METHOD AND APPARATUS FOR ANALOG TO DIGITAL CONVERSION

• US 3,668,690 A
• Filed: 06/08/1970
• Issued: 06/06/1972
• Est. Priority Date: 06/08/1970
• Status: Expired due to Term

• Current Assignee: Alfred Newman Ormond
• Original Assignee: Alfred Newman Ormond
• Inventors: Ormond, Alfred Newman
• Primary Examiner(s): Wilbur, Maynard R.
• Application Number:
  US05/044,245
• Time in Patent Office: 729 Days
• Field of Search: 340/347CC,347NT
• US Class Current: 341/157
• CPC Class Codes: H03M 1/60 with intermediate conversion to frequency of pulses

Abstract

An analog voltage signal is converted to a digital output reading by applying it to a variable frequency oscillator, the output frequency of which is proportional to the applied signal. Initially, the oscillator is set to oscillate at a given reference frequency for zero voltage input signals. The analog signal is then applied to the oscillator for a first given time interval. The output of the oscillator is counted by an up-down counter during this first period to provide a count proportional to the sum of the reference frequency and frequency resulting from the analog signal. The input applied to the oscillator is then changed to either zero volts or the reversed polarity of the analog signal during a second given time interval and the direction of the counter is changed to count the output of the oscillator in a down direction during this second time interval. The resulting counting total after the first and second time intervals registered in the counter is then directly proportional to the value of the analog signal, the count from the reference frequency having been subtracted out over the two time intervals. Any drift effects in the variable frequency oscillator are thus cancelled and a more accurate digital output reading corresponding to the analog signal results. Further features of the method and apparatus contemplate providing an output count in the form of a ratio of a number corresponding to the analog signal and a fixed number provided by a constant input signal so that a scaling factor is provided.

Claims

1. A method of converting an analog signal to a digital signal comprising the steps of:

a. generating a reference frequency;

b. adding to said reference frequency a frequency constituting a function of said analog signal to provide a first modified frequency during a first given time interval;

c. counting the total number of cycles of said first modified frequency in an up direction occuring during said first given time interval;

d. subtracting from said reference frequency said frequency constituting a function of said analog signal such that a second modified frequency is present during a second given time interval equal to said first given time interval; and

e. counting the total number of cycles of said second modified frequency in a down direction occuring during said second given time interval, whereby a first counting total resulting after said first and second time intervals is equal to twice the value of said analog signal.

2. The method of claim 1, including the additional steps of holding said first counting total resulting after said first and second time intervals;

and repeating the up and down counting operations with respect to a constant input signal during third and fourth time intervals equal to said first and second time intervals to provide a second counting total resulting after said third and fourth time intervals equal to k'"'"''"'"' times the value of said constant input signal; and

dividing said first counting total by said second counting total so that the resulting ratio provides a number proportional to said analog signal divided by a fixed number proportional to said constant input signal.

3. An apparatus for converting an analog signal to a digital signal comprising, in combination:

a. a variable frequency oscillator providing an output reference frequency when zero input frequency is applied thereto;

b. an up-down counter connected to said osciLlator to count the output frequency thereof;

c. an interval timer means;

d. first switch means connected to said interval timer means and responsive to the initiation of a first given time interval to apply said analog signal to said oscillator, and to the initiation of a second given time interval, equal to said first time interval, to apply the reversed polarity of said analog signal to said oscillator; and

, e. function selector means responsive to said interval timer means for controlling operation of said first time interval so that the sum of said reference frequency and frequency resulting from said analog signal is counted in an up direction, and the difference between said reference frequency and frequency resulting from said analog signal is counted in a down direction only during said second given time interval, whereby a first counting total resulting after said first and second time intervals is registered in said up-down counter equal to twice the value of said analog signal.

4. An apparatus according to claim 7, including a second switch means connected to said interval timer means and responsive to the initiation of a third given time interval to apply a constant input signal to said oscillator and to initiation of a fourth given time interval to apply a second modified signal to said oscillator, said function selector means being responsive to said interval timer means for controlling operation of said up-down counter to count in an up direction only during said third time interval and in a down direction only during said fourth time interval so that a second counting total resulting after said third and fourth time intervals is registered in said up-down counter equal to k'"'"''"'"' times the value of said constant input signal;

computer means for receiving said first counting total from said up-down counter, said function selector means resetting said up-down counter after said first and second time intervals, said computer means receiving said second counting total after said third and fourth time intervals and including means for dividing said first counting total by said second counting total; and

display means connected to said computer means for displaying a number proportional to said analog signal divided by a fixed number proportional to said constant input signal.

5. An apparatus according to claim 3, including display means connected to said up-down counter for displaying said first counting total after said first and second time intervals, said function selector means resetting said up-down counter after each second time interval to define a cycle of operation, said cycle being repeated at a sufficient rate to provide a visually continuous display of a number corresponding to the value of said analog signal.

6. An apparatus according to claim 4, in which said function selector means resets said up-down counter after each fourth time interval to define a cycle of operation, said cycle being repeated at a sufficient rate to provide a visually continuous display of the number proportional to said analog signal divided by said fixed number proportional to said constant input signal.

7. An apparatus according to claim 6, including a zero zero-cross-over detector connected to said up-down counter and responsive to the counter passing through zero to change the direction of counting from down to up and at the same time display a negative sign on said display means.