MCP3906AT-ISLASHSS datasheet(12/26 Pages) MICROCHIP

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MCP3905A/05L/06A

DS22011A-page 12

4.0

DEVICE OVERVIEW

The MCP3905A/05L/06A devices are energy metering

ICs that supply a frequency output proportional to

active (real) power, and higher frequency output pro-

portional to the instantaneous power for meter calibra-

tion. 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 chan-

nel (Channel 0). The calculation of the active power,

and the filtering associated with this calculation is per-

formed in the digital domain, ensuring better stability

and drift performance. Figure 4-1 represents the simpli-

fied block diagram of the MCP3905A/05L/06A, detail-

ing 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.

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 wave-

forms, 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 4.7

“FOUT0/1

and

HFOUT

Output

Frequencies”).

FIGURE 4-1:

Simplified MCP3905A/05L/06A Block Diagram with Frequency Contents.

HPF

...

DTF

–

ADC

–

PGA

LPF

HPF

CH0+

CH0-

CH1+

CH1-

ADC

FOUT0

FOUT1

HFOUT

MCP3905

Input Signal with

System Offset and

Line Frequency

ADC Output Code

Contains System

and ADC Offset

DC Offset

Removed by HPF

Instantaneous

Power

Instantaneous

Real Power

Frequency

Content

ANALOG

DIGITAL

MCP390X

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