Integrating analog-to-digital converter
AssigneeBeckman Instruments, Inc.
InventorJohn B. Crosby
Life of this patent
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Abstract
An integrating analog-to-digital converter for producing a digital output signal representing the value of an analog input signal which may have either a positive or negative polarity. The converter uses a switching circuit to reverse an integrator capacitor between charge and discharge periods so that the same input-signal-controlled current source can be used to both charge and discharge the capacitor. This results in inherent zero calibration because any offset errors during the charge cycle are cancelled out by equal and opposite offsets during the discharge cycle.
Claims32
Apparatus for converting an input signal which is bipolar with respect to a common potential and has an amplitude representing an analog quantity into a digital signal representing the quantity comprising:
- a capacitor having first and second terminals;
- a controlled current source, having an input terminal adapted for receipt of said input signal and having an output terminal, for producing through said output terminal a unipolar current having a value determined by the value of said input signal during a first fixed-duration time period and having a fixed value during a second variable-duration time period;
- first switch means for alternately connecting said current source input terminal to said input signal during a first fixed time period and to said common potential during a second variable time period;
- a source of supply potential;
- second switch means for connecting said source of supply potential and said current source output terminal to said first and second capacitor terminals, respectively, during said first time period and for connecting said source of supply potential and said current source output terminal to said second and first capacitor terminals, respectively, during said second time period, said controlled current source modifying from an initial charge and in a first direction the charge on said capacitor at a rate determined by said input signal during said first time period and modifying the charge on said capacitor in the opposite direction and at a fixed rate during said second time period;
- means responsive to said capacitor for generating a timing signal when the charge thereon returns to said initial charge;
- means for generating regularly recurring timing pulses,
- counting means responsive to said timing pulses for continuously and alternately generating an up count and a down count between predetermined low and high counts;
- timing circuit means responsive to said counting means and said timing signal generating means for operating said first and second switch means, said low and high counts establishing the starts of said first and second time periods, respectively, said timing signal establishing the end of said second time period;
- and
- means responsive to said timing signal generating means for determining the count in said counting means upon the occurrence of said timing signal, said count being representative of said input signal.
An integrating analog-to-digital converter circuit for successively performing conversion operations such that each conversion operation produces a digital output signal which represents the value of an analog input signal, the time occupied by a conversion operation being termed a conversion cycle, comprising:
- a capacitor, having two terminals, across which appears a capacitor voltage;
- a comparator circuit for indicating when the capacitor voltage crosses a threshold;
- a timing circuit for dividing each conversion cycle into at least two time periods, including a first time period which begins after the conversion cycle begins and which has a fixed duration, and a second time period which begins after the first time period ends and which ends when the comparator indicates the capacitor voltage has crossed the threshold;
- a controlled current source for supplying through a first output terminal a unipolar current having a fixed value during one of said first and second time periods and having a value determined by the input signal during the other of said first and second time periods;
- a capacitor switching circuit for connecting the current source output terminal to the first capacitor terminal during said first time period and to the second capacitor terminal during said second time period, whereby the current supplied by the controlled current source modifies the capacitor voltage in one direction at a fixed rate during one of said two time periods, and whereby the current modifies the capacitor voltage in the opposite direction at a rate determined by the input signal during the other of said two time periods;
- and
- an output circuit for producing the digital output signal as a function of the relative durations of the first and second time periods.
An analog-to-digital converter circuit according to claim 17, 19 or 21, wherein each of the four switches comprises a transistor.
Dependencies are inferred from the claim text.