SA9105E
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 SA9105E integrated circuit are:
C7, C9, C10 and C11 are the outer loop capacitors for the integrated oversampling
A/D converters. The typical value of C7 is 2.2nF and the value of C9, C10 and C11 is
560pF.
Theactualvaluesdeterminethesignaltonoiseandstabilityperformance. Thetolerances
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 SA9105E through current setting resistors (R8 ..R13).
The resistor values should be selected for an input current of 16µA into the SA9105E, at
the rated line current.
The values of these resistors should be calculated as follows:
Phase 1:
R8 = R9 = (IL1/16µA) * R18/2
Phase 2:
R10 = R11 = (IL2/16µA) * R19/2
Phase 3:
R12 = R13 = (IL3/16µA) * R20/2
Where ILX
=
=
Secondary CT current at rated conditions.
R18, R19 and R20
Current transformer termination resistors for the three phases.
R1 + R1A, R4 and R15 set the current for the phase 1 voltage sense input. R2 + R2A, R5 +
P5 andR16 setthecurrentforphase2andR3+R3A,R6+P6 andR17 setthecurrentforphase
3. The values should be selected so that the input currents 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.
R14+ P14 defines all on-chip bias and reference currents. With R14+ P14 = 24kΩ, optimum
conditionsareset. R14 maybevariedwithin± 10%forcalibrationpurposes. Anychanges
to R14 will affect the output quadratically (i.e: ∆R = +5%, ∆f = +10%).
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