Analog-to-digital converter with offset reduction loop
BACKGROUND OF THE INVENTION 
     The present invention relates to analog-to-digital converters, and more particularly to the use of an analog-to-digital converter with an offset reduction loop for detecting the presence/absence of an analog signal. 
     The detection of the presence/absence of analog signals is generally performed by rectifying and peak detecting an analog input signal to produce a d.c. signal. The d.c. signal is compared with a threshold value, and from the comparison the presence of the analog signal is detected. If there is no analog signal, the threshold is not exceeded, and vice versa. 
     Many analog signals, such as audio signals, represent zero mean value processes, i.e., over an extended period of time the direct current (d.c.) component is zero. The analog-to-digital conversion of a zero mean value signal should yield a digital signal which does not exhibit a static d.c. offset when reconstituted to analog form. This is not always true due to internal or external offsets. The sign bit of the analog-to-digital conversion output should represent the polarity of an analog sample from the analog input signal. A zero mean value input signal should yield equal numbers of positive and negative (logical 1 and 0) sign bits on average. 
     Examples of offset compensation circuits are shown in U.S. Pat. No. 4,965,867, issued Oct. 23, 1990 to Tsuchida et. al. entitled &#34;Offset Compensation Circuit&#34;, and U.S. Pat. No. 4,996,529, issued Feb. 26, 1991 to Connell entitled &#34;Auto-Zeroing Circuit for Offset Cancellation.&#34; In Tsuchida et. al. a sign bit from an ADC is integrated to generate a compensation voltage which is used to control an input filter to equalize the probability that the polarities of the sign bit are equal to each other. Connell also uses an integrator in a feedback loop to determine when the inequality of polarities of the sign bit exceeds an overflow threshold, and then provides a bias control signal that is attenuated and applied to an input limiter. 
     SUMMARY OF THE INVENTION 
     The present invention provides an analog-to-digital converter with an offset reduction loop for detecting the presence/absence of an analog input signal. A d.c. offset compensation signal is added to the analog input signal, and the compensated analog input signal is input to an analog-to-digital converter (ADC). The least significant bits from the ADC are compared with a decision threshold value to determine the presence/absence of the analog input signal. To compensate for arbitrary d.c. offsets in the output from the ADC, the sign bit is integrated with respect to a reference voltage that represents a mid-logic level. The reference voltage is generated from a precision 50% duty cycle clock signal. The output from the integrator is attenuated to produce the d.c. offset compensation signal. 
     The objects, advantages and other novel features of the present invention are apparent from the following detailed description when read in conjunction with the appended claims and attached drawing. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The FIGURE is a block diagram view of an analog-to-digital converter with an offset reduction loop according to the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to the FIGURE an analog input signal Ain is input to a summing circuit 12 together with a d.c. offset compensation signal E. The resulting analog signal A is input to an analog-to-digital converter (ADC) 14 which includes an internal dither generator, such as the Analog Devices 1879 manufactured by Analog Devices, Inc. of Norwood, Mass., United States of America, which assures that there is always sign bit activity even with no analog input signal. The output from the ADC 14 is a digital version of the analog signal A. The most significant bit (MSB) from the ADC 14 is a sign bit S. The sign bit S is passed through a logic buffer gate 16, and is pre-filtered by an RC network 18 before being applied as one input to an integrator 20 to compensate for non-ideal integrator characteristics and enhance a desired low frequency component. A precision 50% duty cycle clock generator 22 provides a clock signal C having a frequency low enough so that edge effects are minimal relative to the clock period, and so that a clock divider chain yields a precision 50% duty cycle. The clock signal also is passed through a logic buffer gate 24 which is on the same substrate 23 as the sign bit logic buffer gate 16 to ensure that the logic levels of the two signals track over time and temperature. The clock signal C is lowpass filtered with a simple RC network 26 to yield a mid-logic level integrator reference voltage R for input to the integrator 20. The integrator output I is input to a resistive attenuator 28, the output of which is the d.c. offset compensation signal E that is input to the summing circuit 12. 
     The least significant bits from the ADC 14 are input to a digital comparator circuit 30 where the bits are compared with a decision threshold value T. The integration of the pre-filtered sign bit S with the reference signal R to produce the d.c. offset compensation signal E compensates for any arbitrary offset that exists in the analog input signal or the ADC 14 so that the decision threshold T may be fixed and repeatable. The presence of an analog input signal Ain provides digital values in the least significant bits that exceed the decision threshold T to generate an indicator signal, provided that the decision threshold T is above the noise output level of the ADC 14. If the decision threshold is not exceeded, then the absence of the analog input signal Ain is indicated. The indicator signal may be used to illuminate a light emitting diode (LED) or LED array to provide a visual indication of the presence/absence of the analog input signal Ain. 
     Thus the present invention provides an analog-to-digital converter with an offset reduction loop for detecting the presence/absence of an analog input signal by comparing the least significant bits from the analog-to-digital converter with a decision threshold value, and compensating for any d.c. offset by integrating the pre-filtered sign bit from the ADC with respect to a mid-logic reference voltage to generate a d.c. offset compensation signal which is added to the analog input signal prior to the ADC.