I've had good results with a transformer feeding a Maxim MX536A 
RMS-to-DC converter, which I then read with the 10-bit ADC in a 16F877A.

The MX536A converts an AC input into a DC output equal to the RMS of the 
input, e.g., a 1 V RMS input provides 1V DC output. Hence, in your case, 
to have a 2.5V DC output correspond to 270VAC, you will need a suitable 
step down transformer, which will provide the isolation you need as well.

Jack



Groups wrote:

>I am looking for an isolated method of measuring mains voltage (230VAC).
>
>First thoughts are a Opto coupler running in its linear region ?
>My concerns are that the transfer ratio degrades over time.
>
>The circuit needs to provide a fairly linear response from 150VAC to 270VAC.
>The output required for the Micro is 0-2.5V.
>
>Any ideas for existing circuits much appreciated
>
>Thanks in advance
>
>Regards
>
>David
>
>
>  
>
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