MCP3906 datasheet(14/26 Pages) MICROCHIP

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MCP3905/06

DS21948C-page 14

4.6

Low-Pass Filter and DTF

Converter

The MCP3905/06 low-pass filter is a first-order IIR filter

that extracts 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

ω 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

frequency, the rejection of the 2

ω component (100 Hz)

will be more than 20 dB. This equates to a 2

component containing 10 times less power than the

main DC component (i.e., the average active (real)

power).

FIGURE 4-5:

LPF Magnitude Response

(MCLK = 3.58 MHz).

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

DTF converter. This accumulation is compared to a

different digital threshold for FOUT0/1 and HFOUT,

representing a quantity of real energy measured by the

part. Every time the digital threshold on FOUT0/1 or

HFOUT is crossed, the part will output a pulse (See

Section 4.7 “FOUT0/1 and HFOUT Output Frequen-

cies”).

The equivalent quantity of real energy required to

output 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

integration period acts as another low-pass filter so that

the output ripple due to the 2

ω 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 Microcontroller

Unit (MCU) in the application will then remove the small

sinusoidal content of the output frequency and filter out

the remaining 2

ω 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

component be given more attention. Since a sinusoidal

signal 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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