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AC voltage measurement for ADC

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epp

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Hi!
I need to measure three phase AC voltages (~230VAC, 50Hz) in order to
control AC motor. I have to use resistor divider plus some OpAmp and RC filter
in order to scale and isolate that voltages which will be connected at input of an ADC
(in dsPIC controller).
How should I connect resistors and OpAmp? Please draw it.
I want these values at input of an ADC (0-5V):
-230VAC --> 0V
0VAC --> 2.5V
+230VAC --> 5V
 

If you dont worry about isolation, and you may consider 10:1 or similar isolation transformer, here is a simple circuit that will convert +230 to -230 to 5V-0V ..
You may feed the output directly to ADC pin or you may use a unity gain buffer (opamp) ..
Just add a 5.1V zener diode ..

This circuit has a small drawback: at 0V input the output will be 2.47V instead of 2.5, as part of the current flows backwards through 100kΩ .. but I hope you can accept that "glitch" ..

Regards,
IanP
 
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