Logarithmic analog to digital converter

• US 4,625,197 A
• Filed: 09/28/1984
• Issued: 11/25/1986
• Est. Priority Date: 09/28/1984
• Status: Expired due to Fees

• Current Assignee: Milton ROY Company One Plaza Place St Petersburg FL 33701
• Original Assignee: Milton Roy Company
• Inventors: Holmes, Robert B.
• Primary Examiner(s): Hix, L. T.
• Assistant Examiner(s): Gray, David M.
• Application Number:
  US06/656,058
• Time in Patent Office: 788 Days
• Field of Search: 340/347 NT, 340/347 CC, 340/347 AD
• US Class Current: 341/120
• CPC Class Codes: H03M 1/1235 Non-linear conversion not otherwise provided for in subgroups of H03M1/12

Abstract

A method and apparatus for converting an analog input signal to a digital output signal equal to the logarithm of the ratio of the input signal to a reference signal includes a method and apparatus for counting the cycles from a fixed frequency source for the time required for a capacitor to discharge through a resistor from a reference voltage to a calibration voltage; storing the result; counting the cycles from the same fixed frequency source for the time required for the capacitor to discharge through the resistor from the reference signal to the input signal and dividing the result by the stored result.

Claims

1. A method for converting an analog input signal to a digital output signal equal to the logarithm of the ratio of said input signal to a reference signal comprising the steps of:

counting the cycles from a fixed frequency source for the time it takes to discharge a capacitor through a resistor from the reference signal to a calibration signal;

storing the result;

counting the cycles from said fixed frequency source for the time it takes to discharge said capacitor through said resistor from the reference signal to said input signal;

dividing the result by said stored result.

2. The method of claim 1 further comprising the step of setting said calibration signal equal to 0.1 times said reference signal.

3. A method for converting an analog input signal to a digital output signal equal to the logarithm of the ratio of said input signal to a reference signal comprising the steps of:

counting the cycles from a fixed frequency source for the time it takes to dischrge a capacitor through a resistor from the reference signal to a first calibration signal;

storing the result;

counting the cycles from a fixed frequency source for the time it takes to discharge a capacitor through a resistor from the reference signal to a second calibration signal;

subtracting the second calibration signal from the stored result and replacing said stored result with the difference;

counting the cycles from said fixed frequency source for the time it takes to discharge said capacitor through said resistor from the reference signal to said input signal;

dividing the result by said stored difference.

4. The method of claim 3 wherein said first calibration signal and said second calibration signal are related by the ratio 10:

1.

5. Apparatus for converting an analog input signal to a digital output signal equal to the logarithm of the ratio of said input signal to a reference signal comprising:

means for counting the cycles from a fixed frequency source for the time it takes to discharge a capacitor through a resistor from the reference signal to a calibration signal;

means for storing the result;

means for counting the cycles from a fixed frequency source for the time it takes to discharge said capacitor through said resistor from the reference signal to the input signal; and

means for dividing the result by the stored result.

6. A logarthmic analog to digital converter for producing a digital output signal equal to the logarithm of the ratio of an input signal to a reference voltage comprising:

a voltage comparator having first and second inputs and an output;

a voltage reference source;

a calibrating voltage source;

first switch means connected to said first input operative to selectively connect said first input to said input signal or said calibrating voltage source;

a capacitor;

a resistor connected to said second input;

second switch means operative to selectively connect said capacitor to said voltage reference source to charge said capacitor or to said resistor to discharge said capacitor;

fixed frequency oscillator means for producing output pulses;

counter means responsive to said oscillator means to provide a digital output equal to total the number of pulses applied to said counter means;

gate means coupled between said oscillator means and said counter, said gate means connected to the output of said voltage comparator;

control means connected to said first and second switches for sequentially connecting said capacitor to said voltage reference source during a charging period and to said resistor during a discharge period said control means connected to said counter to reset said counter prior to the beginning of said second measurement period;

said control means operative to sequentially connect said first input to said input signal during a measurement period and to said calibrating voltage source during a calibration period;

digital storage means connected to said counter means for storing the digital output produced by said counter during said calibration period;

arithmetic processing means connected to said counter and said storage means for dividing the output produced by said counter during said measurement period by the output stored during said calibration period.

7. The apparatus of claim 6 wherein said calibrating voltage source comprises a resistive voltage divider connected to said voltage reference source.

8. The apparatus of claim 7 wherein said calibrating voltage source is adapted to provide a calibrating voltage equal to 0.1 times said voltage reference source.

9. The apparatus of claim 6 further comprising means for sensing the offset voltage of said comparator during said discharge period and subtracting said offset voltage from said input signal during said measurement period and from said calibrating voltage during said calibration period.

10. In a logarithmic analog to digital converter of the type including means for charging a capacitor to a reference voltage;

means for exponentially discharging said capacitor through a resistor;

means for continuously comparing one of a plurality of input signals including an analog input signal and at least one calibration input signal to the instantaneous voltage on said capacitor;

oscillator means and counter means for producing a count equal to the number of cycles of said oscillator during the time required for the voltage on said capacitor to decay from said reference voltage to the value of said input signal, said number of cycles being a function of the logarithm of the ratio of said reference voltage to said input signal multiplied by a constant determined by the value of said capacitor, the value of said resistor and the frequency of said oscillator, the improvement comprising;

means for sequentially determining the counts for said analog input signal and said at least one calibration input signal;

means for dividing the count for said analog input signal by the count for said at least one calibration input signal, the result being a function of the logarithm of the ratio of said analog input signal to reference voltage, and independent of the values of said capacitor, said resistor and the frequency of said oscillator.

11. The converter of claim 10 wherein said at least one calibration input signal equals 0.1 times said reference voltage.

12. Apparatus for producing a digital output signal equal to the logarithm of the ratio of an analog signal to a reference signal comprising:

converter means responsive to an input signal and said reference signal for providing an intermediate digital output signal equal to a constant multiplied by the logarithm of the ratio of said input signal to said reference signal;

means for producing a calibration signal;

means for sequentially connecting said converter means to said analog signal and said calibration signal;

storage means for storing the intermediate digital output signals corresponding to said analog signal and said calibration signal;

dividing means for dividing said intermediate digital output signals to produce said digital output signal that is a function only of said logarithm of the ratio of said analog signal to said reference signal.

13. The apparatus of claim 12 wherein said calibration signal equals 0.1 times said reference signal.