MCP3909T-ESLASHSS datasheet(32/44 Pages) MICROCHIP

32 / 44 page

MCP3909

DS22025C-page 32

 2006-2012 Microchip Technology Inc.

6.0

APPLICATIONS INFORMATION

The following application figures represent meter

designs using the MCP3909 device. Some of these

applications ideas are available as fully function meter

reference designs and demo boards. For complete

schematic and for fully function meter designs, visit

Microchip’s web page for demo board and reference

design availability.

6.1

Performing RMS, Apparent Power,

and Active Power using MCP3909

Waveform Data

Figure 6-1 represents power calculations from wave-

form data based on a PIC MCU and MCP3909 device.

The PIC MCU accomplishes the following energy

meter calculation outputs per phase, per line cycle:

- RMS Current

-RMS Voltage

-Active Power

- Apparent Power

Output registers for the power quantities and calibra-

tion registers for phase, offset, gain, and LSB adjust-

ment are available through a serial interface to the PIC

microcontroller. See Microchip’s web page for firmware

solution and demo board.

The example signal flow here shows 4 output power

quantities and 6 calibration registers. For a 60 Hz

design that is using 128 samples per line cycle for the

power calculation the MCP3909 would have a new

data ready pulse every 130 µs. The SPI communica-

tion to gather 16-bits x 2 channels at 10 MHz is approx-

imately 3.2 µs, leaving ~125 µs for the power

calculations before the next sample is ready.

FIGURE 6-1:

Power Calculations from Waveform Sampling Using a PIC MCU. Register names

shown are used on MCP3909 Energy Meter Reference Design.

PHA_W_OFF:32

PHA_W_GAIN:16

PHA_DELAY:8

PHA_V_RMS_OFF:16

PHA_VA_GAIN:16

ADC

CURRENT

VOLTAGE

Active Power

RMS Current

RMS Voltage

Apparent Power

MCP3909

PHA_I_RMS_OFF:16

PIC Microcontroller

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