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

    Sample & Hold not giving expected results



      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

          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 ?