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PRELIMINARY

RM4104ASEB

frequencies. The lowest -3dB cut-off frequency is achieved

when all four input resistors are equal (R18 = R19 = R20 =

both capacitors C12 and C13 must also be equal (C12 =

C

C

C,-3dB

C

R

1

f

π

=

(2)

This frequency should be somewhere between 10kHz and

20kHz to ensure both adequate attenuation at integer

multiplies of the analog to digital converters sampling

frequency, and very low phase shift at mains frequency and

its harmonics. The requirement for the very low phase is

explained under the “Voltage Sense Network” section.

VOLTAGE SENSING NETWORK

The voltage sensing network performs similar functions to

the current sensing network. It senses the mains voltage

and converts it to the input current signal required by the

SA4104A. It also filters all unwanted signals and prevents

them from distorting the performance of the SA4104A. The

voltage sensing network is shown in Figure 3.

The voltage sensing network is composed of an adjustable

voltage divider (resistors R6 to R16) and the current input

resistor that generates the required current input signal for

the SA4104A. The first consideration when designing the

voltage sensing network is that the -3dB cut-off frequency

has to be very closely matched 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 load 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 between the cut-off

frequencies is achieved by using identical capacitors on

both the current and voltage sensing networks, so C11

should equal C12 and C13. The IVP input is a virtual short

circuit

to

analog

ground

(AGND)

so

the

equivalent

resistance associated with C11 is

X

11

C

equ

R

||

17

R

||

16

R

R

=

−

(3)

R15 that can be enabled or disabled to calibrate the meter.

than R16 then

16

R

R

11

C

equ

≈

−

(4)

To match the -3dB cut-off frequency of the Current Sensing

of the current sense network equivalent resistance. This

ensures balanced phase shifts on the current and voltage

input networks so the performance of the meter at non-unity

power factors will not be affected.

The voltage input IVP of the SA4104Ahas to be driven with

a current of 11

This input also saturates at 25

μA peak current, so

the11

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 R17 at 100 times

the value of R16, so it will not significantly affect the -3dB

cut-off frequency of the voltage sense network. Setting

50R

R16

100

R17

C

=

×

=

(5)

sets the required output voltage on the voltage divider to

R16

100

10

11

V

-6

D

×

×

×

=

(6)

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

NOM

X

NOM

X

D

V

R

R16

V

R

R16

R16

V

×

≈

×

+

=

(7)

Combining these equations results in

NOM

4

NOM

X

V

909

10

11

V

R

≈

×

=

−

,

(8)

R10

LIVE IN

R11

SA4104A

IVP

AGND

C11

C

C

15

16

R12

R13

R14

R15

R6

R7

R8

R9

R17

NEUTRAL

R16

½R

C

50R

C

J1

J2

J3

J4

J5

J6

J7

J8

R

trim

V

D

R10

LIVE IN

R11

SA4104A

IVP

AGND

C11

C

C

15

16

R12

R13

R14

R15

R6

R7

R8

R9

R17

NEUTRAL

R16

½R

C

50R

C

J1

J2

J3

J4

J5

J6

J7

J8

R

trim

V

D

Figure 3: Circuit diagram of the voltage sensing network