SA4301A datasheet(18/23 Pages) SAMES

18 / 23 page

SPEC-1538 (REV. 7)

18/23

29-09-2017

SA4301A

TYPICAL APPLICATION

The following description outlines the basic process required

to design a typical three phase energy meter using the

SA4301A. The meter is a 3-phase 4-wire meter capable of

measuring 3x220V/60A/50Hz with a precision better than

Class 1. The meter uses a stepper motor counter with

100imp/kWh and the calibration LED has a constant of

800imp/kWh.

The most important external circuits required for the

SA4301A are the current input networks, the voltage input

networks as well as the bias resistor. All resistors should be

1% metal film resistors of the same type to minimize

temperature effects.

Bias Resistor

A bias resistor of R34 = 47k

 sets optimum bias and

reference currents on chip. Calibration of the meter should be

done using the voltage inputs and not by means of the bias

resistor.

Current Input Networks

Three current transformers are used to measure the three

line currents. The output of each current transformer is

terminated with a low impedance resistor split into two equal

parts to obtain purely differential current input signals. The

voltage across the termination resistors 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 SA4301A.

The voltage drop across the current transformer termination

resistors at maximum rated current should be in the order of

100mVRMS. The current transformers have a low phase shift

and a turns ratio of 1:2500. The value of the termination

resistors R1, R2 is therefore

��1 = ��2 = 100�� ×

��

≈ 2Ω = ��

where NCT is the current transformer ratio (2500) and IMAX is

the maximum input current (60A).

The four current input resistors (R3, R4, R5, R6) should be of

equal size to optimize the input networks low pass filtering

characteristics, so the values can be calculated as follows:

��3 = ��4 = ��5 = ��6 =

��

2 × 16��

= 1.5��Ω = ��

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 = ��2 =

��

�� × 10�� × 1.5��Ω

≈ 22�� = ��

where fCI is the cut-off frequency of the anti-alias filter of the

current input network.

The current input networks for channel 2 and channel 3 are

identical.

Voltage Input Networks

The voltage sense inputs require 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 SA4301A. The phase shift of the current

transformers is compensated by means of this anti-alias filter

as well, by purposefully increasing the cut-off frequency.

The input resistor R22 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 R22 = 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 R23 is 1.27V. Choosing a

 trimpot results in

��23 =

1��Ω

(2 × 0.17)

× (1.27 − 2 × 0.17) ≈ 2.7��Ω

The effect of R22 can be ignored in the above equation, given

the fact that R22 is significantly larger than P1 and R23. Now

let RA = R19 + R20 + R21 and

�� = (��1 + ��23) × (

220��

1.27��

− 1) ≈ 637��Ω

so choose R19 = 240k

, R20 = 220k and R21 = 180k.

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