TIDUF25 june 2023 ADS131M08 , MSPM0G1507
The analog front end for current inputs is different from the analog front end for the voltage inputs. Figure 2-4 shows the analog front end used for a current channel, where the positive and negative leads from a CT for Phase A are connected to pins 1 and 3 of header J4. Again, similar circuitry is used for the CTs on each of the Phases B and C.
The analog front end for current consists of footprints for electromagnetic interference filter beads (R35 and R39), burden resistors for current transformers (R37 and R38), and an RC low-pass filter (R36, R40, C13, C15, and C14) that functions as an anti-alias filter.
As Figure 2-4 shows, resistors R37 and R38 are the burden resistors, which are in series with each other. For best THD performance, instead of using one burden resistor, two identical burden resistors in series are used with the common point being connected to GND. This split-burden resistor configuration makes sure that the waveforms fed to the positive and negative terminals of the ADC are 180 degrees out of phase with each other, which provides the best THD results with this ADC. The total burden resistance is selected based on the current range used and the turns ratio specification of the CT (this design uses CTs with a turns ratio of 2000). The total value of the burden resistor for this design is 12.98 Ω.
Equation 2 shows how to calculate the range of differential voltages fed to the current ADC channel for a given maximum current, CT turns ratio, and burden resistor value.
Based on the maximum current of 100 A, CT turns ratio of 2000, and burden resistor of 12.98 Ω, of this design, the input signal to the current ADC has a voltage swing of ±918 mV maximum (649 mVRMS) when the maximum current rating of the meter (100 A) is applied. This ±918-mV maximum input voltage is well within the ±1.2-V input range of the device for the selected PGA gain of 1 that is used for the current channels.