12 Replies Latest reply on Oct 6, 2015 1:25 PM by user_199038981

    PGA Operation

    user_199038981

      Hi,

         

      Using a PGA to buffer a small signal before input to a mux and ATD. The part is configured to use Internal Vss.

         

      In the program, I accidently forgot to turn it on via PGA_Start() yet it does give me a directly proportional output with a range that is repeatable and can be calibrated. If the part is turned on, then the dynamic range is reduced to an unusable range, though the ATD count is much higher.

         

      Is the part actually in a low power state and receiving power if I fail to do a PGA_Start()?

        • 1. Re: PGA Operation
          user_14586677

          If you are taking G with a PGA then your CM input range is reduced by

             

          the G you are using before you drive the output into distortion/saturation.

             

          So if you used G = 10, on a Vdd of 5, then input range is limited to 0 - .5

             

          before you loose CM range on the output.

             

           

             

          Not sure of effect you are seeing before its started, what freq are you

             

          inputing to PGA. When you start it if power up in with the G and Power

             

          level as configed, unless you use APIs to change them.

             

           

             

          Regards, Dana.

          • 2. Re: PGA Operation
            user_199038981

            I'm trying to find out if the PGA is getting power without issuing a PGA_Start(). It might be a question for a semiconductor / VHDL guru.

               

            If there is no power applied to the PGA, then I'm doing something like driving a diode through the feedback resistors. The signal is basically a DC level around 240mV.

            • 3. Re: PGA Operation
              user_14586677

              You have a schematic and scope capture of in and out ?

                 

               

                 

              Regards, Dana.

              • 4. Re: PGA Operation
                user_199038981

                Attached as document.

                • 5. Re: PGA Operation
                  user_199038981

                  No scope capture, just reading the ADC_DelSig counts. The input is straight DC, typically 240 millivolts. The PGA is in place to boost a photo diode output.

                  • 6. Re: PGA Operation
                    user_14586677

                    I set up attached test bed.

                       

                     

                       

                    With PGA off, 10 Khz in, G = 1 or 50, I measured 100 mV out of feedthru.

                       

                     

                       

                    My guess is that when PGA is off the feedthru occurs thru stray C of fdbk network,

                       

                    which is probably not switched open when PGA is off ? Note I used adjacent pins so

                       

                    that could contribute to pin to pin coupling. So I moved output to end of port pins

                       

                    and feedthru dropped to 50 mV.

                       

                     

                       

                    Regards, Dana.

                    • 7. Re: PGA Operation
                      user_199038981

                      Thank you for trying that, Dana. I've subsequently opened a case to see what they say and the initial response was that the SC/CT block is configured at boot up, which incorporates capacitors. I'll report back what I hear from them.

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                      • 8. Re: PGA Operation
                        user_199038981

                        So I did open a case and they sent me a project that routes the PGA output to a pin. Their result was a fluctuating voltage due to "a high Z condition. Not sure what the goal of trying the project is yet. I guess so that I can see a fluctuating voltage on a scope in a configuration I'm not using? I did read the PSoC3 architecture section that talks about Miller capacitance. Surely there is someone that understands what's happening at the silicon level.

                        • 9. Re: PGA Operation
                          user_14586677

                          Looking at attached, SC/CT Block from which PGAs are made of, you can see

                             

                          quite a few opportunities for feed thru, off switch parasitics, etc.. This of course

                             

                          does not address silicon issues, like source bulk effect currents modulating

                             

                          driver thresholds, etc.. although I think good design and isolation would have

                             

                          addressed most of this. I would have to file a CASE but I thought that when

                             

                          a _Stop() was issued to a CT block all bias networks were turned off, eg. the

                             

                          block is no longer active. Otherwise how could they achive the deep sleep current

                             

                          specs.....

                             

                           

                             

                          That was a great project they sent you....:)

                             

                           

                             

                          Are you using, in addition to PGA, a TIA to condition the photodiode ? You can

                             

                          take a lot of conversion G there properly setup.

                             

                           

                             

                          Regards, Dana.

                          • 10. Re: PGA Operation
                            user_199038981

                            The thing is, I didn't issue the PGA_Stop(). I just powered up and started the reading the output in the ADC_DelSig, so there might be an encounter of the third kind, ha ha,.

                               

                            Your suggestion about the TIA sounds like the right idea and is what we may be trying next but with an available external op-amp. My problem was, in measuring the light, I was only getting about a 10 ADC count range from fully on to all the way off. The ADC is set to 16 bit and the readings were in up around 10,000. I put in the PGA and it gave me a repeatable range of around 400 counts, which allowed me to calibrate but it was in the range of 700 - 1100 counts. If I hear anything back on my case I'll drop it in here.

                               

                            Thanks for your help.

                            • 11. Re: PGA Operation
                              user_14586677

                              Basically you are working with photo diode currents which are small. But the TIA

                                 

                              has a conversion G quite high since the Rfb can be in the M Ohm range. Just try

                                 

                              it.

                                 

                               

                                 

                              https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&ved=0CB4QFjAAahUKEwj9pc3l9J_IAhXPth4KHf31Cag&url=http%3A%2F%2Fwww.ti.com%2Flit%2Fan%2Fsboa122%2Fsboa122.pdf&usg=AFQjCNGurHa5ICZ2qctQauVDX7A7rUk6HQ&cad=rja

                                 

                               

                                 

                              Regards, Dana.

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                              • 12. Re: PGA Operation
                                user_199038981

                                Hi Dana,

                                   

                                I did try it and the TIA is definitely the solution but I also put in a PGA_INV so the counts I'm reading are in phase. The downside is the amplification also amplifies the noise level but we'll deal with that next. Appreciate your looking into this and guidance. It has indeed been a help.

                                   

                                Nick