MCP3906 Microchip Technology, MCP3906 Datasheet - Page 18

• Manufacturer Part Number: MCP3906
• Description: (MCP3905 / MCP3906) Energy-Metering ICs
• Manufacturer: Microchip Technology

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MCP3905/06
4.6
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
signal has harmonic content (coming from the 2ω
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 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 component (i.e., the average active (real)
power).
FIGURE 4-5:
(MCLK = 3.58 MHz).
DS21948E-page 18
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4-5. Due to the fact that the instantaneous power
-5
0
0.1
Low-Pass Filter and DTF
Converter
1
LPF Magnitude Response
Frequency (Hz)
10
100
1000
The output of the low-pass filter is accumulated in the
DTF converter. This accumulation is compared to a
different digital threshold for F
representing a quantity of real energy measured by the
part. Every time the digital threshold on F
HF
(See Section 4.7
Frequencies”).
The equivalent quantity of real energy required to
output a pulse is much larger for the F
than the HF
for the F
integration 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 Microcontroller
Unit (MCU) in the application will then remove the small
sinusoidal content of the output frequency and filter out
the remaining 2ω ripple.
HF
due to its instantaneous power content. The shorter
integration period of HF
component be given more attention. Since a sinusoidal
signal average is zero, averaging the HF
steady-state conditions will give the proper real energy
value.
OUT
OUT
is intended to be used for calibration purposes
is crossed, the part will output a pulse
OUT0/1
OUT
. This is such that the integration period
outputs is much larger. This larger
“F
OUT0/1
© 2009 Microchip Technology Inc.
OUT
and
demands that the 2ω
OUT0/1
HF
OUT0/1
OUT
OUT
and HF
OUT0/1
signal in
Output
outputs
OUT
or
,