Signal processing circuit and method using analog voltage signal to pulse width modulation conversion

A signal processor and processing method are provided for measuring current received from a photo-detector. Generally, the processor includes a transimpedance amplifier (TIA) to integrate a current received from a photo-detector in the optical navigation system to generate a voltage signal having a slope that is proportional to the received current, and a comparator having a first input coupled to an output of the TIA to receive the voltage signal, and a second, inverting, input coupled to a threshold voltage. The comparator is configured to compare the voltage signal to the threshold voltage and to generate an output pulse having a predetermined voltage and a duration or width that is a function of the received current.

TECHNICAL FIELD

The present invention relates generally to signal processing, and more particularly to a signal processing circuit and method for measuring current received from a photo-detector in an optical navigation device.

BACKGROUND OF THE INVENTION

Signal processors are used in a wide range of applications including, for example, measuring a current output from a photo-detector of an array in an optical navigation system. Optical navigation systems, such as an optical computer mouse or trackball, are well known for inputting data into and interfacing with personal computers and workstations. Such devices allow rapid relocation of a cursor on a monitor, and are useful in many text, database and graphical programs. A user controls the cursor, for example, by moving the mouse over a surface to move the cursor in a direction and over distance proportional to the movement of the mouse. Alternatively, movement of the hand over a stationary device may be used for the same purpose.

One embodiment of an optical computer mouse uses a coherent light source, such as a laser, to illuminate a rough surface, and an array of a number of photo-sensors or detectors, such as photodiodes, to receive light scattered from the surface. Light from the coherent source scattered off of the surface generates a random intensity distribution of light known as speckle. The varying intensity of scattered light detected by the photo sensors in the array as the mouse is moved across the surface is used to detect movement of the mouse.

Although a significant improvement over prior art optical mice, these speckle-based devices have not been wholly satisfactory for a number of reasons. In particular, processing signals from the photodiodes involves measuring the current output from the photodiode through an integrating transimpedance amplifier (TIA). The TIA converts current to voltage by producing a voltage output with a slope proportional to the current. Typically, the TIA is either single ended or differential, and is followed by one or more analog-to-digital-converters (ADCs). The disadvantages of this approach using one or more ADCs include a relatively high complexity and high power consumption. In addition, the above approach does not insure that the measurements are made while the TIA is operating continuously in a linear region, and is not saturating before being read by the ADC.

Another problem arises from the fact that the TIA output is reset at the beginning of each sampling period and then allowed to ramp up for a fixed amount of time. At the end of the ramp time, the voltage output is converted to a digital value in the ADC and the next sampling period begins. Ideally, the TIA output should be reset to exactly the same voltage at the beginning of each sampling period. In actuality, there is a variation in the beginning voltage output from the TIA after reset commonly referred to as reset noise. Because the digital value is derived from the output voltage measured at the end of the ramp, this reset noise shows up as noise in the digital value measured, reducing the accuracy of measurement, and therefore the performance of the optical mouse.

One technique for dealing with the above reset noise problem is to measure a digital value at the beginning of the ramp just after reset and then subtract this from the digital value measured at the end of the sampling period, thereby limiting the effects of the reset noise on the measurement. However, this increase in accuracy is accomplished at the expense of requiring twice as many analog to digital conversions, and hence doubling the sampling rate required of the ADCs, and requiring additional digital circuitry to store the beginning voltage value and do the subtraction.

Accordingly, there is a need for a signal processor or circuit and method for measuring current received from a photo-detector in a photo-detector array that uses a circuit having reduced complexity and power consumption. It is further desirable that the circuit and method accomplish this measurement in a way that is substantially independent of and unaffected by reset noise from an integrating transimpedance amplifier output at the beginning of each sample period.

The present invention provides a solution to these and other problems, and offers further advantages over the prior art.

DETAILED DESCRIPTION

The present invention is directed to a signal processor or signal processing circuit and method for measuring current received from a photo-detector. The circuit has reduced complexity and power consumption over conventional signal processing circuits, and is substantially independent of and unaffected by reset noise from an integrating transimpedance amplifier (TIA) at the beginning of each sample period.

The signal processing circuit and method are particularly advantageous for processing signals from a photo-detector, such as a photodiode or other light sensitive element, in a photo-detector array used in an optical navigation system, such as an optical computer mouse or an optical trackball.

For purposes of clarity, many of the details of optical navigation systems in general and signal processing circuits for optical navigation systems in particular that are widely known and are not relevant to the present invention have been omitted from the following description.

The circuit and method will now be described in greater detail with reference toFIGS. 1 to 10B.

In the following description, for purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of the present invention. It will be evident, however, to one skilled in the art that the present invention may be practiced without these specific details. In other instances, well-known circuits, structures, and techniques are not shown in detail or are shown in block diagram form only in order to avoid unnecessarily obscuring an understanding of the invention.

Reference in the description to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the invention. The appearances of the phrase “in one embodiment” in various places in the specification do not necessarily all refer to the same embodiment. The term “to couple” as used herein may include both to directly connect and to indirectly connect through one or more intervening components.

A functional block diagram of one embodiment of an optical navigation system for which the signal processing circuit and method of the present invention is particularly useful is shown inFIG. 1. Generally, the an optical navigation system100includes an illuminator102having a light source104and illumination optics106to illuminate a portion of a surface108, an array110having a number of photo-detectors112, imaging optics114, and a signal processor or signal processing circuit116for combining and processing signals from each one or a combination of the photo-detectors to produce an output signal from the optical navigation system.

Preferably, the photo-detectors112and signal processing circuit116of the optical navigation system100are integrally fabricated using a standard semiconductor fabrication processes. More preferably, the optical navigation system100is a speckle-based optical navigation system. Most preferably, the optical navigation system100is an optically-efficient speckle-based optical navigation system having, for example, structured illumination and telecentric imaging. By speckle it is meant a random intensity distribution of light from a coherent source scattered off of a rough surface to generate an interference pattern known as speckle. Speckle in an interference pattern of light reflected from a rough surface is illustrated inFIG. 2where angle θ202is an angle of incidence with respect to the surface normal of the incident light,204is the scattered light, and206is the speckle pattern of the reflected or scattered light. Speckle-based optical navigation systems are described, for example, in co-pending, commonly assigned U.S. patent application Ser. No. 11/129,967, entitled, “Optical Positioning Device Having Shaped Illumination,” filed on May 16, 2005 by Clinton B. Carlisle et al., and incorporated herein by reference in its entirety.

It has been found that a speckle-based optical navigation system using the signal processing circuit and method of the present invention can meet or exceed all performance criteria typically expected of such systems, including maximum displacement speed, accuracy, and path error rates, while reducing the amount of electrical power dedicated to signal processing and displacement-estimation in the system.

Briefly, the signal processing circuit of the present invention uses transimpedance amplifiers (TIAs) having an internal capacitor or capacitors to integrate current from the photo-detectors to create a voltage signal having a fixed ramp or slope. Comparators at the output of each TIA determine when the ramp or slope of a voltage signal crosses a threshold thereby converting the slope to a pulse having a predetermined voltage and a width or duration that is a function of the received current. This pulse can then be used to latch a counter thereby performing an analog to digital conversion of the current.

By eliminating the standard analog-digital-converter (ADC) or converters of a conventional signal processor in an optical navigation system, the inventive signal processing circuit insures that any measurement of the voltage signal is made in the linear region of the TIA, and that the TIA is not saturating before being read by an ADC.

The signal processing circuit302in its simplest form, shown inFIG. 3, includes a single comparator304at the output of each TIA306to measure the amplitude of current received from a photo-detector, such as a photodiode, or a photo-detector array308. Referring toFIG. 3A, power is supplied through a driver310to a light source312, such as a semiconductor micro-laser diode or Vertical Cavity Surface Emitting Laser (VCSEL), which provides reflected light to the photo-detector308. The photodiode generates a current in response to the received light, which is then coupled to an input of the TIA306. The TIA306integrates the received current over a predetermined sample period to generate a voltage signal having a monotonically increasing ramp or slope as described above. In the embodiment shown, the voltage signal is coupled to a first, non-inverting input of the comparator304, which is configured to compare the voltage signal to a threshold voltage or level coupled to a second, inverting input of the comparator, and to generate an output pulse having a predetermined voltage and a width that is a function of the received current.

In the embodiment shown, the threshold level is provided as a digital signal coupled to the second input of the comparator304through a digital-to-analog converter314(DAC). However, it will be apparent that the threshold level can also be directly applied to the comparator304as an analog voltage from a voltage divider or other voltage source.

Optionally, an output of the comparator304is coupled through a control element, such as a RS flip-flop316(RSFF), to the driver310supplying power to the light source312. Thus, when the voltage signal reaches or is equal to the threshold level, the circuit302turns off the light source312, thereby reducing power consumption in the optical navigation system.

FIG. 3Bis a timing diagram illustrating the comparator output318pulse and the RSFF output320in relation to an initialization or reset pulse322for the signal processing circuit302ofFIG. 3A.

A graph illustrating three (3) signals324A-C of the infinite possible voltage signals output from the TIA306for the circuit302ofFIG. 3Ais shown inFIG. 3C. It will be noted that the time for a voltage signal324A-C output from the TIA306to reach a predetermined threshold level326is dependent primarily on the current received from the photo-detector308. A higher current results in a voltage signal having a higher or steeper slope or ramp. It will also be noted that the time for the voltage signal324A-C to reach the threshold level326is also dependent on the variation in a beginning voltage output from the TIA306following a reset322. This variation in the beginning voltage output from the TIA after reset commonly referred to as reset noise328.

As shown inFIG. 1, optical navigation systems100frequently include photo-detector arrays110having multiple photo-detectors112, current signals from each of which must be measured or processed in parallel. Several of the blocks in the circuit ofFIG. 3A, including the DAC314, the RSFF316, the driver310and the light source312can be common or shared by multiple channels receiving signals from separate photo-detector elements or groups of elements in the array110.

Optionally, in one embodiment (not shown) where the signal processing circuit302processes signals from multiple channels, the comparator304outputs from each channel can be combined using a number of logic elements so that the light source312is turned off only after the measurement of the last channel is finished.

In one embodiment, shown inFIG. 4A, the circuit402further includes a sample and hold (S/H) circuit404and a differential amplifier406(Diff Amp) to remove reset noise. As in the embodiment ofFIG. 3A, the signal processing circuit402includes a comparator408at the output of each Diff Amp406to measure the amplitude of current received from a photo-detector412, and power is supplied to a light source414through a driver416controlled by a RSFF418. The threshold level is provided through a DAC419.

The S/H circuit404has an input coupled to the TIA410output to sample and hold a voltage of the voltage signal at a predetermined time following a reset. The Diff Amp406has a first input coupled to an output of the S/H circuit404and a second input coupled directly to the TIA410output. The Diff Amp406is configured to subtract the TIA reset level stored in the S/H circuit404from the TIA output410, thereby measuring the slope or ramp of the voltage signal independently of the reset noise from the TIA.

FIG. 4Bis a timing diagram illustrating the comparator output420and the RSFF output422pulses in relation to an initialization or reset pulse424and a SH sample pulse426for the signal processing circuit402ofFIG. 4A.

A graph illustrating three (3) signals428A-C of the infinite possible voltage signals output from the Diff Amp406for the circuit402ofFIG. 4Ais shown inFIG. 4C. It will be noted that the time for a voltage signal428A-C output from the Diff Amp406to reach the threshold level430following a reset is substantially independent of and unaffected by reset noise.

FIG. 5Ais a graph showing a simulation of a Diff Amp output versus time to threshold for a number of different input currents to a circuit having a fixed threshold502, as in the circuit ofFIG. 4A. In the example shown three (3) different input currents generate three (3) possible voltage signals504A-C output from the Diff Amp. Although an improvement over conventional signal processing circuits, it is noted that using a fixed threshold level may be less than optimal, since if the received photo-detector currents into the TIA are too low the time it takes it to integrate may take longer than the interval between reset pulses, commonly known as the reset interval. It is also noted from graph or trace506ofFIG. 5Bthat the integration time required to reach the threshold, which is equal or proportional to pulse width, is a 1/× function.

One solution that insures the threshold is reached within the reset period uses a variable threshold voltage or threshold ramp602as shown inFIG. 6A. In the example shown three (3) different input currents generate three (3) signals604A-C of the infinite possible voltage signals output from the TIA.

Also note that as shown inFIG. 6Athis ramped threshold approach can work for substantially all levels of received photo-detector current even down to a zero input current. That is, unlike with the fixed threshold ofFIG. 5Athere will be an output pulse having a finite width or duration less than the reset interval even for a zero input current. It should also be noted that although a steep slope of the threshold voltage gives a nearly linear response, it does so at the expense of time resolution. That is, as shown by the graph or trace606ofFIG. 6B, the difference in time to threshold (and therefore in pulse width) for materially different received currents of from 1.4 nA to 5.8 nA is reduced to less than 3 μs.

One method of improving time resolution involves adjusting or decreasing the slope of the threshold ramp702as shown inFIG. 7A, thereby achieving improved time resolution as shown by the graph or trace704inFIG. 7B. In the example shown three (3) different input currents generate three (3) voltage signals706A-C output from the TIA. It is also noted from graph or trace704ofFIG. 7Bthat although time resolution is improved there is still a non-linear response time versus input current in this embodiment.

In yet another embodiment, shown inFIG. 8A, the threshold voltage802is a shaped threshold voltage that decreases or ramps non-linearly at a rate selected to provide a desired response out from the signal processor. In the example shown three (3) different input currents generate three (3) voltage signals804A-C output from the TIA. Note that in this example the light source was not extinguished when the threshold was reached but allowed to integrate until the amplifier saturation was reached, thus each of the 3 voltage signals804A-C continue to increase after crossing the threshold802. However, it will be appreciated that this is not a requirement of this technique, but rather the light source could have been shut off when the threshold voltage was crossed, for example by an RS flip-flop as in the preceding embodiments ofFIGS. 3A and 4A.

In still another embodiment, shown inFIGS. 8C and 8D, the threshold voltage (Vt), shown as a graph or trace808inFIG. 8C, to achieve a linear response is defined by a second (2nd) order polynomial defined by the following equation:
Vt=(mt2+bt)/C

where m is the slope of the linear response, shown as a graph or trace810inFIG. 8D, in Amps per second, t is time in seconds, b is the current at time zero, and C is the integrator capacitance. The graph or trace of the linear response810illustrates the further improvement in time resolution within a limited ranges made possible by a shaped threshold as compared to the threshold ramps of the embodiments ofFIGS. 6 and 7.

FIG. 8Cshows an exemplary shaped threshold curve808for a voltage signal having a time from the reset to the threshold of from 10 us to 30 us (in this example reset ends at about 1 us) and received current of from to 10 nA to 0 nA, and a current at zero time (b) of b=1.5 nA or 1.5E-8A (the current if you extended the response line down to 0 us) to provide linear or nearly response having a slope (m) of about −0.5 nA/us or 0.0005 A/s. In this example, the integrator is assumed to have a capacitance (C) of about 42 fF (femtoFarads) or 4.2E-14F. In addition to the shaped threshold curve808FIG. 8Cshows (5) different voltage signals812A-F output from the TIA and produced from (5) five different input currents. Note that in this example the light source was not extinguished when the threshold was reached but allowed to integrate until the amplifier saturation was reached, thus each of the 5 voltage signals812A-F continue to increase after crossing the threshold808. However, it will be appreciated that this is not a requirement of this technique, but rather the light source could have been shut off when the threshold voltage was crossed, for example by an RS flip-flop as in the preceding embodiments ofFIGS. 3A and 4A.

In one embodiment, illustrated inFIG. 9A, a double threshold is used to remove the reset noise. In this embodiment, the signal processing circuit902further includes a second comparator904, a second DAC906to provide a threshold level to the second comparator, and a D-type flip-flop908(DFF). As in the previous embodiments, the signal processing circuit902includes a TIA910to integrate the amplitude of current received from a photo-detector912, and a first comparator914. Power is supplied to a light source916through a driver918. The first threshold level is provided to the first comparator914through a first DAC919.

Although not shown it will be appreciated that the circuit902can further include a control element, such as an RSFF coupled, between the driver918and an output from the DFF908to turn off the light source916, thereby reducing power consumption in the optical navigation system.

Referring toFIG. 9A, the second comparator904has a first, non-inverting input coupled to the output of the TIA910, and a second, inverting, input coupled to the second threshold level through the second DAC906. The DFF908has an input coupled to the output of the first comparator914, and a reset coupled to an output of the second comparator904. The DFF908is set when the first threshold level is reached and reset when the second threshold level is reached. The pulse width of the output of the DFF908is a measure of the input current. By placing the second threshold, parallel to the first, lower threshold the difference in time it takes to cross both thresholds is insensitive to reset noise.

FIG. 9Bis a timing diagram illustrating the DFF output920in relation to the first comparator914output922and second comparator904output924and an initialization or reset pulse926for the signal processing circuit902ofFIG. 9A.

A graph illustrating three (3) signals928A-C of the infinite possible voltage signals output from the TIA910for the circuit902ofFIG. 9Ais shown inFIG. 9C. It will be noted that, provided the first and second threshold levels930,932are substantially parallel, the time for a voltage signal, such as one of the signals928A-C, output from the TIA910to reach the second threshold level932following reach the first threshold level930is substantially independent of an unaffected by reset noise934.

FIGS. 10A and 10Billustrate the results of a simulation of the double threshold circuit. In particular,FIG. 10Ais a graph illustrating three (3) signals1006A-C of the infinite possible voltage signals output from the TIA versus time to a first ramped threshold1002and a second ramped threshold1004for a number of different input currents to the circuit902ofFIG. 9A.FIG. 10Bshows a graph or trace1008time between the first and second thresholds1002,1004, versus input current, and illustrates the method for reducing sensitivity to reset noise by the use of a double threshold circuitFIG. 9A.

The advantages of the signal processor and method of the present invention over previous or conventional approaches include: (i) reduced complexity and increased performance (speed) through the elimination of the ADC(s); (ii) reduced power consumption through control of the driver to the VCSEL or laser; and (iii) the measurement of the voltage signal from the TIA output is independent of and substantially unaffected by reset noise.

The foregoing description of specific embodiments and examples of the invention have been presented for the purpose of illustration and description, and although the invention has been described and illustrated by certain of the preceding examples, it is not to be construed as being limited thereby. They are not intended to be exhaustive or to limit the invention to the precise forms disclosed, and many modifications, improvements and variations within the scope of the invention are possible in light of the above teachings. It is intended that the scope of the invention encompass the generic area as herein disclosed, and by the claims appended hereto and their equivalents.