Signal input selecting circuits

A signal input selecting circuit comprising:

• a plurality of signal input terminals each comprising a signal electrode and a reference electrode,
• first switch means for selecting one of the signal electrodes of the signal input terminals to be connected with a signal processing block,
• second switch means for selecting one of the reference electrodes of the signal input terminals to be connected with a reference potential point,
• third switch means for connecting the second switch means with a current supplying portion, and
• voltage detecting means for detecting a voltage corresponding to an impedance of the second switch means,
• wherein when the voltage detected by said voltage detecting means has a level thereof less than a predetermined level under a condition in which said second switch means is in operation to connect the reference electrode of a selected one of the signal input terminals with the reference potential point and said third switch means is in operation to connect said second switch means with the current supplying portion, said third switch means is turned to disconnect said second switch means from the current supplying portion and said first switch means is caused to connect the signal electrode of the selected one of the signal input terminals with the signal processing block.