PM4104APDA
Current Input Filtering
For best immunity to electromagnetic disturbances the
SA4104A/B requires low-pass filters on the current sense
inputs. Referring to Figure 2 and Figure 3, these filters are
realized by means of the capacitors C1 and C2. The typical
cut-off frequency of these filters should be between 10kHz
and 20kHz. The equivalent resistance associated with each
capacitor is RC/2 so the capacitor value should be in the
order of
The input resistor R12 sets the current input into the device.
This resistor should not be too large else the capacitor for
the low-pass filter will be quite small. This could cause
inaccurate phase shift due to parasitic capacitances and
affect the performance of the energy meter at low power
factor. Therefore R12 = 100kΩ is chosen. For the purpose
of the evaluation module a very large tuning range on the
trimpot is selected, so let R11 and P1 be 1kΩ each and the
voltage over the combination of R11 and P1 be 1.76V. This
allows the input currents on the voltage sense inputs of the
1
C1= C2 =
= CC
πfCIRC
device to be scaled from 17.6μARMS down to 8.8μARMS.
The following equations can be used to obtain the remaining
resistor values:
where fCI is the required cut-off frequency of the low-pass
filters of the current input networks.
2kΩ
1.76V = 220V
The current input network on the PM4104APDA has been
set up for a shunt with input resistors RC = 200Ω. The
filtering capacitors have been set to CC = 100nF for
simplicity. The resulting cut-off frequency is 15.9kHz.
R7 + R8 + R9 + R10 + 2kΩ
and
2kΩ
1.76V = 110V
Voltage Sense Input Network
R7 + R8 + 2kΩ
The voltage input network on the PM4104APDA has been
designed for operation at both 220V and 110V mains
voltage. The mains voltage is selected by means of a
jumper (J1). The voltage input network attenuates the mains
voltage signal to a lower voltage by means of a voltage
divider. A trimpot is used to tap this voltage divider at
different levels to effect calibration.
This results in R7 + R8 = 123kΩ and R9 + R10 = 125kΩ and
the values are chosen as R7 = 75kΩ, R8 = 47kΩ and R9 =
R10 = 62kΩ. The effect of R12 can be ignored in the above
equations, given the fact that R12 is significantly larger than
the combination of P1 and R11.
A low-pass filter is required on the voltage sense input to
remove any high frequency signals that could affect the
performance of the SA4104A/B. If a current transformer is
used as a current sensing element then this low pass filter is
used to compensate for the phase shift of the current
transformer as well by purposefully increasing the cut-off
frequency.
Figure 4 shows the voltage sense input network as
implemented on the PM4104APDA. The voltage sense input
of the SA4104A/B saturates at an input current of
±17.6μARMS (±25μAPEAK). The current into the voltage sense
input should therefore be set between 11μARMS and
12μARMS at nominal mains voltage (VNOM) to allow for a
mains voltage variation of up to +30% and –50% without
saturating the voltage sense input.
JS2 “L”
R7
R8
R9
R10
VNOM
Live
(220V / 110V)
P1
R12
(11µARMS
J1
110V / 220V
IVP
SA4104A
JS2 “N”
)
Neutral
C3
SA4104B
R11
GND
GND GND
AGND
GND
Figure 4: Voltage input network on the PM4104APDA
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