MCP3909 datasheet(19/44 Pages) MICROCHIP

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DS22025B-page 19

MCP3909

4.0

DEVICE OVERVIEW

The MCP3909 is an energy metering IC that serves two

distinct functions that can operate simultaneously:

- Active Power Pulse Output

- Waveform Output via SPI Interface

For the active power output, the device supplies a

frequency output proportional to active (real) power,

and higher frequency output proportional to the

instantaneous power for meter calibration.

For the waveform output, it can be used serially to

gather 16-bit voltage channel and current channel A/D

data, or 20-bit wide multiplier output data. Both

channels use 16-bit, second-order, delta-sigma ADCs

that oversample the input at a frequency equal to

MCLK/4, allowing for wide dynamic range input signals.

A Programmable Gain Amplifier (PGA) increases the

usable range on the current input channel (Channel 0).

Figure 4-1 represents the simplified block diagram of

the MCP3909, detailing its main signal processing

blocks.

Two digital high-pass filters cancel the system offset on

both channels such that the real-power calculation

does not include any circuit or system offset. After

being high-pass filtered, the voltage and current signals

are multiplied to give the instantaneous power signal.

This signal does not contain the DC offset components,

such that the averaging technique can be efficiently

used to give the desired active-power output.

4.1

Active Power

The instantaneous power signal contains the active-

power information; it is the DC component of the

instantaneous power. The averaging technique can be

used

with both sinusoidal and non-sinusoidal

waveforms, as well as for all power factors. The

instantaneous power is thus low-pass filtered in order

to produce the instantaneous real-power signal.

A digital-to-frequency converter accumulates the

instantaneous active real power information to produce

output pulses with a frequency proportional to the

average real power. The low-frequency pulses present

at the FOUT0 and FOUT1 outputs are designed to drive

electromechanical counters and two-phase stepper

motors displaying the real-power energy consumed.

Each pulse corresponds to a fixed quantity of real

energy, selected by the F2, F1 and F0 logic settings. The

HFOUT output has a higher frequency setting and less

integration period such that it can represent the

instantaneous real-power signal. Due to the shorter

accumulation time, it enables the user to proceed to

faster calibration under steady load conditions (see

Section 4.8 “Active Power FOUT0/1 and HFOUT

Output Frequencies”).

FIGURE 4-1:

Active Power Signal Flow with Frequency Contents.

HPF

...

DTF

ADC

PGA

LPF

HPF

CH0+

CH0-

CH1+

CH1-

ADC

FOUT0

FOUT1

HFOUT

INSTANTANEOUS

POWER

INSTANTANEOUS

REAL POWER

MCP3909

DC Offset

removed by HPF

ADC Output code

contains System

and ADC offset

Input Signal with

System offset and

line frequency

Frequency

Content

ANALOG

DIGITAL

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