A variety of types of sensors exist for application in a wide variety of situations. Among these sensors are, for example, photodetectors/photosensors, infrared sensors, laser sensors, microwave sensors, proximity sensors, ultrasonic sensors, inductive sensors, magnetic sensors, among others. Many of these sensors operate by sensing/receiving analog signal inputs. The sensors in turn typically process these analog signal inputs in various ways.
In particular with respect to photodetectors, for example, such devices are employed in a wide variety of applications for a wide variety of purposes. In some embodiments, a light signal is provided by a light emitting device at one position and a photodetector is employed at another position to detect whether that light signal has been interrupted or not, either because the light signal is being turned on and off or because something has cut or interrupted the light path between the light emitting device and the photosensitive device. Photodetectors implemented in this manner can be utilized in a variety of applications such as industrial conveyor systems, in which it typically is necessary to detect whether items being conveyed have passed into or left a given region along the conveyor system, or in industrial systems that are designed to determine whether particular conditions are or are not met (e.g., light curtains).
In many applications, information is conveyed from a light emitting device to a photodetector by rapidly switching or pulsing the light emitting device on and off so. Depending upon the circumstance, this pulsed signal can take the form of a square wave, the form of an AC (or effectively-AC) signal, or some other form. Based upon the frequency of the pulsing, the duration of the pulses, the magnitude of the pulses, the duty cycle, and a variety of other factors (e.g., possibly, the color of the light being transmitted), a variety of information can be transmitted to the photodetector. The coding of this information can involve, for example, amplitude-modulation, frequency-modulation, phase-modulation, polarity-modulation.
Due to the many uses of photodetector circuits, such circuits have become ubiquitous. To reduce the circuits' size and cost, the circuits have increasingly been implemented in the form of integrated circuits rather than out of discrete components. Despite such size and cost improvements, however, conventional photodetector circuits nevertheless suffer from certain inadequacies. First, to the extent that the pulsed or AC information received by the photodetector contains information that is of interest, it is necessary that the AC information be recoverable. Yet conventional recovery circuits, such as conventional rectification or peak detection circuits, typically utilize diodes or transistors that have significant forward-conductive voltage drops (e.g., 0.7 Volts) across them. Consequently, the resulting signals output by those recovery circuits include an undesirable offset. Further, to the extent that such recovery circuits provide an output signal that represents both the positive (e.g., positive with respect to a neutral level of the AC signal) and the negative (e.g., negative with respect to the neutral level) swings of the received signal, discontinuities are created at the cross-over points between the positive and negative portions of the output signal as a result of the forward-conductive voltage drops.
Additionally, regardless of the aforementioned issues relating to the forward-conductive voltage drops within recovery circuits, conventional photodetectors have additional inadequacies. In particular, it is common that the AC signals received by photodetectors include a DC offset. This offset, which can be magnified during propagation within the photodetector circuit, can significantly distort the resulting output signal. Although some conventional photodetector circuits employ DC offset removal circuitry to address this problem, conventional removal circuitry typically involves the use of bypassing or decoupling capacitors that are too large for practical implementation on integrated circuits. Consequently, conventional photodetector circuits having DC offset removal circuitry, when implemented on integrated circuits, typically require discrete capacitors coupled to the integrated circuits. The use of these discrete capacitors increases manufacturing costs and can impact robustness.
Further, to the extent that any DC offset may have been introduced into the signal received by the photodetector circuit itself rather than introduced as part of the input to the photodetector circuit, conventional DC offset removal circuitry fails to eliminate such DC offsets. Thus, even though conventional DC offset removal circuitry does ameliorate the DC offset problem (albeit through the use of discrete capacitors), such conventional circuitry cannot by its nature eliminate all DC offsets.
Still another disadvantage associated with conventional photodetector circuits generally is that it can be relatively difficult in practice for technicians to calibrate the circuits. Photodetector circuits commonly are implemented in situations where it is important that the circuits be capable of differentiating between high and low levels of light corresponding effectively to “on” or “off”. During setup of the photodetector circuits, the circuits are exposed to levels of light intended to be representative of levels that are likely to be experienced in practice, and the gain or amplification of the circuits is then adjusted/calibrated so as to arrive at an output signal that is representative of the light exposure. The calibration process should result in an amplification level that provides a strong output signal but at the same time does not excessively exaggerate unwanted signal components, particularly noise.
A common conventional practice for conducting this calibration is for a technician to hold down a button for a specific period of time during the calibration process to, where the period of time determines the eventual amount of gain. For example, by holding down the button for an amount of time lower than a threshold, the amplification might be set to one level and, by holding down the button for an amount of time higher than the threshold, the amplification might be set to a second, different level. While this procedure has been used in practice, the procedure has proven to be somewhat unreliable, since the amount of gain is dependent upon the skill of the technician performing the adjustment, for example, upon the ability of the technician to hold down the button for an appropriate amount of time. As a result, it is sometimes if not often difficult to achieve consistency in the calibration of photodetectors, particularly insofar as calibrations can be performed differently by different technicians.
In view of the above, it would be advantageous if a new photodetector could be developed that addressed one or more of the inadequacies associated with conventional photodetectors. In particular, it would be advantageous if a new photodetector circuit could have an AC recovery circuit that successfully recovered AC information from an introduced signal without introducing significant distortions into that information due to diode-type voltage drops within the AC recovery circuit. It also would be advantageous if a new photodetector circuit could be designed that was capable of lessening or entirely eliminating DC offsets introduced to the photodetector circuit in the signals input thereto, where such DC offset removal circuitry could be more easily implemented on integrated circuits without the use of large, discrete capacitor components. It further would be advantageous if such DC offset removal circuitry not only served to reduce or eliminate DC offsets introduced by the signals input to the photodetector circuits, but also served to reduce or eliminate additional DC offsets introduced by internal operation of the photodetector circuits themselves. It additionally would be advantageous if the calibration process of photodetector circuits could be improved to reduce the difficulty with which technicians perform the process and improve the repeatability of the calibration process. It would likewise be advantageous if similar deficiencies to those discussed above with respect to photodetectors found in other types of sensors could similarly be ameliorated or eliminated.