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Pic Adc Input Signal

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richard.c

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Happy New Year to One and All.


I'm using an amplified ADC input circuit very similar to the one in Nigels tutorials, but feeding into the 10 bit adc of a 18f4520 chip.

The signal, for test purposes, is 0 - 5v and this converts fine, however the lowest 1.25v and the top 1.25v of the signal are effectively useless for my needs. Original signal source approx -50mv to +50mv.

If I could loose these parts of the voltage and then amplify the resultant 2.5 upto 5 v , I could achieve twice the resolution on the range I'm really interested in.

My problem is how can I do this ? - is there an easy way to limit the signal in this way ?

thanks

Richard
 
richard.c said:
Happy New Year to One and All.


I'm using an amplified ADC input circuit very similar to the one in Nigels tutorials, but feeding into the 10 bit adc of a 18f4520 chip.

The signal, for test purposes, is 0 - 5v and this converts fine, however the lowest 1.25v and the top 1.25v of the signal are effectively useless for my needs. Original signal source approx -50mv to +50mv.

If I could loose these parts of the voltage and then amplify the resultant 2.5 upto 5 v , I could achieve twice the resolution on the range I'm really interested in.

My problem is how can I do this ? - is there an easy way to limit the signal in this way ?

thanks

Richard

hi,
Basically what you have to do is to use a OPA, apply an offset voltage to the OPA so that the lowest voltage of your input range gives a zero voltage output.
Then amplify the signal voltage so that you get a +5V out for your highest input voltage.
Attached is an example on how to do it, this circuit is for an altimeter with a small output and an offset voltage.
You should be able to modify it for your application.:)
 
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