MCP3909T-ESLASHSS datasheet(21/44 Pages) MICROCHIP

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 2006-2012 Microchip Technology Inc.

DS22025C-page 21

MCP3909

The multiplier output gives the product of the two

high-pass filtered channels, corresponding to instanta-

neous real power. Multiplying two sine wave signals by

the same ω frequency gives a DC component and a 2ω

component. The instantaneous power signal contains

the real power of its DC component, while also contain-

ing 2ω components coming from the line frequency

multiplication. These 2ω components come for the line

frequency (and its harmonics) and must be removed in

order to extract the real-power information. This is

accomplished using the low-pass filter and DTF

converter.

4.7

Active Power Low-Pass Filter and

DTF Converter

For the active power signal calculation, the MCP3909

uses a digital low-pass filter. This low-pass filter is a

first-order IIR filter, which is used to extract the active

real-power information (DC component) from the

instantaneous power signal. The magnitude response

of this filter is detailed in Figure 4-5. Due to the fact that

the instantaneous power signal has harmonic content

(coming from the 2

 components of the inputs), and

since the filter is not ideal, there will be some ripple at

the output of the low-pass filter at the harmonics of the

line frequency.

The cut-off frequency of the filter (8.9 Hz) has been

chosen to have sufficient rejection for commonly-used

line frequencies (50 Hz and 60 Hz). With a standard

input clock (MCLK = 3.58 MHz) and a 50 Hz line fre-

quency, the rejection of the 2ω component (100 Hz) will

be more than 20 dB. This equates to a 2ω component

containing 10 times less power than the main DC com-

ponent (i.e., the average active real power).

FIGURE 4-5:

LPF1 Magnitude Response

(MCLK = 3.58 MHz).

The output of the low-pass filter is accumulated in the

digital-to-frequency converter. This accumulation is

compared to a different digital threshold for FOUT0/1

and HFOUT, representing a quantity of real energy mea-

sured by the part. Every time the digital threshold on

FOUT0/1 or HFOUT is crossed, the part will output a

pulse (See Section 4.8 “Active Power FOUT0/1 and

HFOUT Output Frequencies”).

The equivalent quantity of real energy required to out-

put a pulse is much larger for the FOUT0/1 outputs than

the HFOUT. This is such that the integration period for

the FOUT0/1 outputs is much larger. This larger integra-

tion period acts as another low-pass filter so that the

output ripple due to the 2ω components is minimal.

However, these components are not totally removed,

since realized low-pass filters are never ideal. This will

create a small jitter in the output frequency. Averaging

the output pulses with a counter or a MCU in the appli-

cation will then remove the small sinusoidal content of

the output frequency and filter out the remaining 2



ripple.

HFOUT is intended to be used for calibration purposes

due to its instantaneous power content. The shorter

integration period of HFOUT demands that the 2ω com-

ponent be given more attention. Since a sinusoidal sig-

nal average is zero, averaging the HFOUT signal in

steady-state conditions will give the proper real energy

value.

-40

-35

-30

-25

-20

-15

-10

-5

0.1

100

1000

Frequency (Hz)

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