RM4301ASEA datasheet(3/19 Pages) SAMES

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PRELIMINARY

RM4301ASEA

RX18

RX16

LIVE OUT

LIVE IN

RX19

RX17

SA4301A

IIP

IIN

AGND

CX2

CX3

RX14

RX15

1:N

RX18

RX16

LIVE OUT

LIVE IN

RX19

RX17

SA4301A

IIP

IIN

AGND

CX2

CX3

RX14

RX15

1:N

Figure 2: Circuit diagram of one current sensing network

CI,-3dB

(3)

This frequency should be somewhere between 10kHz and

20kHz to ensure both adequate attenuation at integer

multiples of the analog to digital converters sampling

frequency, and very low phase shift and gain error at mains

frequency and its relevant harmonics. The requirement for

the very low phase shift is explained under the “Voltage

Sense Networks” section.

VOLTAGE SENSING NETWORK

The voltage sensing networks perform similar functions to

the current sensing networks. They sense the mains

voltages and convert them to the input current signals

required by the SA4301A, as well as compensating for the

phase shift of the current transformers used on the current

sensing networks. They also filter all unwanted signals and

thereby ensure that the performance of the SA4301A is not

affected. The voltage sensing network for one phase is

shown in Figure 3. The sensing networks for all phases are

identical.

The voltage sensing network is composed of an adjustable

voltage divider (resistors RX0 to RX12) and the current input

resistor RX13 that generates the required current input

signal for the SA4301A. The first consideration when

designing the voltage sensing network is that the -3dB cut-

off frequency has to be accurately related to that of the

current sensing network. This is important to ensure that the

phase shift experienced by the voltage and current signals

is identical. If this is not the case the energy meter will have

poor

performance

under

non-unity

power

factor

conditions. The high cut-off frequency of the input network

filters does ensure that this matching does not have to be

extremely precise. This allows component tolerances to be

accommodated without seriously affecting the performance

of the meter. The best matching conditions between the cut-

off frequencies is achieved by using identical capacitors on

both the current and voltage sensing networks, so CX1

should equal CX2 and CX3. The IVP input is a virtual short

circuit to analog ground (AGND) so the equivalent

resistance associated with CX1 is

equ

(4)

where RX is the series combination of RX0, RX1, RX2 and

Rtrim. Further, Rtrim is the series combination of the resistors

RX3 to RX11 that can be enabled or disabled to calibrate

the meter. If both RX and RX13 are designed to be

significantly larger than RX12 then

equ

(5)

The overall phase shift of the current sensing networks and

the voltage sensing networks needs to be matched to

ensure that the performance of the meter at non-unity power

factors will not be affected. This is done by purposefully

increasing the -3dB cut-off frequency of the anti-alias filters

on the voltage input networks.

The phase shift of the current input network is

)

arctan(

Mains

(6)

Additionally it is given that the phase shift of the voltage

input network has to be

(7)

where

fCT is the phase shift of the current transformer.

Assuming the result of equation (5) the value of RX12 can

be calculated as

Mains

tan

(8)

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