MCP3909-I/SP Microchip Technology, MCP3909-I/SP Datasheet - Page 19

• Manufacturer Part Number: MCP3909-I/SP
• Description: Power Meter with SPI and Active Power Pulse Output & Internal Oscillator., -40C to +85C, 28-SPDIP, TUBE
• Manufacturer: Microchip Technology

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).
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 F
and HF
measured by the part. Every time the digital threshold
on F
pulse (See Section 4.8 “Active Power F
HF
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 MCU in the
TABLE 4-2:
© 2006 Microchip Technology Inc.
OUT
F1
OUT0/1
0
0
1
1
-10
-15
-20
-25
-30
-35
-40
-5
Output Frequencies”).
0
OUT
0.1
OUT0/1
, representing a quantity of real energy
or HF
OUT
F0
. This is such that the integration period
0
1
0
1
outputs is much larger. This larger
OUT
ACTIVE POWER OUTPUT FREQUENCY CONSTANT F
1
is crossed, the part will output a
LPF1 Magnitude Response
Frequency (Hz)
MCLK/2
MCLK/2
MCLK/2
MCLK/2
F
C
10
(Hz)
21
20
19
18
100
OUT0/1
(MCLK = 3.58 MHz)
OUT0/1
outputs
1000
OUT0/1
F
13.66
and
C
1.71
3.41
6.83
(Hz)
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.
4.8
The thresholds for the accumulated energy are
different for F
different transfer functions). The F
output frequencies are quite low in order to allow
superior integration time (see Section 4.7 “Active
Power Low-Pass Filter and DTF Converter”). The
F
following equation:
EQUATION 4-1:
For a given DC input V, the DC and RMS values are
equivalent. For a given AC input signal with amplitude
of V, the equivalent RMS value is V/ sqrt(2), assuming
purely sinusoidal signals. Note that since the real
power is the product of two RMS inputs, the output fre-
quencies of AC signals are half of the DC inputs ones,
again assuming purely sinusoidal AC signals. The
constant F
settings.
for the different logic settings.
Where:
OUT0/1
OUT
F
V
OUT
REF
F
V
V
with Full-Scale
is intended to be used for calibration purposes
G
C
0
1
output frequency can be calculated with the
F
Active Power F
Output Frequencies
Table 4-2
Frequency (Hz)
DC Inputs
OUT
C
=
=
=
=
=
depends on the F
0.74
1.48
2.96
5.93
(
Hz
OUT0/1
the RMS differential voltage on Channel 0
the RMS differential voltage on Channel 1
the PGA gain on Channel 0 (current
channel)
the frequency constant selected
the voltage reference
)
C
shows F
=
FOR FOUT0/1 (V
F
OUTPUT EQUATION
8.06 V
---------------------------------------------------------- -
and HF
OUT
OUT
×
OUT0/1
OUT0/1
FREQUENCY
MCP3909
(
F
0
V
demands that the 2ω
OUT0
OUT
OUT
×
REF
with Full-Scale
V
1
output frequencies
)
AC Inputs
Frequency (Hz)
DS22025A-page 19
×
and F
2
and HF
(i.e., they have
G F
OUT0/1
0.37
0.74
1.48
2.96
REF
×
OUT
OUT1
C
= 2.4V)
signal in
allowed
OUT
digital