JAJSIU8B June 2015 – April 2020 ADS131E08S
Figure 58 illustrates 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 a current output transformer, the burden resistors (R1) are used for current-to-voltage conversion. The output of the burden resistors is connected to the ADS131E08S INxP and INxN inputs through an antialiasing RC filter for current sensing. In the case of a voltage output transformer for current sensing (such as certain types of Rogowski coils), the output terminals of the transformer are directly connected to the ADS131E08S INxP and INxN inputs through an antialiasing RC filter. The input network must be biased to mid-supply if using a unipolar-supply analog configuration (AVSS = 0 V, AVDD = 2.7 V to 5.5 V). The common-mode bias voltage [(AVDD + AVSS) / 2] can be obtained from the ADS131E08S by either configuring the internal op amp in a unity-gain configuration using the RF resistor and setting the OPAMP_REF bit of the CONFIG3 register to 1, or generated externally with a resistor divider network between the positive and negative supplies.
Select the value of resistor R1 for the current output transformer and turns ratio of the transformer such that the ADS131E08S full-scale differential input voltage range is not exceeded. Likewise, select the output voltage for the voltage output transformer to not exceed the full-scale differential input voltage range. In addition, the selection of the resistors (R1 and R2) and turns ratio must not saturate the transformer over the full operating dynamic range. Figure 58a illustrates differential input current sensing and Figure 58b illustrates single-ended input voltage sensing. Use separate external op amps to source and sink current because the internal op amp has very limited current sink and source capability. Additionally, separate op amps for each channel help isolate individual phases from one another to limit crosstalk.