The problem arises if the residual current required by the FBM219 is higher than the current that the digital I/O module can provide. The digital I/O module may then incorrectly detect an off state as an on state because the sensor is drawing more current than it is prepared to supply. Most 2-wire sensors in industrial applications have off-state leakage or residual currents no higher than 1.7 mA.

The FBM219 complete test system has a total of n unit measurement circuits, which can realise the multi-channel measurement of the sensor. Each unit measurement circuit is designed in a four-wire system, and this four-wire structure requires a precision constant-current source; in addition, due to the weak output signal of the unit measurement circuit, it is also necessary to amplify the output signal for DC amplification, and then carry out A/D conversion after amplification.

FBM219 In order to improve the measurement accuracy and reduce the error brought by the peripheral circuit during measurement, this design adopts a multi-channel electronic switch Ka, which makes the n-channel unit measurement circuit share a precision constant-current source of 0.5mA, and at the same time, makes the n-channel unit measurement circuit share a common amplification circuit, i.e., during the measurement of Pt temperature sensors, the nth switch of the electronic switch groups Ka and Kb can only be selected when the nth switches of the electronic switch groups Ka and Kb are switched on at the same time. This means that the nth Pt temperature sensor can be selected and its parameters measured only when the nth switch of the electronic switch groups Ka and Kb is switched on at the same time.