This is the full circuit of a V/F convertor interface for 80C31 SBC. The Mux is configured for selecting between two inputs. It uses analog switch 4066. LF353 along with LM331 form the VCO, you can further study such configuration in the Application Notes of TI/NS.
Mixed Circuits Analog with Digital
This design is built above an application note. Two Pt-100 RTDs are switched to the A/D using the FET Switches. Here a 1mA Precision current source is shown at the bottom for RTD and a 50mA Current source is shown above for special uses as an example.
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