RM4301ASEA datasheet(4/19 Pages) SAMES

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PRELIMINARY

RM4301ASEA

and the resulting -3dB cut-off frequency of the voltage

network anti-alias filter is

CV,-3dB

(9)

The value of the cut-off frequency of the voltage input

network is less critical than that of the current input network

because the dynamic range required on the voltage input

network is small. A cut-off frequency between 10kHz and

25kHz is acceptable.

The voltage input IVP of the SA4301A has to be driven with

a current of 11μARMS at the nominal rated mains voltage of

220V. This input also saturates at 25μA peak current, so the

11μARMS input current allows for 50% overdrive capability

while maintaining linearity. This ensures that the device will

not saturate with a ±20% variation in mains voltage. The

simplest method is to set the input resistor RX13 at 100

times the value of RX12, thereby satisfying the condition for

equation (5). This choice sets the required output voltage

on the voltage divider to

RX12

100

-6

(10)

because the IVP pin has a virtual short to ground. Given that

RX13 and RX are large compared to RX12

NOM

RX12

(11)

Combining these equations results in

NOM

909

(12)

which should easily satisfy the condition that RX should be

significantly larger than RX12. It must be noted that RX

changes during calibration of the meter, so if the stated

condition is not met, the -3dB cut-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 SA4301A is the component that

will exhibit the widest tolerance and has a gain variation of

±10%. All resistors should be 1% metal film resistors so that

the resistor tolerance will not add significant overall

deviation. The total tuning range can therefore be designed

as ±15%. A binary weighted series combination of nine

resistors will then allow calibration to an accuracy of

30%/512 = 0.06% which is considered sufficient for a low-

cost class 1 energy meter. To achieve approximately 15%

tuning range in either direction around the nominal point the

highest resistance in the calibration network (RX3) should

be 15% of RX 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

RX0 + RX1 + RX2 = RX - 15%RX = 0.85RX ,

(13)

RX3 = 15%RX ,

(14)

RX4 = 0.5 x RX3, RX5 = 0.5 x RX4, etc.

(15)

This topology of current and voltage input networks has

some advantages for mass production. The first is that the

value of RX which is used to determine the calibration

network is independent of RX12. This is important because

the value of RX12 is related to that of the current input

resistors (RX16 to RX19) which will be specific to the

current transformer. The CT can therefore be changed

without having to adapt the calibration network, i.e. the

calibration network is universal. Secondly, the value of

RX12 can be changed to adapt the phase compensation

without affecting the calibration network. Any change in

RX12 is the simply reflected in RX13 and the input networks

are guaranteed to be correct.

RX5

NEUTRAL

RX6

SA4301A

IVP

AGND

CX1

RX7

RX8

RX9

RX11

RX1

RX2

RX3

RX4

RX13

LIVE IN

RX12

100R

JX2

JX3

JX4

JX5

JX6

JX7

JX8

JX10

trim

RX0

JX9

RX10

RX5

NEUTRAL

RX6

SA4301A

IVP

AGND

CX1

RX7

RX8

RX9

RX11

RX1

RX2

RX3

RX4

RX13

LIVE IN

RX12

100R

JX2

JX3

JX4

JX5

JX6

JX7

JX8

JX10

trim

RX0

JX9

RX10

Figure 3: Circuit diagram of one voltage sensing network

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