Low pass digital averaging filter

Abstract

A circuit for extracting a low frequency signal component from a composite signal is disclosed which uses a digital averaging technique for filtering out higher frequency components. An analog to digital converter receives a composite analog signal and provides a digital output signal. The composite analog signal is sampled during periodic intervals or sample periods. The digital output signal of the analog to digital converter is coupled to one input port of an adder/subtractor circuit and to the input of a sample register. The sample register stores the digital output signal and outputs the stored signal, delayed by a predetermined number of sample periods, to a second input port of the adder/subtractor. A storage register is used to store a running average which is equal to the sum of the digital signals received during the most recent predetermined number of sample periods. The average signal stored by the storage register is fed back to a third input port of the adder/subtractor. During each sample period, the adder/subtractor causes the current output of the analog to digital converter to be added to the previous average signal and causes the output of the sample register to be subtracted from the average signal. The result generated by the adder/subtractor is then stored in the storage register until the next sample period, during which the procedure is repeated.