Radiant energy – Photocells; circuits and apparatus – Photocell controlled circuit
Reexamination Certificate
1998-09-29
2001-03-20
Le, Que T. (Department: 2878)
Radiant energy
Photocells; circuits and apparatus
Photocell controlled circuit
C250S208100
Reexamination Certificate
active
06204496
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to focal plane readout unit cells, and particularly to techniques for sensing induced pulses and diverting them from a unit cell's signal path.
2. Description of the Related Art
A focal plane array is made from an array of photodetectors that are coupled to respective “readout unit cells”. The unit cells integrate the photocurrent produced by their respective photodetector over a specific integration period, and the integrated currents are multiplexed and amplified to produce a single video output.
One problem faced by focal plane arrays, particularly infrared arrays operating in the presence of nuclear radiation, for example, is that of ionizing radiation which is absorbed by the photodetectors. The radiation, which can include beta (&bgr;) and gamma (&ggr;) rays, deposits a charge packet on the photodetector's p-n junction capacitance. The charge packet causes a voltage step to appear across the junction, inducing extremely short pulses or spikes in the photocurrent output. In a typical application, as many as 4-5 pulses can be induced in the course of a typical integration period (~8 ms), which introduce undesirable noise into the unit cell's signal path.
Efforts have been made to reduce or eliminate the negative impact of such induced pulses. One method is referred to as “sub-frame averaging”, and is described in U.S. Pat. No. 5,119,321 to Burton, Jr. et al., for example. Many frames of video data are taken within the integration period, and signal processing is used to eliminate the frames that are corrupted by noise pulses. However, detecting corruption due to noise pulses is difficult for the small but non-negligible pulses typically induced by radiation absorption. Also, the elimination of corrupted video data makes it difficult or impossible to preserve the systems' radiometric calibration. Nor can the circuitry required to perform sub-frame averaging be practically realized within the area of a unit cell, arrays of which are typically integrated together on a common substrate.
A need exists for a circuit or method that reduces or eliminates the negative impact of pulses induced in a photodetector's photocurrent output, which can be realized within the area allotted a unit cell.
SUMMARY OF THE INVENTION
An induced pulse diversion circuit is presented which senses induced pulses in an input signal and diverts them from a main signal path. The circuit is particularly well-suited for removing &bgr; and &ggr; pulses induced into the photocurrent input to a focal plane readout unit cell.
The pulse diversion circuit includes a diversion control circuit which, in a typical application, receives an input signal that includes a constant or slowly varying photocurrent. The diversion control circuit monitors the input signal for the presence of induced pulses. When no pulses are sensed, nearly all of the input is directed to the signal path, where it is integrated and processed. However, upon sensing a pulse, the diversion control circuit diverts the input signal to the diversion path. The photocurrent input is directed back onto the signal path once the induced pulse has been removed from the signal path.
In a preferred embodiment, a pair of gate devices, preferably MOSFETS, control access to the signal and diversion paths, respectively. The diversion control circuit includes a pass transistor which feeds the input signal to both gate devices. The diversion control circuit also includes an inverting amplifier which is driven by the input signal, with the inverted output connected to the control inputs of both the pass transistor and the MOSFET gate device controlling access to the diversion path.
The inverting amplifier is arranged such that a positive pulse produces a negative-going output which, when sufficiently negative, turns on the diversion path's gate device and turns off the signal path's gate device. The pulse is thus directed onto the diversion path. After the pulse has been diverted, the inverting amplifier's output rises, terminating access to the diversion path and restoring access to the signal path.
The inverting amplifier's negative-going output is also coupled through the pass transistor back to the amplifier's input, and the resulting positive output voltage can turn off the pass transistor. If the amplifier is slow to recover, the turn-off time becomes unacceptably long and “toggling” can occur, i.e., entire frames of video data are lost, causing a displayed image to toggle on and off. The invention controls the occurrence of toggling by limiting the negative-going transitions of the inverting amplifier so that it is prevented from being driven into saturation.
The relatively few components needed to implement the invention fit within the area allotted to an integrated unit cell. Arrays of such cells are integrated together to form a focal plane array.
Further features and advantages of the invention will be apparent to those skilled in the art from the following detailed description, taken together with the accompanying drawings.
REFERENCES:
patent: 5119321 (1992-06-01), Burton, Jr. et al.
patent: 5436451 (1995-07-01), Silver et al.
Levy Miguel E.
Madajian David S.
Le Que T.
Lenzen, Jr. Glenn H.
Raytheon Company
Schubert William C.
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