Datasheet

MCP3909
DS22025C-page 20  2006-2012 Microchip Technology Inc.
4.4 Ultra-Low Drift V
REF
The MCP3909 contains an internal voltage reference
source specially designed to minimize drift over tem-
perature. This internal V
REF
supplies reference voltage
to both current and voltage channels ADCs. The typical
value of this voltage reference is 2.4V ±100 mV. The
internal reference has a very low typical temperature
coefficient of ±15 ppm/°C, allowing the output frequen-
cies to have minimal variation with respect to tempera-
ture since they are proportional to (1/V
REF
)².
The output pin for the voltage reference is REFIN/OUT.
Appropriate bypass capacitors must be connected to
the REFIN/OUT pin for proper operation
(see Section 6.0 “Applications Information”). The
voltage reference source impedance is typically 4 kΩ
which enables this voltage reference to be overdriven
by an external voltage reference source.
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 volt-
age source goes from 2.2V to 2.6V for accurate mea-
surement error. A V
REF
value outside of this range will
cause additional heating and power consumption due
to the source resistor, which might affect measurement
error.
4.5 Power-On Reset (POR)
The MCP3909 contains an internal POR circuit that
monitors analog supply voltage AV
DD
during operation.
This circuit ensures correct device startup at system
power-up and system 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
DD
pin, providing additional transient immunity
(see Section 6.0 “Applications Information”).
The threshold voltage is typically set at 4V, with a toler-
ance of about ±5%. If the supply voltage falls below this
threshold, the MCP3909 will be held in a Reset condi-
tion (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 approx-
imately 1s (with MCLK = 3.58 MHz), thereby prevent-
ing potential metastability due to intermittent resets
caused by an unsettled regulated power supply.
Figure 4-3 illustrates the different conditions for a
power-up and a power-down event in the typical
conditions.
FIGURE 4-3: Power-On Reset Operation.
4.6 High-Pass Filters and Multiplier
The active real-power value is extracted from the DC
instantaneous power. Therefore, any DC offset compo-
nent 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 instan-
taneous power signal, a high-pass filter has been intro-
duced 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 fre-
quency 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 set-
tling time for the desired applications. The two high-
pass filters can be disabled by applying logic ‘0’ to the
HPF pin.
FIGURE 4-4: HPF Magnitude Response
(MCLK = 3.58 MHz).
AV
DD
5V
4.2V
4V
0V
DEVICE
MODE
RESET
PROPER
OPERATION
RESET
NO
PULSE
OUT
Time
1s
-40
-35
-30
-25
-20
-15
-10
-5
0
0.1 1 10 100 1000
Frequency (Hz)
Normal Mode Rejection (dB)