I have Autotransformer which output 0-250 v and i want to read Ac Voltage by arduino but how to read accurately
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How accurately? +/-1V, +/-0.5V, +/-0.1V?
Be careful what you ask for since more accuracy means more money.
This is a real cheap solution
0.1 volt so how to do that ?
That would be +/- 0.04 % error.
It will take me several days to come up with a circuit that can measure RMS voltage that accurately. It would require a custom PCB
Can i use zmpt101b ?
Yes but I have no idea about the accuracy but probably not 0.04%
Many people use an AC transformer with Arduino to safely reduce the 250 VAC to a reasonable value, as described here:
https://docs.openenergymonitor.org/electricity-monitoring/voltage-sensing/index.html
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Have a look at multimeter specifications to see what is reasonable , a typical fluke is around 1% of reading on AC volts .
Have a look at voltage transducers with a 0-5v DC output
First, you need to isolate yourself, the Arduino, and your computer if you plug-in USB, from the lethal voltages!
...Hand-held multimeters are obviously isolated (battery operated and enclosed in plastic) and AC powered multimeters have isolated power supplies (i.e. The black lead isn't "ground".) You can do that with an Arduino but it's not easy to keep it "safe" during development, and the USB isn't isolated. In this kind of applicationl, it's better to use a transformer on the high voltage input.
A transformer will provide isolation, while stepping-down the voltage to something more in the range that an Arduino can read. And can be inexpensive since you need virtually zero current out of it.
You shouldn't need an op-amp since you are starting with 200V! 
Transformers aren't necessarily super-accurate (a 220-to-12V transformer may not put-out exactly 12V) but they are linear and repeatable as long as the load doesn't change so it's easy to correct/calibrate.
The basic Arduino (Uno, etc.) has a 10-bit ADC (0-1023). It's linear & accurate to within 1 or 2 counts, so that's 0.1 or 0.2% of full scale.
Voltage Divider (2-resistors to bring the voltage down further to the 5V or 3.3V range. (The resistors should sum-up to around 10K, although it's the ratio that's important.)
You'll need a Protection Circuit (or a bias circuit) to protect from negative voltages, and possibly from over-voltage. Assuming you use a voltage divider, you can leave-out the current limiting resistor, as the voltage divider will do double-duty.
Assuming this a sine wave (the power line?) you can just find the peak, and the RMS is 0.707 x Peak.
To expand on what Hammy posted. If you have deep enough pockets I would go with a nice AC voltage transducer. Something like this. CR4810 Average RMS AC Voltage Transducer - Single Phase - with 0~5VDC Output. That gets you a 0.5% uncertainty and is average responding RMS indicating so only for use with a nice clean true sine wave.
Now if you want a nice true RMS responding RMS indicating version you need slightly deeper pockets. CR4510 True RMS AC Voltage Transducer - Single Phase - 0~5VDC Output. You still have a basic accuracy of 0.5% and I doubt short of going much higher in cost you will beat that uncertainty or accuracy.
Finally if you really want cheap or inexpensive (inexpensive sounds better) then yes there is the ZMPT101B AC Voltage Transformer Module, Single Phase Active Output Voltage Sensor Board Power Supply Voltage 5-30V for Household Appliances. I have no clue what the uncertainty is but any of what I mentioned is easy enough to get a basic calibration on using a good known accurate AC voltmeter for comparison.
You can also roll your own using a transformer and a handful of parts so it is up to you but your uncertainty requirement of 0 to 250 VAC @ I imagine 50 Hz is unrealistic unless you have very deep pockets. There may be off the boat (Chinese Manufacture) AC voltage transducers like those I mentioned for a lower cost so just Google AC Voltage Transducer and see what is available. My links are merely examples and transducers I have used.
I should also mention the first two links to transducers come in a wide range of outputs. You can have 0 to 5 VDC, 0 - 20 mA, 4 - 20 mA, 0 to 10 VDC and about anything else you want including custom.
Ron
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