Datasheet

(a) Current Output CT with Differential Input
(b) Voltage Output CT with Single-Ended Input
INP
INN
Device
EMI
Filter
I
I
LN
R2
R2
+
-
OPAMPOUT
OPAMPN
OPAMPP
Rf
(AVDD + AVSS)
2
OPAMP_REF
C
R1
R1
INP
INN
Device
EMI
Filter
V
LN
C
+
-
OPAMPOUT
OPAMPN
OPAMPP
Rf
(AVDD + AVSS)
2
OPAMP_REF
R2
To PGA To PGA
ADS131E04
ADS131E06
ADS131E08
SBAS561A –JUNE 2012–REVISED APRIL 2013 www.ti.com
CURRENT SENSING
Figure 44 shows a simplified diagram of typical configurations used for current sensing with a Rogowski coil,
current transformer (CT), or an air coil that outputs a current or voltage. In the case of the current output
transformers, the burden resistors (R1) are used for current-to-voltage conversion. The output of the burden
resistors is connected to the ADS131E0x INP and INN inputs through an antialiasing RC filter for current
sensing. In the case of the voltage output transformers (such as certain types of Rogowski coils), the output
terminals of the transformers are directly connected to the ADS131E0x INP and INN inputs through an
antialiasing RC filter for current sensing. The common-mode bias voltage (AVDD + AVSS) / 2, can be obtained
from the ADS131E0x by either configuring the internal op amp in a unity-gain configuration using the R
F
resistor
and setting bit 3 of the CONFIG3 register, or it can be generated externally with a simple resistor divider network
between the positive and negative supplies.
The value of resistor R1 for the current output transformer and turns ratio of the transformer should be selected
so as not to exceed the ADS131E0x full-scale differential input voltage (FSDI) range. Likewise, the output
voltage (V) for the voltage output transformer should be selected to not exceed the FSDI. In addition, the
selection of resistor (R) and turns ratio should not saturate the transformer over the full operating dynamic range
of the energy meter. Figure 44a shows differential input current sensing and Figure 44b shows single-ended
input sensing.
Figure 44. Simplified Current Sensing Connections
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