Focal plane readout unit cell background suppression circuit and method
First Claim
1. A background suppression circuit suitable for use with a focal plane readout unit cell, comprising:
- an integrating capacitor arranged to receive a first current for a predetermined integration period and to store the charge developed from said first current, said capacitor having a maximum charge capacity,a transistor having a current circuit connected to said capacitor and a control input connected to receive a skimming pulse at least once per integration period, said transistor reducing the quantity of charge stored on said capacitor when said skimming pulse is received, anda pulse generation circuit arranged to generate at least one of said skimming pulses per integration period, said at least one skimming pulse having a duration substantially shorter than said integration period and an amplitude sufficient to turn said transistor on,said reduction of stored charge enabling an increased quantity of said first current to be integrated during said integration period when said first current would otherwise exceed the charge capacity of said capacitor.
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Accused Products
Abstract
A background suppression technique uses well-controlled and repeatable charge skimming operations to increase the charge capacities of the integration capacitors of integrated focal plane readout unit cells. A MOSFET (Q1) is connected to an integration capacitor (Cint) from which the quantity of stored charge is to be reduced. During each photocurrent integration period, the MOSFET is driven with a "skimming pulse" (Vsk) to draw charge from the capacitor. The skimming pulse is substantially shorter than an integration period, reducing the amount of noise contributed by the MOSFET'"'"'s noise mechanisms, and has an amplitude great enough to drive the MOSFET into its strong inversion mode, making the quantity of the removed charge relatively insensitive to variations in MOSFET threshold voltage. The charge skimming pulse is arranged to reduce the charge on the capacitor almost, but not quite, to zero, so that the entire integration period remains utilized.
116 Citations
30 Claims
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1. A background suppression circuit suitable for use with a focal plane readout unit cell, comprising:
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an integrating capacitor arranged to receive a first current for a predetermined integration period and to store the charge developed from said first current, said capacitor having a maximum charge capacity, a transistor having a current circuit connected to said capacitor and a control input connected to receive a skimming pulse at least once per integration period, said transistor reducing the quantity of charge stored on said capacitor when said skimming pulse is received, and a pulse generation circuit arranged to generate at least one of said skimming pulses per integration period, said at least one skimming pulse having a duration substantially shorter than said integration period and an amplitude sufficient to turn said transistor on, said reduction of stored charge enabling an increased quantity of said first current to be integrated during said integration period when said first current would otherwise exceed the charge capacity of said capacitor. - View Dependent Claims (2, 3)
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4. A background suppression circuit suitable for use with an integrated focal plane readout unit cell, comprising:
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an integrating capacitor arranged to receive a first current representing a photocurrent for a predetermined integration period and to store the charge developed from said first current, said capacitor having a maximum charge capacity, a field-effect transistor (FET) having a current circuit connected to said capacitor and a control input connected to receive a skimming pulse at least once per integration period, said FET operating as a controlled current source which reduces the quantity of charge stored on said capacitor by a controlled amount when said skimming pulse is received, and a pulse generation circuit arranged to generate at least one of said skimming pulses per integration period, said at least one skimming pulse having a duration substantially shorter than said integration period and an amplitude sufficient to drive said FET into its strong inversion mode, said capacitor, FET and pulse generation circuit integrated together on a common substrate, said reduction of stored charge enabling an increased quantity of photocurrent to be integrated during said integration period when said photocurrent would otherwise exceed the charge capacity of said capacitor. - View Dependent Claims (5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18)
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19. A focal plane readout unit cell with a background suppression circuit, comprising:
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a photodetector interface circuit connected to receive a photocurrent at an input and to produce a first current at an output, an integrating capacitor arranged to receive said first current for a predetermined integration period and to store the charge developed from said first current, said capacitor having a maximum charge capacity, a field-effect transistor (FET) having a current circuit connected to said capacitor and a control input connected to receive a skimming pulse, said FET operating as a controlled current source which reduces the quantity of charge stored on said capacitor by a controlled amount when said skimming pulse is received, and a pulse generation circuit arranged to generate at least one of said skimming pulses per integration period, said at least one skimming pulse having a duration substantially shorter than said integration period and an amplitude sufficient to drive said FET into its strong inversion mode, said photodetector interface circuit, integrating capacitor, FET and pulse generation circuit integrated together on a common substrate, said reduction of stored charge enabling an increased quantity of photocurrent to be integrated during said integration period when said photocurrent would otherwise exceed the charge capacity of said integrating capacitor. - View Dependent Claims (20, 21)
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22. An integrated focal plane array with improved background suppression, comprising:
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a plurality of focal plane readout unit cells arranged into a row and column array, each unit cell comprising; a photodetector interface circuit connected to receive a photocurrent at an input and to produce a first current at an output, an integrating capacitor arranged to receive said first current for a predetermined integration period and to store the charge developed from said first current, said capacitor having a maximum charge capacity, and a field-effect transistor (FET) having a current circuit connected to said capacitor and a control input connected to receive a skimming pulse, said FET operating as a controlled current source which reduces the quantity of charge stored on said capacitor by a controlled amount when said skimming pulse is received; clock generation circuitry arranged to generate at least one of said skimming pulses per integration period for each of said FETs in said array, each skimming pulse having a duration substantially shorter than said integration period and an amplitude sufficient to drive the receiving FET into its strong inversion mode, and a plurality of column multiplexers connected to respective columns of said unit cells for assembling the charges from said array of unit cells into a video frame, said unit cells, clock generation circuitry and column multiplexers integrated together on a common substrate. - View Dependent Claims (23, 24, 25)
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26. A method of removing charge from the integrating capacitor of a focal plane readout unit cell to allow said capacitor to accommodate a higher maximum photocurrent, comprising the steps of:
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integrating a photocurrent on an integrating capacitor over an integration period, said integrating capacitor having a maximum charge capacity, and drawing current from said capacitor during said integration period to reduce the quantity of charge stored on said capacitor, said current drawn from said capacitor for a period substantially shorter than said integration period, said charge reduction enabling an increased quantity of photocurrent to be integrated during said integration period when said photocurrent would otherwise exceed the charge capacity of said capacitor. - View Dependent Claims (27, 28, 29, 30)
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Specification