A/D photodiode signal conversion apparatus

Claim 2

A photodiode signal conversion circuit comprising: a source of circuit ground; a photodiode having an anode and a cathode, the anode being connected to said source of circuit ground; a plurality of capacitors, each having first and second plates, and having capacitance values of a predetermined relationship; a sense node; means connecting the first plates of said plurality of capacitors to said sense node; a reference voltage source; switching means for selectively connecting the second plates of said plurality of capacitors to either said source of circuit ground or said reference voltage source; charging means operable to alternately charge said sense node to a threshold voltage and then allow said sense node to electrically float for a first time interval; comparator means connected to said sense node, said comparator means having an output terminal at which is produced a characteristic signal if the voltage on said sense node is larger than the threshold voltage; control means responsive to the output signal of said comparator means to supply control signals to said switching means, said control means being operable during the time said sense node is being charged to the threshold voltage to connect the second plates of said plurality of capacitors to said source of circuit ground, said control means further being operable after a predetermined delay during the first time interval to sequentially connect the second plates of said plurality of capacitors to said reference voltage source, and, if connection of the second plate of any one of said plurality of capacitors in production of a characteristic signal by said comparator, to reconnect the second plate of that capacitor to the source of circuit ground; and digital output means responsive to the state of said switching means to produce a digital representation thereof, the digital representation being indicative of the light intensity at said photodiode.

Claim 3

The conversion circuit of claim 1 wherein: the capacitance values of the capacitors in said plurality of capacitors are weighted in a binary relationship; and said control means is operable to sequentially connect the second plates of the capacitors to said reference voltage source in descending order of capacitance values.

Claim 4

The conversion circuit of claim 2 wherein: said comparator means comprises an inverter; and said charging means comprises a switch operable to periodically electrically connect the output terminal of said comparator means to an input terminal thereof.

Claim 5

The conversion circuit of claim 3 wherein said means connecting the first plates of said plurality of capacitors to said sense node comprises a coupling capacitor.

Claim 6

An optical image sensing system providing direct conversion of photodiode sensing element output signals to digital form, comprising: a sense node; a source of circuit ground; an array of photodiodes, each having an anode and a cathode, the anodes being connected to said source of circuit ground; a plurality of capacitors, each having first and second plates, and having capacitance values of a predetermined relationship; means connecting the first plates of said plurality of capacitors to said sense node; a reference voltage source; a multiplexer operable to sequentially connect the cathodes of the photodiodes in said array to said sense node; switching means for selectively connecting the second plates of said array of capacitors to either said source of circuit ground or said reference voltage source; charging means operable to alternately charge said sense node to a threshold voltage and then allow said sense node to electrically float for a first time interval; comparator means connected to said sense node, said comparator means having an output terminal at which is produced a characteristic signal if the voltage on said sense node is larger than the threshold voltage; control means responsive to the signal produced at the output terminal of said comparator means to supply control signals to said switching means, said control means being operable during the time said sense node is being charged to the threshold voltage to connect the second plates of said plurality of capacitors to said source of circuit ground, said control means further being operable after a predetermined delay during the first time interval to sequentially connect the second plates of said plurality of capacitors to said reference voltage source, said control means being responsive to production of a characteristic signal by said comparator means to switch the second plate of said capacitor from said reference voltage source to said source of circuit ground; and digital output means responsive to the state of said switching means to produce a digital representation thereof, the state of said switching means being indicative of the light intensity at the photodiode connected to said sense node.

Claim 7

An optical image sensing system providing direct conversion of photodiode sensing element output signals to digital form, comprising: a sense node; a source of circuit ground; an array of photodiodes, each having an anode and a cathode, the anodes being connected to said source of circuit ground; a plurality of capacitors, each having first and second plates, and having capacitance values of a predetermined relationship; means connecting the first plates of said plurality of capacitors to said sense node; a reference voltage source; a multiplexer operable to sequentially connect the cathodes of the photodiodes in said array to said sense node; switching means for selectively connecting the second plates of said array of capacitors to either said source of circuit ground or said reference voltage source; charging means operable to alternately charge said sense node to a threshold voltage and then allow said sense node to electrically float for a first time interval; comparator means connected to said sense node, said comparator means having an output terminal at which is produced a characteristic signal if the voltage on said sense node is larger than the threshold voltage; control means responsive to the signal produced at the output terminal of said comparator means to supply control signals to said switching means, said control means being operable during the time said sense node is being charged to the threshold voltage to connect the second plates of said plurality of capacitors to said source of circuit ground, said control means further being operable after a predetermined delay during the first time interval to sequentially connect the second plates of said plurality of capacitors to said reference voltage source, said control means being responsive to production of a characteristic signal by said comparator means to switch the second plate of said capacitor from said reference voltage source to said source of circuit ground; and digital output means responsive to the state of said switching means to produce a digital representation thereof, the state of said switching means being indicative of the light intensity at the photodiode connected to said sense node.

Claim 8

The optical image sensing system of claim 5 wherein: said comparator means comprises an inverter; and said charging means comprises a switch operable to periodically electrically connect the output terminal of said comparator means to an input terminal thereof.

Claim 9

The optical image sensing system of claim 6 wherein: the capacitance values of the capacitors in said plurality of capacitors are weighted in a binary relationship; and said control means is operable to sequentially connect the second plates of the capacitors to said reference voltage source in descending order of capacitance values.

Claim 10

The optical image sensing system of claim 7 wherein said means connecting the first plates of said plurality of capacitors to said sense node comprises a coupling capacitor.

Claim 13

The method of claim 9 wherein: the capacitance values of the plurality of capacitors are weighted in a binary relationship; and the step of sequentially increasing the voltage on the second plates of the capacitors is done in descending order of capacitance values.