Analog to digital converter using timed level changes

An analog to digital converter comprising:

• a sampling circuit responsive to a sample command being supplied thereto to sample an analog input signal;
• a subtracting circuit arranged to subtract the last preceding sample of the analog input signal from the analog input signal to produce a difference signal;
• means including a plurality of first comparators each arranged to compare the analog input signal with a respective predetermined level for producing signals on respective ones of first lines in response to the analog input signal crossing said respective predetermined levels;
• means including a plurality of second comparators each arranged to compare the difference signal with a respective predetermined level for producing signals on respective ones of second lines in response to the difference signal crossing said respective predetermined levels;
• a source of clock pulses;
• a counter for counting said clock pulses;
• means for producing a signal on a third line in response to the counter reaching a predetermined count;
• means for detecting said signal(s) on any of said first, second, and third lines and for producing a digital code in dependence upon said signal(s);
• and
• control means responsive to detection of said signal(s) by said detecting means to supply a sample command to the sampling circuit, to cause the count of the counter and said digital code to be supplied as a digital representation of the analog input signal, and to reset the counter.

A method of converting an analog signal into a digital representation thereof, comprising the steps of:

• comparing the analog signal with at least one reference level;
• sampling the analog signal in response to a sample command;
• comparing the difference between the analog signal and the last preceding sample thereof with at least one reference level;
• determining the elapsed time following the last preceding sample command;
• in response to one or more of the results of the comparisons indicating that a reference level has been crossed by the analog signal or said difference, producing a digital signal representative of said level crossing(s) and the elapsed time, and producing said sample command;
• and
• in response to said elapsed time reaching a predetermined period, producing a digital signal representative of this and producing said sample command;
• said digital signals constituting said digital representation of the analog signal.