Charge detector with long integration time

• US 5,920,199 A
• Filed: 11/21/1997
• Issued: 07/06/1999
• Est. Priority Date: 04/23/1996
• Status: Expired due to Fees

• Current Assignee: Sarnoff Corporation (SRI International, Inc.)
• Original Assignee: Sarnoff Corporation (SRI International, Inc.)
• Inventors: Sauer, Donald Jon
• Primary Examiner(s): BROWN, GLENN WARNER
• Application Number:
  US08/975,750
• Time in Patent Office: 592 Days
• Field of Search: 324/452, 324/457, 324/613, 324/678, 324/72
• US Class Current: 324/678
• CPC Class Codes: G01R 29/24 Arrangements for measuring quantities of charge

Abstract

The invention relates to an apparatus and method for detecting electrical charge with a long integration time and in particular to a sampling method which reduces noise that affects the accuracy of the measurement of the total charge. The apparatus samples the charge on the capacitor at the start of the integration period to obtain a sample proportional to a first noise component. It then samples the charge on the capacitor at the end of an integration period and subtracts the noise component sample from the integrated charge sample to obtain a measure of integrated charge to the relative exclusion of the noise component. The circuit uses a folded cascode amplifier and at least one correlated double sampling circuit. The charge detector can be used with any apparatus which generates electrical charge in response to an input including for example, a photodetector, photomultiplier, ion detector, e beam detector and piezoelectric charge detector and arrays of such devices.

Claims

1. A method of measuring a charge integrated on a sensor during a time interval comprising the steps of:

connecting the sensor to a source of reference potential to generate a measure of a noise component;

storing the measured noise component;

disconnecting the sensor from the source of reference potential;

integrating the charge on the sensor;

connecting the sensor to the source of reference potential to generate a signal representing a measure of the integrated charge contaminated by the noise component; and

subtracting the stored noise component from the generated signal to generate a measure of the charge integrated on the sensor.

2. A method according to claim 1 wherein:

the step of storing the measured noise component includes the steps of;

digitizing the generated measure of the noise component to produce a digital noise value; and

storing the digital noise value; and

the step of subtracting the stored noise component from the generated signal includes the steps of;

digitizing the generated signal to produce a digital signal value; and

subtracting the digital noise value from the digital signal value to produce a digital measure of the charge integrated on the sensor.

3. A method according to claim 1, wherein:

the step of connecting the sensor to the source of reference potential to generate the measure of the noise component includes the steps of;

connecting the sensor to the source of reference potential to generate a first signal including a measure of the reference potential and a measure of a l/f noise component when the sensor is connected to the source of reference potential;

disconnecting the sensor from the source of reference potential to generate a second signal including a measure of the reference potential, a measure of the l/f noise component when the sensor is disconnected from the source of reference potential and a measure of a kTC noise component; and

subtracting the first signal from the second signal to produce a signal representing the kTC noise component to the relative exclusion of the reference potential or any l/f noise component.

4. A method according to claim 3, wherein the step of connecting the sensor to the source of reference potential to generate a signal representing a measure of the integrated charge contaminated by the noise component includes the steps of:

connecting the sensor to the source of reference potential to generate a third signal including the measure of the integrated charge, a measure of the reference potential and a measure of a l/f noise component when the sensor is connected to the source of reference potential;

disconnecting the sensor from the source of reference potential to generate a fourth signal including a measure of the reference potential, a measure of the l/f noise component when the sensor is disconnected from the source of reference potential and a measure of a kTC noise component; and

subtracting the third signal from the fourth signal to produce a signal representing an inverse of a quantity which includes the measure of integrated charge and the kTC noise component to the relative exclusion of the reference potential and any l/f noise component.

5. A method according to claim 4 wherein:

the step of storing the measured noise component includes the steps of;

digitizing the signal representing the kTC noise component to produce a digital noise value; and

storing the digital noise value; and

the step of subtracting the stored noise component from the generated signal includes the steps of;

digitizing the signal representing the inverse of the quantity which includes the measure of integrated charge and the kTC noise component to produce a digital signal value; and

adding the digital noise value to the digital signal value to produce a digital value which represents an inverse of the measure of the charge integrated on the sensor to the relative exclusion of the kTC noise component.

6. Apparatus which measures a charge integrated on a sensor during an integration time interval comprising:

switching means for connecting the sensor to a source of reference potential to generate a measure of a noise component during a first time interval prior to the integration time interval and for connecting the sensor to the source of reference potential to generate a signal representing a measure of the integrated charge contaminated by the noise component during a second time interval, after the integration time interval; and

means for subtracting the measured noise component from the generated signal to produce a measure of the charge integrated on the sensor.

7. Apparatus according to claim 6, wherein:

the switching means includes a correlated double sampling circuit.

8. Apparatus according to claim 7, wherein the switching means includes a folded cascode amplifier coupled between the sensor and the correlated double sampling circuit.

9. Apparatus according to claim 6, wherein the switching means includes a first correlated double sampling circuit which generates the measure of the noise component during the first time interval and a second correlated double sampling circuit which generates the measure of the integrated charge contaminated by the noise component during the second time interval.

10. Apparatus according to claim 9, wherein the switching means further includes:

means for connecting the sensor to the source of reference potential and to the first correlated double sampling circuit to cause the first correlated double sampling circuit to store a first value, including a measure of the reference potential and a measure of a l/f noise component when the sensor is connected to the source of reference potential;

means for disconnecting the source of reference potential from the sensor and the first correlated double sampling circuit to cause the first correlated double sampling circuit to store a second value, including a measure of the reference potential, a measure of the l/f noise component when the sensor is disconnected from the source of reference potential and a measure of a kTC noise component; and

the first correlated double sampling circuit includes means for subtracting the first value from the second value to produce an output value representing the kTC noise component to the relative exclusion of the reference potential or any l/f noise component.

11. Apparatus according to claim 10, wherein the switching means further includes:

a low-pass filter coupled between the sensor and the first correlated double sampling circuit, the low-pass filter being configured to have a narrow-band frequency response characteristic or a wide-band frequency response characteristic responsive to a control signal; and

means for switching the low-pass filter between the narrow-band frequency response characteristic and the wide-band frequency response characteristic at the same time that the source of reference potential is disconnected from the sensor and the first correlated double sampling circuit to allow rapid acquisition of the kTC noise component.

12. Apparatus according to claim 11, wherein the switching means includes a folded cascode amplifier coupled between the sensor and the low-pass filter.

13. Apparatus according to claim 12, wherein the switching means further includes:

means for connecting the sensor to the source of reference potential and to the second correlated double sampling circuit to generate a third value including the measure of the integrated charge, a measure of the reference potential and a measure of a l/f noise component when the sensor is connected to the source of reference potential after the integration time interval;

means for disconnecting the source of reference potential from the sensor to generate a fourth value including a measure of the reference potential, a measure of the l/f noise component when the sensor is disconnected from the source of reference potential after the integration time interval and a measure of the kTC noise component; and

the second correlated double sampling circuit includes means for subtracting the third value from the fourth value to produce an output value representing an inverse of a quantity which includes the measure of integrated charge and the kTC noise component to the relative exclusion of the reference potential and any l/f noise component.

14. Apparatus according to claim 13 wherein:

the switching means further comprises;

a first analog-to-digital converter which digitizes the output value of the first correlated double sampling circuit to produce a digital noise value;

a first register into which the digital noise value is stored;

a second analog-to-digital converter which digitizes the output value of the second correlated double sampling circuit to produce a digital signal value representing the measure of integrated charge, contaminated by the noise component; and

a second register into which the digital signal value is stored; and

the means for subtracting the measured noise component from the generated signal includes an adder having first and second input ports coupled to the first and second registers respectively and having an output port at which is produced a digital measure of the charge integrated on the sensor to the relative exclusion of the kTC noise component.

15. An apparatus for measuring an amount of electric charge integrated on a sensor during a time interval comprising:

the sensor,

a reference potential setting circuit which is electrically coupled to the sensor,

a sampling circuit electrically coupled to the sensor to form successive difference values, each difference value representing a difference between signals provided by the sensor at first and second instants;

at least one analog to digital converter electrically coupled to receive and digitize successive difference values provided by the sampling circuit and to provide successive digitized difference values;

at least one register electrically coupled to the analog to digital converter to hold a first one of the successive digitized difference values, and

a summing circuit coupled to the register to add a subsequent one of the successive digitized difference values to the stored first digitized difference value.

16. Apparatus according to claim 15, wherein:

the sampling circuit is a correlated double sampling circuit which forms a first difference value representing a noise component and a second difference value representing an inverse of a measure of the electric charge integrated on the sensor between the first and second instants contaminated by the noise component; and

summing circuit is an adder which adds the first difference value to the second difference value to produce an output value representing an inverse of the measure of electric charge integrated on the sensor between the first and second instants, to the relative exclusion of the noise component.