The conducting phase current and decay current flows in opposite directions. If the CSA input is connected such that conducting phase current develops a positive input voltage across the shunt resistor, the decay current will develop a negative input voltage for the CSA. The decay current will consequently push the CSA output toward ground. The negative input voltage due to decaying phase current is illustrated in Figure 7-6.

If a unidirectional CSA is used, it will be pushed into saturation by the negative shunt voltage. A deep saturation may take such a long time to overcome that the CSA is not ready to respond when the next conduction cycle comes, where it is tasked to provide a valid output value that is proportional to the phase current. For this reason, the CSA should be configured as bi-directional, so that it stays in linear operation region throughout the entire commutation cycle.

It should be noted that shunt resistor parasitic inductance can exacerbate the effect of decay current by creating a differential voltage spike, which adds to the negative differential voltage. For this reason, it is better to choose shunt resistors of low inductance. Smaller passive components tend to offer smaller parasitic inductance.