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SA9108FFA

TWO PHASE BIDIRECTIONAL POWER/ENERGY METERING IC WITH INSTANTANEOUS PULSE OUTPUT

South African Micro Electronic Systems 

SA9108F

TYPICAL APPLICATION

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

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

sensing.

The most important external components for the SA9108F integrated circuit are:

C2, C6 and C7 are the outer loop capacitors for the integrated oversampling A/D

converters. The typical value of C2 is 2.2nF and the value of C6 and C7 is 560µF.

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

tolerances should be within ± 10%.

C , C and C are the inner loop capacitors for the integrated oversampling A/D

1

3

4

converters. The typical value of C1, C3 and C4 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 SA9108F through current setting resistors (R , R and R , R ).

56

78

The resistor values should be selected for an input current of 16µARMS into the SA9108F,

at the rated line current.

The values of these resistors should be calculated as follows:

Phase 1:

R5 = R6 = (IL1/16µARMS) * R3/2

Phase 2:

R

7

=

R

8

=

(IL2/16µARMS)

*

R /2

4

Where ILX

= Secondary CT current at rated conditions.

R3 and R4

= Current transformer termination resistors for the two phases.

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

1

1A 13

9

2

2A 12

P and R set the currents for phase 2. The values should be selected so that the input

2

10

currents into the voltage sense inputs (virtual ground) are set to 14µARMS for nominal line

voltage. Capacitors C8 and C9 are for decoupling and phase compensation.

R11 and P1 defines all on-chip bias and reference currents. With a total resistance of

24kΩ, optimum conditions are set. R11 + P1 may be varied within ± 10% for calibration

purposes. Any change in resistance will affect the output quadratically (i.e: ∆R = +5%,

∆f = +10%).

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

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

A/D converters.

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