1 Reply Latest reply on Nov 26, 2014 11:30 PM by senthilnathanl_

    Sample & Hold not giving expected results

    user_284146

      Hi,

         

      I'm using the sample & hold component in my proyect since I need to sample a signald comming from a photosensor which receives light from a very short pulse of light which comes from an LED.

         

      The  problem is that when I use the sample and hold circuit I get a noisy signal compared to the one I get when sampling without the H&S.

         

      In my case I'm turing on the LED, activating the H&S with a Control register connected to the clk input of the sample and hold , then turning of the LED and lastly I use the SAR ADC to sample 64 times the voltage in the sample and hold. I'm sampling 64 times to get a better resolution. 

         

      When I omit the S&H the signal is good but then it takes too much time for the ADC to make the 64 samples. If I add the S&H it gets very noisy. 

         

      I've checked the TRM which in page 271 shows the Track and Hold diagram and it has a hold cap of 12pF and a voltage follower. 

         

      If I use the S&H making just one sample at a time it gets a "smooth" signal.

         

      I think the might be that the S&H might not be holding the voltage enough time to do all the conversions.

         

      I've tried adding a unity gain PGA between the S&H and the ADC with the same results.

         

       

         

      Does anyone have any idea why this is happening?

         

       

         

      Thank you.

        • 1. Re: Sample & Hold not giving expected results
          senthilnathanl_

          The droop rate of the sample and hold determines how long the sampled voltage will be held without much drop in the voltage. Hold times of 100 us with < 100 uV drop in the sampled voltage is possible with the S&H component.

             

          For how long does the sample and hold is holding the sampled voltage for SAR ADC to convert ?

             

          Can you send the screen shot of the S&H output ? What is the BW of the input signal that is fed to S&H ?