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PRELIMINARY

RM4104ASEB

changes during calibration of the meter, so if the stated

condition is not met, the -3dBcut-off frequency of the voltage

sensing network will change during calibration of the meter

which is not desirable.

A calibration mechanism is required to compensate for

component tolerances. The SA4104A and the shunt resistor

are the components that will exhibit the widest tolerances.

The combination of these two can have variations in the

order of ±10%. All resistors should be 1% metal film

resistors so that the resistor tolerance is low and can be

ignored. The total tuning range is therefore 20%. A binary

weighted series combination of eight resistors will therefore

allow calibration to an accuracy of 20%/256 = 0.78% which

is considered sufficient for a low-cost class 1 energy meter.

To achieve approximately 10% tuning range in either

direction around the nominal point the highest resistance in

with each subsequent resistor having half the value of the

previous one. The values of the voltage divider and

calibration network are then designed using

(9)

(10)

R9 = 0.5 x R8, R10 = 0.5 x R9, R11 = 0.5 x R10, etc

(11)

This topology of current and voltage input networks has

some advantages for mass production. The first is that the

network is independent of R16. This is important because

the value of R16 is half that of the current input resistors

(R18 to R21) which will be specific to the shunt. The shunt

can therefore be changed without having to adapt the

calibration network, i.e. the calibration network is universal.

Secondly, one of the input networks can be used to

intentionally introduce a phase shift between the voltage

and the current signal networks by reducing the cut-off

frequency. This allows phase shift compensation to be

performed for certain shunts that may exhibit phase shift

due to high shunt inductance.

SA4104A RELATED CIRCUITRY

All aspects discussed in this section are illustrated on the

complete meter schematic (Figure 6). The SA4104Arequires

a split supply of +2.5V (VDD) and 2.5V (VSS) around the

meter ground node, which has to be connected to the

AGND pin of the SA4104A. These three supply lines have to

be properly decoupled using capacitors C8 and C9 (220nF

ceramic each) between the supplies and ground and C10

(1

μF ceramic) between the two supplies. These capacitors

are required to achieve good performance and have to be

placed as close to the device as possible. Their placement

is as important as their presence, placing them more then a

few millimeters away from the device renders them useless.

The on-chip reference current is derived from a 47k

Ω

resistor (R22) connected between VREF and VSS. This

resistor must be a 1% tolerance metal film type or similar.

The metal film ensures that less noise will be induced into

the device pin. A 220nF ceramic capacitor (C14) must also

be connected between the VSS side of the resistor and

ground for noise filtering the reference current.

The internal pulse dividers of the SA4104A are configured

through the pins R3, R2, R1, R0 and FMS. These pins must

either be tied to VDD or VSS. The FMS pin can be left

floating to enable a fast pulse output mode which is typically

not applicable to a low-cost energy meter of this type. These

setup pins can be directly connected to VDD or VSS, no

pull-up or pull-down resistors are required.

The stepper motor used to display the consumed energy

should be connected to the MOP and MON pins through two

small

current

limiting

resistors

(R24

and

R25).

For

calibration and performance verification purposes an LED

and opto-isolator are connected in series to the LED output

pin of the SA4014A. This output is active low so the LED

and opto-isolator are connected through a current limiting

resistor to VDD.

For more elaborate information on setting up and using the

SA4104A, refer to the SA4104A datasheet available on the

SAMES website at www.sames.co.za or from any SAMES

representative.

SETUP OF R0, R1, R2, R3 AND FMS FOR

DIFFERENT RATED CONDITIONS

The following equations and table state the basic pulse

constants

and

motor

constants

obtainable

with

the

SA4104A.

The pulse rate of the LED output is:

MAX

NOM

LED

I

V

1000

3600

LED

_

DF

1

5000

16

IVP

p/kWh

×

×

×

×

×

=

(12)

The motor output pulse rate is

MO

_

DF

1

p/kWh

p/kWh

MOTOR

MOTOR

×

=

(13)