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sample and hold

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picozero

New Member
Hi
I have to implement a sample and hold circuit. Assuming that I have to inputs :
a voltage value from a source (a photodiode) and another voltage value from another source pulsed with a delay (another photdiode). I have to compute the ration of both.

Of course their values can change during the time but their frequency no, they should behave like the signals in the second and third row of this picture:
**broken link removed**

How can I implement the hold/buffer for the first lasting enough for capturing the second one ?

How do you usually compute a "ratio" of voltage ?

Thanks in advance
 

crutschow

Well-Known Member
Most Helpful Member
For the sample/hold you could use a chip such as the LF398.

You can always compute the digital ratio by converting the voltages to digital with an A/D converter and use a microprocessor. To generate an analog ratio you could use a multiplier/divider chip in the divider mode. Analog Devices sells many different models.
 
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picozero

New Member
Thank you I ll look up this divider.:p

But assuming I'd like to do everything for the moment without PIC and DAC.

How can I implement a buffer with operational for the sample and hold ?
 

crutschow

Well-Known Member
Most Helpful Member
How can I implement a buffer with operational for the sample and hold ?
Do the photodiodes have amplifiers or are those the buffers you need?
 

picozero

New Member
I am using some photodiodes with transimpedence connected :

https://www.electro-tech-online.com/custompdfs/2011/01/getfileaspxtypepressfiletsl250r-e30.pdf

Sorry I don't know why I skimmed your answer about the LF398., thank you !

The Analog Devices devider are too expensive and I don't think the available models of tranference functions fit my purpose. I think the original advice of the DAC conversion and then operations in the digital domain ( a processor or specific IC) is better.
Thank you;)
 

crutschow

Well-Known Member
Most Helpful Member
I think the original advice of the DAC conversion and then operations in the digital domain ( a processor or specific IC) is better.
Thank you;)
If you use an A/D (not DAC) converter you may not need a separate sample and hold. Many A/D's include a S&H as part of their input circuitry.
 

picozero

New Member
What kind of ADC do you advice me if I have an input between 0 and 9 V and I'd like to have at least 7 bit output ?

possibly not smd and not expensive
 
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