SA4106A
TYPICAL APPLICATION
Voltage Input Network
The following description outlines the basic process required
to design a typical single phase energy meter using the
SA4106A and a shunt resistor as a current sensing element.
The meter is capable of measuring 220V/40A/50Hz with a
precision better than Class 1. It uses a stepper motor counter
with 100imp/kWh and the calibration LED has a constant of
1600imp/kWh.
The voltage sense input requires an input current of 11μARMS
at VNOM (220V) according to Table 1. 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 SA4106A. A shunt typically has very little
phase shift so phase compensation is not required.
The most important external circuits required for the
SA4106A 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.
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 and the voltage at the centre
of the trimpot should be 1.1V (11μA x 100k). The calibration
range of the voltage input network should be about ±15% to
ensure that all component tolerances can be catered for, so
the total tuning range can be set to ±0.17V. Therefore the
voltage across the trimpot and R9 is 1.27V. Choosing a 1k
trimpot results in:
Bias Resistor
A bias resistor of R10 = 47k sets optimum bias and
reference currents on chip. Calibration of the meter should be
done using the voltage input and not by means of the bias
resistor.
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 SA4106A.
1푘Ω
( )
× 1.ꢄ7 − ꢄ × 0.17 ≈ ꢄ.7푘Ω
ꢈ9 =
(
)
ꢄ × 0.17
The effect of R8 can be ignored in the above equation, given
the fact that R8 is significantly larger than P1 and R9. Now let
RA = R5 + R6 + R7 and
ꢄꢄ0푉
(
)
ꢈ퐴 = 푃1 + ꢈ9 × ꢒ
− 1ꢓ ≈ 637푘Ω
1.ꢄ7푉
so choose R5 = 240k, R6 = 220k and R7 = 180k.
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:
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
ꢈ푆퐻
ꢈ1 = ꢈꢄ = ꢈ3 = ꢈ4 = 퐼푀퐴푋
×
= ꢄ00Ω = ꢈ퐶
4 × 16휇ꢏ
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
1
1
=
(
)
휋ꢐ퐶 × ꢈ퐶 ꢄ휋 푃1 + ꢈ9 × ꢐ3
and so C3 = 2.7nF.
1
1
ꢐ1 = ꢐꢄ =
=
≈ 100푛퐹 = ꢐ퐶
휋푓 ꢈ퐶 휋 × 15푘ꢑ푧 × ꢄ00Ω
퐶ꢃ
where fCI is the cut-off frequency of the anti-alias filter of the
current input network.
SPEC-1587 (REV. 5)
29-09-2017
14/18
SAMES [ SAMES ]
