Datasheet

MCP3909
DS22025C-page 18  2006-2012 Microchip Technology Inc.
4.0 DEVICE OVERVIEW
The MCP3909 is an energy metering IC that serves two
distinct functions that can operate simultaneously:
- Active Power Pulse Output
- Waveform Output via SPI Interface
For the active power output, the device supplies a fre-
quency output proportional to active (real) power, and
higher frequency output proportional to the
instantaneous power for meter calibration.
For the waveform output, it can be used serially to
gather 16-bit voltage channel and current channel A/D
data, or 20-bit wide multiplier output data. Both chan-
nels 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 channel (Channel 0).
Figure 4-1 represents the simplified block diagram of
the MCP3909, detailing 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.
4.1 Active Power
The instantaneous power signal contains the active
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 instanta-
neous 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 F
OUT0
and F
OUT1
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
HF
OUT
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 4.8 “Active Power FOUT0/1 and HFOUT
Output Frequencies”).
FIGURE 4-1: Active Power Signal Flow with Frequency Contents.
HPF
...1010..
DTF
-
+
????ADC
-
+
PGA
LPF
HPF
X
CH0+
CH0-
CH1+
CH1-
????ADC
F
OUT0
F
OUT1
HF
OUT
INSTANTANEOUS
POWER
INSTANTANEOUS
REAL POWER
0 ? 0 ?
MCP3909
DC Offset
removed by
HPF
ADC Output code
contains System and
ADC offset
Input Signal with
System offset and line
frequency
0 ?
?
?
0 ?
?
?
0 ?
Frequency
Content
ANALOG DIGITAL