Capcitor selection value
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From: john1987 on 22 Jul 2010 14:15 Hi, I have seen the following peak detector document on the internet and have following questions http://nov55.com/amr/tu.html 1. Is it a half wave rectifier? 2. I expect to read different voltage values at different times and than sample them using ADC sampling at 500KHz. The diagram suggest that there is no path to discharge the capacitor 0.47uF once it gets charged. So, If I read +5 volts than how can I discharge the cap to read a differnet value later? I thought that may be I can add the switch across the capacitor to discharge it and control the timing of the ON and OFF by using the microcontroller. So, lets suppose after reading the value of 5 volts acorss the capacitor, I discharge it by turning ON the switch and then after micro read the value, it will turn OFF the switch and then wait for another value across the capacitor. But How will the micro know that the different value is available across the capacitor? 3. What is the role of the diode in the feedback loop? 4. How should I choose the value of the capacitor? my input analog signal is 5 volts sine wave peak to peak with the fequency of 100KHz. Thanks John
From: George Herold on 22 Jul 2010 17:05
On Jul 22, 2:15 pm, john1987 <conphil...(a)hotmail.com> wrote: > Hi, > > I have seen the following peak detector document on the internet and > have following questions > > http://nov55.com/amr/tu.html > > 1. Is it a half wave rectifier? No it's a peak detector. The output is the highest voltage seen at the input. > > 2. I expect to read different voltage values at different times and > than sample them using ADC sampling at 500KHz. The diagram suggest > that there is no path to discharge the capacitor 0.47uF once it gets > charged. The 10M ohm resistor is there to discharge the cap. Something like a 5 second time constant. So, If I read +5 volts than how can I discharge the cap to > read a differnet value later? I thought that may be I can add the > switch across the capacitor to discharge it and control the timing of > the ON and OFF by using the microcontroller. So, lets suppose after > reading the value of 5 volts acorss the capacitor, I discharge it by > turning ON the switch and then after micro read the value, it will > turn OFF the switch and then wait for another value across the > capacitor. But How will the micro know that the different value is > available across the capacitor? Sound like you might want a sample and hold circuit. > > 3. What is the role of the diode in the feedback loop? I believe it gets rid cancels out the other diode drop, but I didn't analyze it. > > 4. How should I choose the value of the capacitor? Reset time constant is RC. > > my input analog signal is 5 volts sine wave peak to peak with the > fequency of 100KHz. > > Thanks > John I don't think you want a peak detector. That web site is a bit weird. He defines his current opposite to the rest of the world. George H. |