How do you measure Power using HFout in MCP3909?

Fout0, Fout1, and HFout are output frequencies proportional to the average output power. The difference is that HFout, compared to Fout0 and Fout1, has a higher frequency setting and less integration time for instantaneous real power measurement. The equation of frequency and power for FoutN and HFout is provided in equation 4-1 and equation 4-2 of the datasheet, respectively.

Below are some example scenarios:

  • Channel0 (V0) is the current sense. Channel1 (V1) is the voltage sense. Multiplying the voltage and current, you'll get the instantaneous power. Since there is a maximum voltage for each channel, there should be a divider circuit. The voltages V0 (Channel0) and V1 (Channel1) is converted relative to the reference voltage (Vref). Integrating (accumulating) the pulse with-respect-to time will give you the energy consumed.
  • When V = 220 V and I = 10 A the power should be 2200 W. This 220 V needs to be scaled down to 660 mV (for channel 1) at most, based on the electrical specification. You can use a 1:10000 resistor divider to scale down 220 V to 220 mV.
  • If you use PGA Gain (G) = 1, the maximum voltage on channel 0 should be 470 mV based on the electrical specification. For simplicity, let's say the sense resistor is 10 mΩ. If there's 1 A current, the voltage on 10 mΩ should be 10 mV which gives 10 mV/A. Thus, 10 A would be 100 mV.
  • Let Vref = 2.4V 4. Setting F0, F1, and F2 to 0 (GND), will make HFc equal to 109.25 Hz (see table 4-3 of the datasheet). You now have all the parameters to solve for the HFout. HFout = (8.06 x V0 x V1 x G x HFc) / Vref^2 = (8.06 x 220 mV x 100 mV x 109.25 Hz) / 2.4V2 = 3.3632 Hz. That means 2200 W / 3.3632 Hz is the power-to-frequency conversion. This power-to-frequency conversion is not the same for each circuit. Note that this conversion depends on the resistor divider and the sense resistor. To measure energy(Pulse / KWh) by accumulating the pulses for one hour: P/KW = 3.3632 Hz/2.2 W = 1.5287 Hz/KW. The number of Pulse in 1 KWh is P = 1.5287 x 3600s = 5503 Pulses/KWh.

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