Datasheet

MCP3901 LOW-COST POWER

MONITOR REFERENCE DESIGN

 2010 Microchip Technology Inc. DS51915A-page 11

Chapter 2. Installation and Operation

2.1 POWER MONITOR FIRMWARE DESCRIPTION

2.1.1 Samples Acquisition

Using the external ADC, the current and voltage samples must be acquired before the

correct values of the desired parameters can be computed. The MCU reads the values

of the samples from the ADC through the SPI bus.

The sampling speed of the ADC is controlled by the clock frequency of the MCP3901.

The MCU uses the Output Compare 1 module to generate a 50% pulse-width

modulation (PWM) signal that has a frequency of 901.120 kHz. This frequency can be

easily changed by modifying values in the Timer2 Period Register (PR2) and the

Compare Register 1 (CCPR1).

The sampling speed of the ADC is 1024 times lower than the master clock in the

MCP3901, meaning 880 sps at an Over Sampling Rate (OSR) of 256.

2.1.2 Signal Processing

In order to obtain the desired parameter values out of the acquired samples, a signal

processing technique must be assumed. Since this design uses an 8-bit MCU, the

signal processing technique that is implemented must be fast enough to avoid limiting

the sampling speed, so that a time-domain analysis can be performed. The signal

processing technique is graphically described in Figure 2-1.

FIGURE 2-1: Block Diagram of the Signal Processing Algorithm.

Initially, the acquired samples go through a first-order Infinite Impulse Response

high-pass filter (IIR HPF), which has the following roles:

1. Removes the offset of the ADC

2. Compensates for the Sinc filters transfer function

LPF

Voltage Sample

Current Sample

HPF

Active Power

Apparent Power

RMS

Active Power Scaling Factor

RMS Current Scaling Factor

RMS Voltage Scaling Factor

LPF

SQRT

Power Factor

LPF