MCP3906 Microchip Technology, MCP3906 Datasheet - Page 17

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

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If an external voltage reference source is connected to
the REFIN/OUT pin, the external voltage will be used
as the reference for both current and voltage channel
ADCs. The voltage across the source resistor will then
be the difference between the internal and external
voltage. The allowed input range for the external
voltage source goes from 2.2V to 2.6V for accurate
measurement error. A V
will cause additional heating and power consumption
due to the source resistor, which might affect
measurement error.
4.4
The MCP3905/06 contains an internal POR circuit that
monitors analog supply voltage AV
This circuit ensures correct device startup at system
power-up/power-down events. The POR circuit has
built-in hysteresis and a timer to give a high degree of
immunity to potential ripple and noise on the power
supplies, allowing proper settling of the power supply
during power-up. A 0.1 µF decoupling capacitor should
be mounted as close as possible to the AV
providing
Section 5.0 “Applications Information”).
The threshold voltage is typically set at 4V, with a
tolerance of about ±5%. If the supply voltage falls below
this threshold, the MCP3905/06 will be held in a Reset
condition (equivalent to applying logic ‘0’ on the MCLR
pin). The typical hysteresis value is approximately
200 mV in order to prevent glitches on the power
supply.
Once a power-up event has occurred, an internal timer
prevents the part from outputting any pulse for
approximately 1s (with MCLK = 3.58 MHz), thereby
preventing potential metastability due to intermittent
resets caused by an unsettled regulated power supply.
Figure 4-3
power-up and a power-down event in the typical
conditions.
FIGURE 4-3:
© 2009 Microchip Technology Inc.
DEVICE
MODE
AV
4.2V
5V
4V
0V
DD
Power-On Reset (POR)
RESET
illustrates the different conditions for a
additional
PULSE
OUT
NO
1s
Power-on Reset Operation.
REF
OPERATION
transient
PROPER
value outside of this range
DD
during operation.
immunity
RESET
DD
(see
Time
pin,
4.5
The active (real) power value is extracted from the DC
instantaneous power. Therefore, any DC offset
component present on Channel 0 and Channel 1
affects the DC component of the instantaneous power
and will cause the real-power calculation to be
erroneous. In order to remove DC offset components
from the instantaneous power signal, a high-pass filter
has been introduced on each channel. Since the
high-pass filtering introduces phase delay, identical
high-pass filters are implemented on both channels.
The filters are clocked by the same digital signal,
ensuring a phase difference between the two channels
of less than one MCLK period. Under typical conditions
(MCLK = 3.58 MHz), this phase difference is less than
0.005°, with a line frequency of 50 Hz. The cut-off
frequency of the filter (4.45 Hz) has been chosen to
induce minimal gain error at typical line frequencies,
allowing sufficient settling time for the desired
applications. The two high-pass filters can be disabled
by applying a logic ‘0’ to the HPF pin.
FIGURE 4-4:
(MCLK = 3.58 MHz).
The multiplier output gives the product of the two high-
pass-filtered channels, corresponding to instantaneous
active (real) power. Multiplying two sine wave signals
by the same ω frequency gives a DC component and a
2ω component. The instantaneous power signal
contains the active (real) power of its DC component,
while also containing 2ω components coming from the
line frequency multiplication. These 2ω components
come for the line frequency (and its harmonics) and
must be removed in order to extract the real-power
information. This is accomplished using the low-pass
filter and DTF converter.
-10
-15
-20
-25
-30
-35
-40
-5
0
0.1
High-Pass Filters and Multiplier
1
MCP3905/06
HPF Magnitude Response
Frequency (Hz)
10
DS21948E-page 17
100
1000