Analog input circuit for microcontroller apparatus

An analog input circuit for inputting voltage signals to a controller comprising a circuit board, primary and secondary sides of the circuit disposed on the circuit board separated from one another by an earth drain disposed therebetween on the circuit board,

• the primary side comprising a plurality of independent channels isolated from each other and from common mode environment each having differential solid state relay switches with positive and negative inputs and outputs, the positive output coupled to an input stage of an isolation amplifier, the negative output coupled to a signal common, a gain producing means including a multiplexer, the gain producing means coupled to the input stage of the isolation amplifier, and
• the secondary side comprising an output of the isolation amplifier isolated by a transformer coupling from the input thereto, the output of the isolation amplifier coupled to a sample and hold circuit, the sample and hold circuit having an output coupled to an analog to digital converter, an analog input register coupled to an input of an optical isolator, the optical isolator having an output coupled to the multiplexer, the analog input register further coupled to the sample and hold circuit to multiplex the independent channels and cause the sample and hold output to be processed by the analog to digital converter, the analog to digital converter having an output connectable to a controller, whereby the channels are isolated from one another and from common mode voltage with the amplifier floating with reference to the input common mode voltage for each selected channel.