MCP3905/06
4.0
DEVICE OVERVIEW
The MCP3905/06 is an energy-metering IC that
supplies a frequency output proportional to active (real)
power, and higher frequency output proportional to the
instantaneous power for meter calibration. 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).
The calculation of the active (real) power, as well as the
filtering associated with this calculation, is performed in
the digital domain, ensuring better stability and drift
performance. Figure 4-1 represents the simplified
block diagram of the MCP3905/06, 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 (real) power output.
The instantaneous power signal contains the real-
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
instantaneous power is thus low-pass filtered in order
to produce the instantaneous real-power signal.
A DTF converter accumulates the instantaneous active
(real) power information to produce output pulses with a
frequency proportional to the average active (real)
power. The low-frequency pulses presented at the
F
OUT0
and F
OUT1
outputs are designed to drive electro-
mechanical 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 lower integration
period such that it can represent the instantaneous
active (real) power signal. Due to the shorter accumula-
tion time, it enables the user to proceed to faster calibra-
tion under steady load conditions (refer to
CH0+
CH0-
+
–
PGA
MCP3905/06
ΔΣ
ADC
DIGITAL
HPF
ANALOG
CH1+
CH1-
X
LPF
..0101...
F
OUT0
F
OUT1
HF
OUT
DTF
+
–
ΔΣ
ADC
HPF
Frequency
Content
0
0
0
0
0
Input Signal
with System
Offset and
Line Frequency
ADC Output
Code Contains
System and
ADC Offset
DC Offset
Removed
by HPF
Instantaneous
Power
Instantaneous
Active (Real) Power
FIGURE 4-1:
Simplified MCP3905/06 Block Diagram with Frequency Contents.
©
2007 Microchip Technology Inc.
DS21948D-page 11
MICROCHIP [ MICROCHIP TECHNOLOGY ]
