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US 8,283,620 B2Granted
Optical sensor and sensing method with stray illumination compensation
AssigneeSTMicroelectronics (Research & Development) Limited
InventorsJeffrey M. Raynor, David P. Baxter
Life of this patent
Estimating the term…
Abstract
An optical sensor includes an infra red LED, a photodetector and a lock in charge amplifier. The photodetector passes charge to a feedback capacitor of the charge amplifier following an illumination cycle during which the LED is emitting and also following an illumination cycle during which the LED does not emit. An amplifier presents an output indicative of the difference between the charge generated at the photodetector during the illuminated and non-illuminated cycles.
Claims25
A lock in charge amplifier circuit configured to be coupled to an output of a photodetector which generates input charge in response to illumination by an illumination source, the lock in charge amplifier circuit comprising:
- a switch network, a controller, and a charge amplifier, said charge amplifier further comprising a feedback capacitor connected between an input of the charge amplifier and an output of the charge amplifier;
- wherein said controller is configured to provide a control signal to switch the switch network between at least first and second states and to switch on the illumination source during only one of said first and second states;
- said first and second states being operable to apply a first input charge obtained over a first period of time and a second input charge obtained over a second period of time to said feedback capacitor such that after operation of said first and second states a difference between the first and second input charges is stored on the feedback capacitor, the charge amplifier configured to output an output voltage dependent upon the charge stored on the feedback capacitor;
- and
- wherein said first state is operable to apply said first input charge directly to a first plate of said feedback capacitor and said second state is operable to apply said second input charge directly to a second plate of said feedback capacitor.
2 more elements
Apparatus, comprising:
- an illumination source;
- a photodetector;
- a lock in charge amplifier including a feedback capacitor, the amplifier coupled to an output of the photodetector;
- a control circuit configured arranged to turn on the illumination source within a first part of a cycle but not a second part of a cycle;
- and
- a switching circuit configured to selectively pass charge from the illuminated photodetector to the lock-in charge amplifier during a first time period within the first part of the cycle and selectively pass charge from the non-illuminated photodetector to the lock-in charge amplifier during a second time period within the second part of the cycle so that after completion of the second time period a difference between the first time period charge and second time period charge is stored on the feedback capacitor, the charge amplifier configured to output an output voltage dependent upon the difference charge stored on the feedback capacitor;
- wherein the switching circuit is further configured to repeat the cycle including the first and second parts, and
- further comprising an averaging circuit configured to produce an averaged output.
5 more elements
Apparatus, comprising:
- an illumination source;
- a photodetector;
- a lock in charge amplifier including a feedback capacitor, the amplifier coupled to an output of the photodetector;
- a control circuit configured to turn on the illumination source within a first part of a cycle but not a second part of a cycle;
- and
- a switching circuit configured to selectively pass charge from the illuminated photodetector to the lock-in charge amplifier during a first time period within the first part of the cycle and selectively pass charge from the non-illuminated photodetector to the lock-in charge amplifier during a second time period within the second part of the cycle so that after completion of the second time period a difference between the first time period charge and second time period charge is stored on the feedback capacitor, the charge amplifier configured to output an output voltage dependent upon the difference charge stored on the feedback capacitor;
- wherein said switching circuit is coupled at the input of the lock-in charge amplifier and further comprises an additional switch capacitor configured such that said charge is passed from said illuminated and non-illuminated photodetector to the lock in charge amplifier via said additional switch capacitor.
4 more elements
A circuit, comprising:
- a differential amplifier having a first input, a second input and an output, the second input connected to a reference voltage terminal;
- a feedback capacitor connected between the output and the first input;
- a photosensor having an output terminal;
- a switched capacitor having a first plate and a second plate;
- a first switch selectively connecting the first plate to the first input;
- a second switch selectively connecting the first plate to the second input;
- a third switch selectively connecting the photosensor output terminal to the first plate;
- a fourth switch selectively connecting the second plate to the first input;
- a fifth switch selectively connecting the second plate to the second input;
- and
- a control circuit which is configured to control selective actuation of the first through fifth switches, the control circuit having first through fifth control signal outputs for application to control the first through fifth switches in accordance with a multi-phase cycle including:
- a first phase which selectively activates the third and fifth switches;
- a second phase which selectively activates the first and fifth switches;
- a third phase which selectively activates the third and fifth switches;
- and
- a fourth phase which selectively activates the second and fourth switches.
13 more elements
A circuit, comprising:
- a charge amplifier including a feedback capacitor connected between an input of the charge amplifier and an output of the charge amplifier;
- a switch network coupled to the feedback capacitor of the charge amplifier;
- a controller is configured to control operation of the switch network between at least a first state and a second state;
- said first state configured to apply a first input charge obtained over a first period of time directly to a first plate of said feedback capacitor;
- said second state configured to apply a second input charge obtained over a second period of time directly to a second plate of said feedback capacitor;
- wherein a charge difference between the first input charge and second input charge is stored on the feedback capacitor;
- said charge amplifier configured to generate an output voltage dependent upon the charge difference stored on the feedback capacitor;
- wherein a photodetector is configured to supply the first input charge and second input charge.
5 more elements
Dependencies are inferred from the claim text.