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SA9110AFA

MONOCHIP THREE PHASE BIDIRECTIONAL KILOWATT HOUR METERING IC

South African Micro Electronic Systems 

SA9110A

TYPICAL APPLICATION

In the Application Circuit (Figure 1), the components required for a three phase power

metering application are shown. Terminated current transformers are used for current

sensing.

The most important external components for the SA9110A integrated circuit are:

C , C , C and C are the outer loop capacitors for the integrated oversampling

7 9 10

11

A/D converters. The typical value of C is 2.2nF and the value of C , C and C is

7

9 10

11

560pF.

The actual values determine the signal to noise and stability performance. The tolerances

should be within ± 10%.

C4, C5, C6 and C8 are the inner loop capacitors for the integrated oversampling A/D

converters. The typical value of C4, C5, C6 and C8 is 3.3nF. Values smaller than 0.5nF

and larger than 5nF should be avoided.

Terminated current sensors (current transformers) are connected to the current sensor

inputs of the SA9110A through current setting resistors (R8 ..R13).

The resistor values should be selected for an input current of 16µARMS into the SA9110A,

at the rated line current.

The values of these resistors should be calculated as follows:

Phase 1:

R8 = R9 = (IL1/16µARMS) * R18/2

Phase 2:

R

10

=

R

11

=

(IL2/16µARMS)

*

R /2

19

Phase 3:

R12 = R13 = (IL3/16µARMS) * R20/2

Where ILX

= Secondary CT current at rated conditions.

R18, R19 and R20 = Current transformer termination resistors for the three phases.

R , R and R set the current for the phase 1 voltage sense input. R , R and R set

14

15

25

16

the current for phase 2 and R , R and R set the current for phase 3. The values should

36

17

be selected so that the input currents into the voltage sense inputs (virtual ground) are

set to 14µARMS for nominal line voltage. Capacitors C1, C2 and C3 are for decoupling and

phase compensation.

R defines all on-chip bias and reference currents (I ).

7

R

With

R

7

=

24kΩ,

optimum

conditions are set. R7 may be varied within ± 10% for calibration purposes. Any changes

to R

7

will affect the output quadratically (i.e:

∆R = +5%, ∆f = +10%).

XTAL is a colour burst TV crystal (f = 3.5795MHz) for the oscillator. The oscillator

frequency is divided down to 1.7897MHz on-chip to supply the digital circuitry and the

A/D converters.

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sames

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