Analog to digital converter

An analog to digital converter comprising:

• charge storage means,
• means for selectively applying a first reference current in a first sense to said storage means in a first time interval, the charge in said storage means at the end of said first time interval having a first value,
• means for selectively applying a second reference current in a second sense to said storage means for a second time interval determined by the time it takes the charge in said storage means to shift from said first value to a reference value,
• means for selectively applying a third current in said first sense to said storage means in a third time interval the same in duration as said first time interval, said third current being proportional to an analog signal having a value to be determined, the charge in said storage means at the end of said third interval having a second value,
• means for selectively applying said second reference current in said second sense to said storage means for a fourth time interval determined by the time it takes the charge in said storage means to change from said second value to said reference value, and
• interval comparing means including means for generating first and second signals respectively representing the periods of said second and fourth intervals and for comparing the first and second signals to produce as an output therefrom a digital representation of said compared signals.

In combination:

• a source of a first reference current,
• a source of a second reference current,
• means responsive to an analog signal applied as an input thereto for generating a third current proportional to the amplitude of said analog signal,
• charge storage means,
• switch means for selectively coupling said first, second and third currents to said storage means, said first and third currents being applied to said storage means in a first sense and said second current being applied to said storage means in a second sense,
• comparison means coupled to said charge storage means for producing an output signal therefrom manifesting the magnitude of the charge in said storage means,
• control means responsive to said comparison means output signal for producing a switch means control signal for sequentially connecting initially said first and then said second current sources to said storage means and then initially said third current generating means and then said second current source to said storage means, the time interval during which said first and third current sources are connected being the same, the time intervals during which said second current source is connected being determined according to the magnitude of the charge in said storage means, said control means producing a timing signal representing the period of the respective time intervals during which said second current source is connected to said storage means, the difference between said second current time interval periods manifesting the amplitude of said analog signal, and
• interval comparing means responsive to said timing signal to produce a digital signal representing the amplitude of said analog signal.

An analog to digital converter comprising:

• means for producing first and second reference currents,
• means for producing a third current the magnitude of which represents the magnitude of a signal to be measured,
• means for producing a first timing signal representing the time it takes said second current to reach an accumulated energy level by corresponding to the energy level of said first current accumulated in a given time interval,
• means for producing a second timing signal representing the time it takes said second current to reach an accumulated energy level corresponding to the accumulated energy level achieved by said third current in a time interval the same in duration as said given time interval, and
• means for producing a digital output signal representing the difference in duration between said first and second timing signals.

In combination:

• reactance means for storing energy,
• a first charging circuit coupled to said reactance means for charging said reactance means with a first reference current for a given time interval,
• a second charging circuit coupled to said reactance means for charging said reactance means with a current proportional to an input signal to be measured for a time interval the same as said given time interval,
• means including a discharge circuit coupled to said reactance means for withdrawing energy at a substantially constant rate from said reactance means until a predetermined energy level in said reactance means is reached,
• means including timing means for coupling said discharge circuit means to said reactance means following said given and said same time intervals, and
• digital signal generating means coupled to said discharge circuit for producing a digital signal representing the difference in durations of the time intervals for withdrawing said energy from said reactance means.

A method employing a charge storage device for determining the value of an analog current level I.sub.x comprising the steps of:

• applying to said storage device a charging current of known value for a time interval t.sub.a after the voltage across said device is at a first level, whereby during said interval said voltage across said storage device changes to a second voltage level;
• measuring the time t.sub.1 required for the voltage across the storage device to change from the second back to the first level when discharged at a known rate I.sub.D ;
• supplying said analog current I.sub.x to said storage device as a charging current for said interval t.sub.a measured from the time the voltage across it is at said first level, whereby the voltage across said device attains a level V.sub.x ;
• measuring the time t.sub.2 required for the voltage across said storage device to change from said level V.sub.x to said first level when discharged at said known rate I.sub.D ;
• and
• subtracting t.sub.1 from t.sub.2 to obtain an indication of the value of I.sub.x.