# Accurately Measuring AC Voltage

I’m curious to see if anyone has any experience on the accuracy differences in measuring 120V-277VAC in the following ohmic ways. In all cases, the voltage would be put through a voltage divider to provide a [-2.5V,2.5V] signal for an ADC:

1. Up shift the AC wave from +-2.5V to 0-5V, then read the 0-5V signal.
2. Rectify the signal to read 0-2.5V for the entire waveform.
3. Read the -2.5V to 2.5V signal directly.

I realize 3. is probably the most accurate, but curious to see how 1. and 2. impact the measurement.

• Anytime signal is max and error is minimum improves result – Tony Stewart Sunnyskyguy EE75 Feb 20 '20 at 0:04
• What do you want avg, rms or pp? – Tony Stewart Sunnyskyguy EE75 Feb 20 '20 at 0:09
• 2. Demands "precision rectification". || Do you want RMS or ... ? (as Tony also asks). || Divider error can be calculated. 1 & 3 depend on equipment capabilities. Achievable accuracy is mainly affected by \$ aspect and design competence. What accuracy do you require and why? – Russell McMahon Feb 20 '20 at 1:12
• Was mainly looking for accuracy in sampling the entire waveform, regardless of the final calculated value (RMS, Vpp, Avg) – cdubs Feb 20 '20 at 4:28

120V-277VAC are typically rms voltages, so I will assume you want to measure the rms value. All three of your methods should be able to do that with a sufficiently fast ADC.

Method 1 is able to capture the entire waveform without needing a bipolar ADC input, so it is suitable for use with ADCs that can only measure positive voltage. It relies on the midpoint voltage being set accurately, but keeps away from the ends where the ADC may be less accurate.

Method 2 could get up to 1 extra bit of resolution from the ADC if the reference is set for less than 5 V full scale. Whether that will be significant or not depends on the ADC's resolution and noise. An 8 ADC would probably benefit, but a 16 bit ADC might be all noise in the lower bits anyway.

The rectification circuit could introduce error. A simple diode rectifier would be very bad of course, but even a 'precision' rectifier could distort fast changing parts of the waveform.

Method 2 loses the sign, which could be a problem for rms power measurement. On the other hand if the ADC is very slow then it may be the only method that works at all, since it can at least get a reasonable average over a large number of samples (whereas the others would average to zero volts).

Method 3 is like method 1 except that it avoids possible error caused by the level shifting. However it requires a bipolar ADC input to capture the whole waveform. If the ADC input is unipolar then the bottom half of the waveform will be lost, and then accuracy will be reduced if the waveform is not symmetrical.

In most cases method 1 is probably the most suitable. It works with the majority of ADCs, and should be able to get good accuracy when measuring non-sinusoidal waveforms such as are produced by light dimmers etc. It is also suitable for measuring true power (with simultaneous current measurement), and requires very little circuitry to implement (1 resistor!).

• This is a great answer, thank you so much! – cdubs Feb 20 '20 at 4:26