The circuit shown is from the National Semiconductor Linear Applications Handbook of 1994. using ADC0808. The circuit can handle data from up to 4 transducers with an 8-bit output.
The circuit is simple, as it uses only one transistor and a relay that can be MC2RC1 or MC2RC2,...
We found this circuit in a 1990 documentation. It works with the power of the digital circuit...
The filter shown in the figure can operate with frequencies up to several tens of megahertz thanks...