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:
C
2
, C
6
and C
7
are the outer loop capacitors for the integrated oversampling A/D
converters. The typical value of C
2
is 2.2nF and the value of C
6
and C
7
is 560µF.
The actual values determine the signal to noise and stability performance. The
tolerances should be within ± 10%.
C
1
, C
3
and C
4
are the inner loop capacitors for the integrated oversampling A/D
converters. The typical value of C
1
, C
3
and C
4
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
5
, R
6
and R
7
, R
8
).
The resistor values should be selected for an input current of 16µA
RMS
into the SA9108F,
at the rated line current.
The values of these resistors should be calculated as follows:
Phase 1:
R
5
= R
6
= (I
L1
/16µA
RMS
) * R
3
/2
Phase 2:
R
7
= R
8
= (I
L2
/16µA
RMS
) * R
4
/2
Where I
LX
= Secondary CT current at rated conditions.
R
3
and R
4
= Current transformer termination resistors for the two phases.
R
1
+ R
1A
, R
13
and R
9
set the current for the phase 1 voltage sense input. R
2
+ R
2A
, R
12
+
P
2
and R
10
set the currents for phase 2. The values should be selected so that the input
currents into the voltage sense inputs (virtual ground) are set to 14µA
RMS
for nominal line
voltage. Capacitors C8 and C9 are for decoupling and phase compensation.
R
11
and P
1
defines all on-chip bias and reference currents. With a total resistance of
24kΩ, optimum conditions are set. R
11
+ P
1
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.
8/12
sames
SAMES [ SAMES ]
