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Ambient light rejection circuit don't reject it very well

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David Hess:

--- Quote from: pwlps on April 15, 2019, 06:31:40 am ---
--- Quote from: David Hess on April 14, 2019, 11:14:36 pm ---Reject the ambient signal by driving a current back into the input of the photodiode amplifier as shown in figure 4 of this Burr-Brown datasheet.
--- End quote ---

Driving a current back is only to protect the transimpedance from saturating (but this is apparently not the problem here), otherwise this circuit is equivalent to a transimpedance+highpass.
--- End quote ---

This idea is more common with instrumentation and difference amplifiers where it allows a high-pass response without compromising the common mode rejection of the amplifier but I knew it was also used with photodiode amplifiers to reject ambient light so that is the example I gave.

Of course it results in an AC coupled response.  But because the feedback is to the input of the first amplifier, it prevents that amplifier from saturating due to ambient lighting.  Rejecting the ambient lighting is the whole point.

pwlps:

--- Quote from: hamster_nz on April 15, 2019, 09:45:16 am ---Have you tried a coding system that can be AC coupled?

 8b/10b comes to mind - it is pretty easy to implement and is pretty much the industry standard.

Maybe a variant of Manchester encoding could work too.

--- End quote ---

For Manchester encoding it is possible to do it in the software, without any specialized IC, coding the data bytes in such a way that a standard RS232 UART will be able to send/receive it as a normal data stream :
http://www.quickbuilder.co.uk/qb/articles/Manchester_encoding_using_RS232.pdf

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