MCP3906 datasheet(15/30 Pages) MICROCHIP

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DS21948E-page 15

MCP3905/06

4.0

DEVICE OVERVIEW

The MCP3905/06 is an energy-metering IC that

supplies a frequency output proportional to active (real)

power, and higher frequency output proportional to the

instantaneous power for meter calibration. 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).

The calculation of the active (real) power, as well as the

filtering associated with this calculation, is performed in

the digital domain, ensuring better stability and drift

performance. Figure 4-1 represents the simplified

block diagram of the MCP3905/06, 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 (real) power output.

The instantaneous power signal contains the real-

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 DTF converter accumulates the instantaneous active

(real) power information to produce output pulses with a

frequency proportional to the average active (real)

power. The low-frequency pulses presented 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 lower

integration period such that it can represent the

instantaneous active (real) power signal. Due to the

shorter accumulation time, it enables the user to

proceed to faster calibration under steady load

conditions (refer to Section 4.7 “FOUT0/1 and HFOUT

Output Frequencies”).

FIGURE 4-1:

Simplified MCP3905/06 Block Diagram with Frequency Contents.

HPF

...

DTF

–

ADC

–

PGA

LPF

HPF

CH0+

CH0-

CH1+

CH1-

ADC

FOUT0

FOUT1

HFOUT

MCP3905/06

Frequency

Content

ΔΣ

ADC Output

Code Contains

System and

ADC Offset

DC Offset

Removed

by HPF

Instantaneous

Power

ANALOG

DIGITAL

Instantaneous

Active (Real) Power

Input Signal

with System

Offset and

Line Frequency

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