Cirrus Logic CDB5480U - Manual (Page 12)

CDB5480U
12 DS893DB5
1.6.2 Current Transformer Power Meter Example
A slightly more expensive option is to use a current transformer (CT) to connect the AC current to the
CDB5480U evaluation board. Figure 7 depicts the voltage and current connections for a CT sensor and
its associated filter configurations.
NEVER “open circuit” a CT. Make sure that all signals are well connected before the power source is
turned on. Extreme care should be taken when connecting high-voltage signals to the CDB5480U
evaluation board.
The burden resistor (R11/R22) is necessary in a CT application to convert the secondary current into
voltage. Knowledge of the current transformers turns ratio (N) is key to determining the proper CS5480
input voltage (V
burden
) that the meter places on the system. The optimum secondary voltage (V
burden
) at
the maximum current input should be 10% less than the maximum channel voltage of 250mVp with I-
channel PGA = 10x. The secondary voltage (V
burden
) is determined by converting the primary current to
the secondary current. Then the secondary current (I
burden
) can be converted into a voltage by Ohm's
Law.
The secondary voltage (V
burden
) is sourced to the CS5480 through a simple low-pass, anti-alias filter, and
this voltage should not exceed the 250mVp.
Figure 7. Current Transformer Power Meter
V
burden
I
burden
R
burden

I
primary
N
------------------ R
burden
 = =
IIN1-/IIN2-
IIN1+/IIN2+
GND
GND
GND
LINE
CS
5480
CDB5480U
PH
AS
E
J1/J12
J4
J7/J13
J8/J14
J11
J6
R5
1K
C5/C11
0.033UF
C6/C12
0.033UF
C9
0.027UF
C4
0.027UF
R11/R22
2.2
R1/R21
100
R2/R22
100
R7 1K
R6 1K
R9/R23
1K
R13/R24
1K
R8 422K R12 422K R14 422K R15 422K
R49/R52 1K
R50/R53 1K
C34/C1
0.033UF
C35/C2
0.033UF
J44/J51
J46/J52
R51/
R54
0
J45
J53/J56
J54/J55
IIN1+/IIN2+
IIN1-/IIN2-
VIN-
VIN+