analog voltage to ADC of atmega64a through analog optocoupler

Hello readers,

I planned to give 3phase voltage to ADC of micro controller , for doing this i have done the voltage divider network and i have tapped 5v AC.Now this 5V AC is to be given to ADC of micro controller.Can i give this ac voltage directly to ADC or i should use any isolation in between.If isolation to be done then how can i do that.please help me with circuit.

You have to further reduce the AC signal to less than 5v peak to peak (for example +2.5v to -2.5v) and then add an offset (for example 2.5v) so that this signal becomes 0 to 5v

Check this thread https://www.edaboard.com/thread224449.html

Alex

---------- Post added at 10:51 ---------- Previous post was at 10:50 ----------

The solutions provided are not isolated so you need to be careful of handling the circuit if your source is from the mains

Thanks for the reply,

I will give you in detail what i am trying to do ,
Depending on the 3phase input voltage my input at ADC should vary .For example if my 3 phase input is 400v then input at ADC should be 3v,if input is 200v then adc input should be 2v .
For doing this i stepped down the voltage to 3v using voltage divider and converted to 3v DC ,after converting to dc can i give this directly to ADC of controller ,or i should use linear optocoupler,if linear optocoupler is to be used then which is the best one(cost effective).please suggest me with the complete design.

Thanks for your reply

I can't provide you a complete design (unless this is a proposition to hire me ) but if your voltage is already DC ( not negative ) then you don't need to add the offset.
The mcu have a recommended input impedance specification for the ADC, it is usually about 10K or lower so depending on the resistor values that you use in your divider it may be a good idea to use an opamp to feed the ADC from a very low impedance source.

Alex

Isolation comes into play, if you need the measured signals isolated from mains voltage. It's not easy to achieve analog isolation with acceptable accuracy, even dual-channel couplers with feedback (Vishay IL300 and similar) need at least individual adjustment, and their accuracy isn't mind-blowing. It would be more reasonable to use a microprocessor (or a multi channel ADC with serial interface) for the measurement and perform the isolation on the digital side. That's how most recent system perform isolated analog measurement. For AC voltage and current measurements, transformers are another isolation option.

You didn't clarify yet, if isolation is actually required for your application.