna.leshinsky

                    Thanks! very useful.

                       

                    I am doing differential measurements and using 2 channels:)

                       

                    but because I am new with psoc parts I have a lot of question and problems :(

                       

                    Thanks!

                       

                    Anna

                    • 8. Re: VDAC8 output
                      anna.leshinsky


                      I attached my project.
                      The problem is that I am getting as an output of the VDAC8 signal that looks in very low sample rate - see example 1 pic
                      I connected PGA that works like buffer and amplifier , a PGA output looks very strange - example 2 pic
                      After removing the caupling capacitor ( C3) from the PGA output I getting something like square wave - example 3 pic

                         

                      change the PGA to OpAmp follower problem 2+3 not relevant but the output still strange. - example 4 pic

                         

                      PLEASE HELP!
                      I have no Idea how to solve it....

                         

                      Thanks!!!!! Anna

                      • 9. Re: VDAC8 output
                        user_14586677

                        Some observations -

                           

                         

                           

                        1) You load the output of PGA1 with 1 uF, Thats not a good idea, the phase margin

                           

                        would lead potentially to oscillation, at minimum severe ringing.

                           

                         

                           

                        2) Your VDAC range is 0 - 4 volts, PGA1 G = 16, so you are asking for 64 volts

                           

                        out of PSOC. The PGA should be set for a G = 1, in fact just use an OpAmp configed

                           

                        as a buffer. PSOC can only produce an analog signal that ranges from Vssa to Vdda.

                           

                         

                           

                        3) In the Piezo path you have 10 uF polarized caps, and very high fdbk and input R's.

                           

                        Electrolytics are fundamentally leaky capacitors, and that leakage x the fdbk and input

                           

                        Rs will create large offsets. To maintain large Zin either replace caps with MLC

                           

                        ceramics or DC couple the path and eliminate the caps.

                           

                         

                           

                        Regards, Dana.

                        • 10. Re: VDAC8 output
                          anna.leshinsky

                          Hi,

                             

                          1. I replaced the PGA with OPAMP follower. The capacitor at the feedback prevents the amplification of the DC that the AC ride on it BUT makes deformations on the signal. I can measure the output of the PGA/OPAMP with the oscilloscope without a cap?   

                             

                          2. I doesn`t get 0-4V from the VDAC, I have ~ 130mV , because of it I made GANE=16 

                             

                          3. I put the ceramic capacitors, but still have a lot of noise. 

                             

                          What do you mean DC couple?  I can? I have an AC input signal and AC output.

                             

                          4. I read a LOT of material about noise on ADC DAC path , you sent me  and I made a lot google but still I don`t have any Idea how to rid off the noise of the VDAC output without filter.

                             

                          I tried to change the sample rate and the input signal...nothing works...

                             

                          You are helping me a lot!!! Thank you very much!

                             

                          I very grateful to you and doesn't have enough words to express my apriciation!

                             

                          Anna

                          • 11. Re: VDAC8 output
                            user_14586677

                            1. I replaced the PGA with OPAMP follower. The capacitor at the feedback prevents the amplification of the DC that the AC ride on it BUT makes deformations on the signal. I can measure the output of the PGA/OPAMP with the oscilloscope without a cap?   

                               

                            Yes, the scope is a Hi Z in, if using 10X probe its 10 Mohm Z, 1X probe 1 Mohm.

                               

                             

                               

                            2. I doesn`t get 0-4V from the VDAC, I have ~ 130mV , because of it I made GANE=16 

                               

                            One way to get the G is do it in the filter settings, then you can just use a OpAmp buffer

                               

                            instead of PGA. If that is not workable what is the offset out of the Vdac with no signal

                               

                            in ? And what are you attaching to the PGA ref pin (other than the cap).

                               

                             

                               

                            3. I put the ceramic capacitors, but still have a lot of noise. 

                               

                            What do you mean DC couple?  I can? I have an AC input signal and AC output.

                               

                            DC couple means a DC path, eg no capacitors in the signal path.

                               

                            Your Vdac output signal, what is it being sent to ?

                               

                            The ceramic caps are to eliminate leakage which would cause offset in the input stage.

                               

                            Any leakage goes thru R 8 and R7, and would cause OpAmp3 to move off its bias point.

                               

                             

                               

                            4. I read a LOT of material about noise on ADC DAC path , you sent me  and I made a lot google but still I don`t have any Idea how to rid off the noise of the VDAC output without filter.

                               

                            If you ground P1_32 what does the noise look like ? If its still present then you have coupled

                               

                            noise in you prototype to some parts of the signal path. If its gone then Piezo wiring picking up

                               

                            noise.

                            • 12. Re: VDAC8 output
                              user_14586677

                              So first adjust the G you want in the input stage OpAmp.

                                 

                               

                                 

                              Then adjust the A/D buffer G to get the maximum dynamic range or signal

                                 

                              for the peak signal in. A starting point is you had the intermediate OpAmp

                                 

                              stage set to a G = 3, the A/D buffer gain has an option of 4, start with that.

                                 

                               

                                 

                              Note I changed the opamp names so you would need to change API names used.

                                 

                               

                                 

                              Regards, Dana.

                              • 13. Re: VDAC8 output
                                user_14586677

                                Keep in mind the connections from Piezo to PSOC should be shielded cable or

                                   

                                a twisted pair, to minimize pickup from AC powerlines in room or other interference.

                                   

                                 

                                   

                                Evaluate one stage at a time to work out signal issues. So start by looking at

                                   

                                OpAmp 3 output, are you satisfied looking at the signal there.

                                   

                                 

                                   

                                I think you can eliminate an opamp and make things simpler by taking

                                   

                                G in the A/D buffer configuration.

                                   

                                 

                                   

                                 

                                   

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