SA9903B
TYPICAL APPLICATION
1
1
The following description outlines the basic process required
to design a typical single phase energy meter using the
SA9903B and a shunt resistor. The meter is capable of
measuring 220V/40A/50Hz with a precision better than Class
1 on active energy and Class 2 on reactive energy.
ꢍ1 = ꢍꢃ =
=
≈ 100푛퐹 = ꢍ퐶
휋푓 ꢉ퐶 휋 × 15푘ꢎ푧 × ꢃ00Ω
퐶ꢂ
where fCI is the cut-off frequency of the anti-alias filter of the
current input network.
Voltage Input Network
The most important external circuits required for the
SA9903B are the current input network, the voltage input
network as well as the bias resistor. All resistors should be
1% metal film resistors of the same type to minimize
temperature effects. Calibration of a microcontroller based
meter is typically done in software so the external circuits do
not require calibration mechanisms.
The voltage sense input requires an input current of 14μARMS
at VNOM (220V). The mains voltage is divided by means of a
voltage divider to a lower voltage that is converted to the
required input current by means of the input resistor. Once
again an anti-alias filter is required to remove any high
frequency signals that could affect the performance of the
SA9903B. A shunt typically has very little phase shift so
phase compensation is not required.
Bias Resistor
A bias resistor of R10 = 24k sets optimum bias and
reference currents on chip.
The input resistor R8 sets the current input into the device.
This resistor should not be too large else the capacitor for the
anti-alias filter will be quite small which could cause
inaccurate phase shift due to parasitic capacitances.
Therefore R8 = 100k is chosen. R9 should be significantly
smaller than R8, but not too small in order to limit the power
dissipation of the voltage input network. Hence R9 = 4.3kis
chosen. Now let RA = R19 + R20 + R21 and
Current Input Network
The voltage drop across the shunt resistor at maximum rated
current should not be less than 5mVRMS and not exceed
100mVRMS. A 320μ shunt is chosen which sets the voltage
drop at maximum rated current to 12.8mV and the maximum
power dissipation in the shunt to 0.5W. The voltage across
the shunt resistor is converted to the required differential
input currents through the current input resistors. Anti-alias
filters are incorporated on these input resistors to filter any
high frequency signal components that could affect the
performance of the SA9903B.
ꢃꢃ0푉
ꢉ퐴 = ꢉ9 × ꢏ
− 1ꢐ ≈ 671푘Ω
1.4푉
so choose R19 = R20 = R21 = 220k.
The cut-off frequency of the anti-alias filter is adjusted so that
it is identical to that of the current input network anti-alias
filters. This ensures that the phase shift caused by the anti-
alias filters is identical on the current and voltage input
networks. Therefore
The four current input resistors (R1, R2, R3, R4) should be of
equal size to optimize the input networks low pass filtering
characteristics, so the values can be calculated as follows:
ꢉ푆퐻
ꢉ1 = ꢉꢃ = ꢉ3 = ꢉ4 = 퐼푀퐴푋
×
= ꢃ00Ω = ꢉ퐶
4 × 16휇ꢌ
1
1
=
휋ꢍ퐶 × ꢉ퐶 ꢃ휋 × ꢉ9 × ꢍ3
For optimum performance the cut-off frequency of the anti-
alias filter should be between 10kHz and 20kHz. The
equivalent resistance associated with each capacitor is RC/2
so the capacitor values should be in the order of
and so set C3 = 2.2nF.
SPEC-0051 (REV. 5)
29-09-2017
13/17
SAMES [ SAMES ]
