Using split CT termination for better THD performance
In the ADS131M02 datasheet, there is a following schematic snapshot for interfacing the ADC differential input channel with the Current Transformer:
Furthermore,
Two identical burden resistors in series are used with the common point being connected to GND instead of using one burden resistor for best THD performance.
How does this work - how does it improve THD performance?
2 answers
How does this work - how does it improve THD performance?
As with any transformer connected to a live wire, there will be an AC voltage common mode signal transferred by winding capacitance to the secondary. If you want that common-mode signal to be balanced (and hence easily cancelled by the ADCs differential input), you would use a split burden resistor.
If your application isn't that important then you would use a single burden resistor and imbalanced ADC input.
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Your question is answered in the very next sentence in the datasheet after the one you quoted:
This split-burden resistor configuration ensures 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.
Any measurement system will have some non-linearities. Such non-linearities create frequencies in the result that were not in the input. We call those fictitious frequency components "distortion".
The non-linear effects are probably not symmetrical about the analog input midpoint. By driving both inputs with an equal but opposite signal, the result does become symmetric, cancelling out some of the non-linear effects.
Added
I see Andy's answer mentions another effect. His answer isn't technically about THD (total harmonic distortion), but is nonetheless another issue you should consider. You want to ignore common mode signals caused the the capacitance between the primary and secondary in the current transformer, while measuring only the differential mode signal.
The suggested circuit addresses both issues simultaneously (THD and common mode noise rejection).
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