6 Replies Latest reply on Sep 8, 2014 9:18 AM by user_14586677

    Multiple Analog outputs

    user_343349849

       I am looking at setting up a project that requires 4 x Analog outputs. The system receives digital data values from 0-1000 via a serial port. The values then need to be converted to 0-2.5 DC voltage and output to a pin.

         

      The DAC appears to be 8-bit resolution and I have seen some apps for increasing resolution using 2 Dacs. However, would it be better to use 4 x PWM output and some op amps?

         

      The internal opamps appear to provide rail to rail output. I was thinking of having PWM to Digital output pin and then a 2 pole Low pass filter connected back into PSOC to an opamp (to form a Sallen & Key filter to convert the PWM to DC voltage), finally a voltage follower and DC out a pin?

         

      Or is there a more effcient way of doing this? I don't need much current, the 0-2,5V DC linear voltage is being used to control the setting of each of 4 x  4-20mA generation IC's

         

       

         

      While I am on the topic of analog, Is there a component that can act as a shunt resistor to ground to convert 0-20mA signal input to 0-2V signal for measurement by ADC. Or would external resistor to ground be necessary?

         

      Thanks

        • 1. Re: Multiple Analog outputs
          user_1377889

          The 8-bit DAC only delivers a precision of ~0.5%, your input range seems to be 1 0/00 precise, so with a PWM you will be better off. When you select the clock frequency high enough a rather simple method of filtering, just an RC, could be sufficient.

             

          There is no shunt-component, so you'll need an external (precisionn) resistor.

             

           

             

          Are you designing for naval purposes?

             

          Bob

          • 2. Re: Multiple Analog outputs
            user_14586677

            The DAC appears to be 8-bit resolution and I have seen some apps for increasing resolution using 2 Dacs. However, would it be better to use 4 x PWM output and some op amps?

               

                

               

                      

               

            See the following - http://www.cypress.com/?rID=47478     AN64275

               

             

               

             

               

            The internal opamps appear to provide rail to rail output. I was thinking of having PWM to Digital output pin and then a 2 pole Low pass filter connected back into PSOC to an opamp (to form a Sallen & Key filter to convert the PWM to DC voltage), finally a voltage follower and DC out a pin?

               

            Yes this is fine, also OpAmp serves buth as filetr and buffer. What you give up is step response and

               

            settling time, this you can calc from simple LaPlace intital conditions. Your final value would be 1 LSB

               

            (or you choose) for purposes of calc. Note a simple RC would not be appropriate at high res.

               

            Or is there a more effcient way of doing this? I don't need much current, the 0-2,5V DC linear voltage is being used to control the setting of each of 4 x  4-20mA generation IC's

               

             

               

            While I am on the topic of analog, Is there a component that can act as a shunt resistor to ground to convert 0-20mA signal input to 0-2V signal for measurement by ADC. Or would external resistor to ground be necessary?

               

             

               

            External is needed. Especially if you want to maintain some degree of acuracy.

               

             

               

            Regards, Dana.

            • 3. Re: Multiple Analog outputs
              user_14586677

               While I am on the topic of analog, Is there a component that can act as a shunt resistor to ground to convert 0-20mA signal input to 0-2V signal for measurement by ADC. Or would external resistor to ground be necessary?

                 


                 

              One could consider using the TIA but that eats up an OpAmp and the absolute value tolerance

                 

              of the fdbk R is very poor.

                 

               

                 

              Attached is some aps info on PWM settling time and other considerations, like ripple.

                 

               

                 

              Regards, Dana.

              • 4. Re: Multiple Analog outputs
                user_14586677

                Some better stuff on PWM filters, attached.

                   

                 

                   

                Regards, Dana.

                • 5. Re: Multiple Analog outputs
                  user_343349849

                   Thank you very much for your prompt responses, I appreciate your comments. (The application is not Naval).

                     

                  I did find the TIA but the minimum resitance is 20k (too high for 4-20mA)

                     

                  My previous projects used PWM and external op amp with 2-stage S&K filter and worked ok for scale 0.1% resolution.

                     

                  Will look at the zip file supplied.

                     

                   

                     

                  Thanks again

                  • 6. Re: Multiple Analog outputs
                    user_14586677

                    Glad to have been of assistance.

                       

                     

                       

                    Regards, Dana.