one or more probe cells are use to sense voltage and current accurately and without affecting performance of the switching device or the load . in addition , power , resistance , and temperature can be determined from the voltage and current . voltage sensing is accomplished by placing a large value resistor between the probe cell and its low voltage connection . since the resistor is much greater than the cell resistance , the voltage across the resistor is nearly equal to the voltage across the power chip . current probe cells are isolated from switching cells in mosfet power chips . the cell locations adjacent the probe cells are occupied by cells that are inactive by virtue of their not having had a source region implanted therein during the chip fabrication . this isolation prevents any crosstalk between probe and switching cells .