This is the Analog front end of the 80C39 Process Controller. The analog input is protected by a Zener barrier, low leakage. You could use clamping diodes too.
The non-inverting low-offset amp offers high input impedance. After further amplification it reaches the VCO LM331. The pulse train from the VCO reaches the uC port and is gated and measured by the MCS48 firmware. The voltage is deduced from the Frequency or Pulse width.
The circuit of the Analog Front End was a V-F Converter
You can get a resolution near to a 8-10 bit A/D converter. It does not work for negative voltages. It is a low cost Voltmeter or Process Display solution.
80C39 and MCS48 based Process Controller is the main circuit that has the LED 7 segment display for output and push keys for input.
80C39 is a Cmos Version of 8048 the one that preceded 8031-8051. MCS48 is the set for that, MCS51 is the current set.
Here is an example project for 80C39-8748-8749 microcontroller. This code was written by me, and it works. But the documentation is not complete or may have errors.
80C39-8749 MCS-48 Examples and code.
Process Controller code here 80C39 Code for MCS48 The circuit for these are on this page along with other circuits.
This may be difficult to put together now. But the code and hardware is near compatible to 8051 uC. Now there are may types of ADC and uC.
This was supposed to have Analog in and Analog out. Some parts of the circuit may be in the 80C51 page. This has ramp-up and ramp-down settings for the Increment and Decrement buttons. This helps setting setpoint quickly. There is also debouncing for the pushbutton.