SA9105A
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 SA9105A integrated circuit are:
C
7
, C
9
, C
10
and C
11
are the outer loop capacitors for the integrated oversampling
A/D converters. The typical value of C
7
is 2.2nF and the value of C
9
, C
10
and C
11
is
560pF.
The actual values determine the signal to noise and stability performance. The tolerances
should be within ± 10%.
C
4
, C
5
, C
6
and C
8
are the inner loop capacitors for the integrated oversampling A/D
converters. The typical value of C
4
, C
5
, C
6
and C
8
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 SA9105A through current setting resistors (R
8
..R
13
).
The resistor values should be selected for an input current of 16µA into the SA9105A at
the rated line current.
The values of these resistors should be calculated as follows:
Phase 1:
R
8
= R
9
= (I
L1
/16µA) * R
18
/2
Phase 2:
R
10
= R
11
= (I
L2
/16µA) * R
19
/2
Phase 3:
R
12
= R
13
= (I
L3
/16µA) * R
20
/2
Where I
LX
= Secondary CT current at rated conditions.
R
18
, R
19
and R
20
= Current transformer termination resistors for the three phases.
R
1
+ R
1A
, R
4
and R
15
set the current for the phase 1 voltage sense input. R
2
+ R
2A
, R
5
+
P
5
and R
16
set the current for phase 2 and R
3
+ R
3A
, R
6
+ P
6
and R
17
set the current for phase
3. The values should be selected so that the input current into the voltage sense inputs
(virtual ground) are set to 14µA for nominal line voltage. Capacitors C1, C2 and C3 are
for decoupling and phase compensation.
R
14
+ P
14
defines all on-chip bias and reference currents. With R
14
+ P
14
= 24kΩ, optimum
conditions are set. R
14
may be varied within ± 10% for calibration purposes. Any changes
to R
14
will affect the output quadratically (i.e:
∆R
= +5%,
∆f
= +10%).
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SAMES [ SAMES ]
