SA4120A
FUNCTIONAL DESCRIPTION
Theory of Operation
Analog Inputs
The input circuitry of the current and voltage sensor inputs is
illustrated in Figure 7. These inputs are protected against
electrostatic discharge through clamping diodes. The
feedback loops from the outputs of the amplifiers AI and AV
generate virtual short circuits between IIP and IIN as well as
IVP and AGND. The current sense inputs (IIP and IIN) are
identical and balanced. The AD converters convert the
signals on the voltage and current sense inputs to a digital
format for further processing. All internal offsets are
eliminated through the use of various cancellation
techniques.
The SA4120A includes all the required functions for single
channel single phase power and energy measurement. Two
AD converters sample the voltage and current input signals.
The two digital signals, accurately representing the voltage
and current inputs, are multiplied using digital multiplication.
The output of the multiplier represents the instantaneous
power. The pulse generation circuit creates a pulse output
where the instantaneous frequency is proportional to the
instantaneous power measured.
For given voltage and current signals the instantaneous
power is calculated by:
VDD
( )
푝 푡 = 푣(푡) × 푖(푡)
IIP
( )
(
)
(
)
푝 푡 = 푉푀 cos 휔푡 + 휃 × 퐼푀 cos 휔푡 + 휓
VSS
CURRENT
CHANNEL
ADC
CURRENT
SENSOR
INPUT
ꢀꢁ
ꢂꢁ
then
2
√
AI
Let 휙 = 휃 − 휓, and 푉푅푀푆
=
and 퐼푅푀푆
=
2
√
VDD
( ) )
(
)
(
푝 푡 = 푉푀 cos 휔푡 + 휃 × 퐼푀 cos 휔푡 + 휃 − 휙
IIN
( )
(
( ( ))
)
푝 푡 = 푉푅푀푆 푅푀푆
퐼
cos 휙 + cos ꢃ 휔푡 + 휃 − 휙
VSS
VDD
where
p(t) is the instantaneous power,
IVP
v(t) is the instantaneous voltage signal,
i(t) is the instantaneous current signal,
VM is the amplitude of the voltage signal,
IM is the amplitude of the current signal,
VOLTAGE
SENSOR
INPUT
VSS
VOLTAGE
CHANNEL
ADC
AV
is the phase angle of the voltage signal and
is the phase angle of the current signal.
AGND
The instantaneous power output is integrated over time to
obtain the output energy by simply counting the output
pulses. This removes the double mains frequency component
cos(2(t+)-) and the average pulse output rate is therefore
equivalent to
Figure 7: Analog input configuration
Digital Outputs
The calculations required for power and energy are
performed and the result is converted to pulses on the FOUT
pulse output. The instantaneous output frequency on the
pulse output is proportional to the instantaneous active power
consumption measured. The pulse output is intended for
interfacing the SA4120A to a microcontroller or similar pulse
processing circuit.
ꢄ
1
푃 = ∫ 푝(푡)푑푡
푇
ꢅ
푃 = 푉푅푀푆 푅푀푆
퐼
cos 휙
where
P is the average power and
cos is the power factor.
Anti-Creep Threshold
An integrated anti-creep function prevents any output pulses
from appearing on the pulse output if the energy measured is
less than 0.02% of FMAX, where FMAX is the energy registered
when the input currents for voltage and current are 14μARMS
and 16μARMS with zero phase shift respectively.
Linearity
The SA4120A is a CMOS integrated circuit, which performs
power/energy calculations across a dynamic range of 500:1
to an accuracy that exceeds the IEC62053 specification.
SPEC-3094 (REV. 2)
29-09-2017
7/14
SAMES [ SAMES ]
