I am building a 3 phase power logger with the ability to measure 3-phase AC and 2 DC inputs as well for a final year project. A typical application would be measuring a small inverter for a solar application on both the AC and DC sides. I am using CT's and a burden resistor for AC currents, a hall effect sensor for DC currents, and simple voltage dividers to bring down the AC and DC voltage to within the range of the ADC. I am using a 12 channel single ended ADC with simultaneous sampling. Sampling the voltage and currents for each phase simultaneously and sending it via I2C to a raspberry Pi where all the calculations etc. is done.

My problem is that the ADC only takes voltages from 0 - 2.5 V as analog inputs. All my signals are voltage signals that ranges from -V to +V. So my signals need to be scaled down and lifted with a DC offset. I have found a couple of ways in doing it but would like some advice. Another thing is that I've read that a Low Pass filter needs to be added before the analog input to eliminate aliasing effects etc. So I need to shrink voltage, add a DC offset and a low pass filter.

I would want to capture at least 30 harmonics for a 50 Hz AC signal so the filter needs to let those through. I simulated a simple 3 resistor voltage divider network that lifts and shrinks the voltage to the correct level but will it work on all the frequencies and is it reliable? and where do I need to place the filter, cause the moment I add a filter the wave-forms change, do I need to add Op-amp buffer between the divider network and the filter? Is it best to use a op amp for the low pass filter?

I've added a picture of my suggested circuit.

Suggested Circuit:

Any advice would be highly appreciated. Thank you

  • 1
    \$\begingroup\$ Take care with sampling mains voltage with potential dividers. Basic suggestion is do not do it if you have not done it before or found out why I have made this suggestion. I recommend you use a small step-down transformer on each phase and then sample the low voltage. Transformers for just this purpose are available. \$\endgroup\$ – KalleMP Sep 4 '16 at 21:16
  • \$\begingroup\$ Look at some of the following ideas for inspiration. - google.com/… \$\endgroup\$ – KalleMP Sep 4 '16 at 21:18
  • \$\begingroup\$ whats the ADC sampling rate? if the sample rate is fast enough you should be able to filter with passive components. If this circuit is only for sampling the AC signals, then you can probably AC couple the input and do away with the buffer \$\endgroup\$ – Mark Ch Sep 4 '16 at 21:20
  • \$\begingroup\$ Read up on Nyquist theorem to decide if you need a hardware low pass, can filter in software, or can even do without a low pass. \$\endgroup\$ – JimmyB Sep 5 '16 at 7:06

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