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Hi!

I got a simple problem. My objective is to measure voltage and power factor of a 16V AC through a differential ADC (ADS1115) that measures voltage difference between 2 channels. I only managed to measure current accurately through a CT.

In the positive cycle of the the circuit shown, the voltage across the 1k resistor is 1.45Vp which is measurable. In the negative cycle, the voltage across the resistor will be -14.54V which will burn the ADC.

Hence, how do I modify this circuit to achieve a safe voltage measurement? I will need to detect the waveform at point 0V in order to compare it to the current then measure power factor. I thought of connecting A2 to the ground of the DC supply instead of the nuetral, but I was not sure if that's the right approach.

Sorry i'm a beginner. Any input/ advice will be highly appreciated.

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    \$\begingroup\$ Do you need to digitize the AC waveform or are you just interested in measuring the peak-to-peak voltage? \$\endgroup\$ – kjgregory Nov 9 '17 at 15:57
  • \$\begingroup\$ Is your ADC ground the same as the "DC Ground" in your schematic? \$\endgroup\$ – The Photon Nov 9 '17 at 17:32
  • \$\begingroup\$ @kjgregory, Just interested in measuring the peak to peak voltage as well as the 0 crossing really for power factor. \$\endgroup\$ – Moha Sawyer Nov 10 '17 at 2:34
  • \$\begingroup\$ @ThePhoton, Yes indeed it is. The bridge rectifier is also supplying the ADC. \$\endgroup\$ – Moha Sawyer Nov 10 '17 at 2:36
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You can capacitively couple and re-bias the AC voltages into the range of your ADC:

schematic

simulate this circuit – Schematic created using CircuitLab

VCC is the power supply voltage of your ADC. I've chosen the resistors assuming this is 3.3 V, and that the AC signals are 16 V peak-peak (based on your saying the A1 and A2 nodes in your circuit go to -14.5 V). If I've misinterpreted, you can adjust the resistor values to change the scaling from the AC input to the ADC input.

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Use an AD4950, AD8138 or similar differential amplifier. These parts will accept a common mode value from your ADC or a vref circuit and will output a differential waveform centered around that voltage reference. The gain can be adjusted in order to keep the maximum Vpp output (you probably need about 1/4 to 1/8 gain or so). You may need to produce a negative DC supply rail in order to bias the AD8138 into its linear region.

Alternatively, use a second transformer to step down the AC voltage to within the ADC's range. You'll want one with three windings so you can set the common mode of the output with a vref circuit.

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