| 摘要 |
An electrode (12) for sensing a surface potential is connected to a non-inverting input of an operational amplifier (14). The output of the operational amplifier (14) is connected to the inverting input thereof in a voltage follower configuration. The output of the operational amplifier (14) is also connected to one or more shield electrodes (19) which are operatively disposed adjacent to the sensing electrode (12). An offset bias voltage of the operational amplifier (14) is adjusted to cancel an input bias current thereof. An operational amplifier (33) may be mounted on a flat substrate (112), with sensing and shield electrodes (113), (114), (116) formed on the substrate (112) by printing. The shield electrodes (114), (116) surround the sensing electrode (113) and also the operational amplifier (33).
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