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It depends on if the AC is a sinewave and what parameter you want to measure (rms, average, or peak). If it is a true sine wave, you can take a series of random samples and find the average (inverting the negative values). Then multiply by a conversion factor to get rms or peak. You can also do the RMS calculations from these samples on any type of waveform if RMS is what you want.
For a little extra hardware complexity you can use an op amp rectifier circuit and a RC low pass filter to get the average and just take one ADC sample.
flatulent is right, if you are using MCU, you can get samples and calculate RMS. But d'ont forget of power isolation (secure, etc..), maybe you must use a transformer that can geave you galvanic isolation....
If you want to meause sine!! wave AC voltage, use single diode and capasitor and voltage divider (by 2 resistor). This is simple vay but 0.6v
voltage drop produce error at the low voltage measurement.
I presume you want to measure rms voltage of a sinewave.
In my view the proper way is:
1. scale the voltage with resistors to match the measuring range. Be aware that overvoltage can harm your circuit, so use protection diodes if needed.
2. Rectify without using any diodes since diodes, schottky included cause too much error, are temperature sensitive etc. Use a fullwave rectifier with two opamps or a special rectifying circuit.
3. measure the rectified dc voltage with an AD converter, the simplest is to use a microcontroller that includes an adc.
4. Perform scaling and if apropriate a lowpass filter using software.
Actually there is no need for the rectifier while we have ADC . As far as I know ADC input range 0 - 2 V or something like that .
Make first voltage shift on ADC input via R-R divider such as voltage must be in middle of input voltage level range . (for 0-2 V ADC input you can have 1 V,
thus giving you possibility to measure 1 V in peak AC signals ).
R-R divider must be supplied with stabilized voltage and will give you constant DC level on ADC input .
Supply AC voltage to the ADC input via capasitor with capasity enough for RC circuit to pass lowest measured frequency > (10-15)/RC where R = Rd1*Rd2/(Rd1+Rd2) (assuming ADC input impedance is much higher) impedance of divider. Use amplifier before ADC input with output impedance much less than R-R divider resulting impedance to avoid divider influence on measurement .
As we are talking about AC sine voltage , when you will measure AC , get let say 1000 samples and calculate average middle - equals to DC constant supplied by divider . Then you can make software rectifier within your MCU via :
1. substract from each sample DC part calculated above
2. Revert sample sign if it is negative
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