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PRELIMINARY

RM4104ASEB

Current Value

Power Factor

Class 1 Error Limits

0.05lb

≤ I < 0.1lb

1

± 1.5%

0.1lb

1

± 1.0%

0.1lb

≤ I < 0.2lb

0.5 inductive (lag)

± 1.5%

0.1lb

≤ I < 0.2lb

0.8 capacitive (lead)

± 1.5%

0.2lb

0.5 inductive (lag)

± 1.0%

0.2lb

0.8 capacitive (lead)

± 1.0%

Table 1: IEC61036 Accuracy Specifications

CIRCUIT DESIGN PRINCIPLES

CURRENT SENSING NETWORK

The primary function of the current sensing network is to

sense the load current and convert it to the input current

signal required by the SA4104A. The current sensing

network is shown in Figure 2.

The amplitude of the input current into the SA4104A at

to 16

25

saturation occurs. The SA4104A can be used with most

available shunts. To ensure proper current sensing it is

advisable to use a shunt that will give a minimum voltage

drop of at least 10mV at maximum current. Lower values

can also be used, but this could affect the accuracy of the

meter at very low load currents. The internal current

feedback present on the inputs IIN and IIP of the SA4104A

creates a virtual short circuit between these two input pins.

This means that the resistor value required to generate the

correct input current can be calculated using:

C

6

SH

MAX

R

4

1

10

16

R

I

R21

R20

R19

R18

=

×

×

×

=

=

=

=

−

(1)

A secondary function of the current sense network is to

attenuate all high frequency components that could disrupt

the accuracy of the SA4104A. These high frequency

components may occur due to high frequency surges (fast

transient burst), may be induced through strong electric

fields or may simply be noise on the power lines. Certain

high frequency components, typically those close to integer

multiples of the sampling frequency of the analog to digital

converters will be mapped close to 50Hz once sampled (a

process known as aliasing) and will distort the accuracy of

the converters. This can be prevented by adequately

attenuating all high frequency signal components. The

typical oscillator frequency is 3.58MHz and the analog to

digital converters of the SA4104A operate at one half of this

frequency, so sufficient attenuation should be present at

1.79MHz. This can readily be achieved by placing a single

order RC low pass filter on each current input as shown in

Figure 2. The capacitors cannot be placed directly on the

input pins IIN and IIP because no differential voltage signal

exists between these pins due to the virtual short circuit

created by the input network of the SA4104A. The input

resistance is therefore split into two equal resistors

(R18/R20 and R19/R21) and the capacitor is placed

between these resistors. Now a differential voltage can

appear

across

the

capacitors

and

hence

filter

high

R19

R

C

R18

R

C

Shunt

R

SH

LIVE OUT

LIVE IN

R21

R

C

R20

R

C

SA4104A

IIN

IIP

AGND

C12

C

C

C13

C

C

1

2

16

R19

R

C

R18

R

C

Shunt

R

SH

LIVE OUT

LIVE IN

R21

R

C

R20

R

C

SA4104A

IIN

IIP

AGND

C12

C

C

C13

C

C

1

2

16

Figure 2: Circuit diagram of the current sensing network