An analog to digital converter circuit includes an isolation circuit, an input circuit including a resistor chain, a plurality of fixed threshold comparators, and an encoder. The resistor chain includes a plurality of resistors connected in series, the isolation circuit being configured to isolate a device supplying the analog input signal from the input circuit. The isolation circuit is connected to the input circuit at a top node of the resistor chain, and wherein 2.sup.N values of the analog input voltage are connected to inputs of the fixed threshold comparators to produce corresponding output signals. Methods of converting an analog signal to a digital signal are also described.