Miscellaneous active electrical nonlinear devices – circuits – and – Specific signal discriminating without subsequent control – By amplitude
Patent
1992-07-29
1996-04-09
Callahan, Timothy P.
Miscellaneous active electrical nonlinear devices, circuits, and
Specific signal discriminating without subsequent control
By amplitude
327 73, 327 94, 327362, 327563, G11C 2702, G06G 712
Patent
active
055065252
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to sample-and-hold circuit for analog electrical signals.
One example of an application for which the present invention is particularly suited is the processing of analog electrical signals derived from photosensitive sensors of the charge-transfer type, or other charge-transfer devices.
2. Discussion of the Background
Although the invention applies in other cases, the example of signals derived from charge-transfer photosensitive sensors will be given.
The constraints in processing these signals are speed and precision of measurement of the signals from these sensors.
The signals are presented in the form of successive voltage square waves separated by a plateaus at a reference voltage level. This corresponds, in fact, to the fact that the signals result from the conversion of packets of photosensitive charges transported by shift registers. The voltage level of one square wave corresponds to a quantity of charges which represents the illumination of a point. The voltage level of a following square wave represents the illumination of the following point. The voltage level of the plateau which separates them corresponds to a black level.
A black level may vary slowly in the course of an analysis of an image, but it is always necessary for the useful signal level of the square waves to be referenced with respect to this black level.
In order to be able to effect processing of the received image point by point, it proves to be necessary to sample and hold the signal level of each square wave for as long a time as possible. That is to say, in practice, during the whole period separating one square wave from the next one. In fact, in order to rapidly process numerous points, it is necessary for them to succeed each other at a high rate, and thus a very short time is available for processing each signal. The processing is, for example, analog-digital conversion.
Hence the signal processing constraints are, on the one hand, the sampling and the holding of the signal over one complete period, and, on the other hand, the establishing of a fixed reference voltage level of the sampled-and-held output signal, knowing that the reference level of the input signal can vary. The other constraints are speed of processing and precision.
FIG. 1 represents, by way of example, the input signal S1 in the form of square waves with the reference plateaus (level Vr1 susceptible to drifting in the course of time) and the useful signal square waves V1, the useful signal value being the difference V1-Vr1 between the level of the square wave and the level of the plateau just before or just after. FIG. 1 also represents the desired output signal S2, namely a signal which is sampled and held during a time interval equal to that which separates two successive square waves of the input signal. The signal S2 is a succession of staircase steps of variable levels V2 which are referenced with respect to a fixed reference voltage V0. Each step level V2 taken with respect to this reference level represents the useful signal level V1 taken with respect to the plateau Vr1. It is assumed, for simplicity's sake, that the sampler which establishes S2 has unity gain.
The conventional solutions used to produce this sampling are the following: the signal input can be applied to one input of a differential amplifier, and applied also to another input of the amplifier, but with a delay corresponding to one input signal half-period (delay produced by a delay line). In this way, one of the inputs receives a useful signal level V1 while the other receives the immediately preceding or following plateau level Vr1. The output of the amplifier is applied to a sample-and-hold device whose reference is the voltage V0. The sample-and-hold device maintains a level V2-V0 which is equal to V1-Vr1 from one sampling instant up to the following sampling.
This circuit operates only on condition that the square waves have very good frequency constancy, so that the delay introduced by the delay l
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Callahan Timothy P.
Phan Trong
Thomson Composants Militaires et Spatiaux
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