TECHNICAL FIELD

Embodiments of the present disclosure generally relate to the field of electronic sensing devices. More particularly, embodiments of the present disclosure relate to biased electronic sensing devices.

BACKGROUND

A sensor is an object that is used to detect events or changes in its environment, and then provide a corresponding output. Sensors may provide various types of output, but typically use electrical or optical signals. For example, a thermocouple generates a known voltage (an output) which corresponds to its temperature (the environment) . Nowadays, the uses of sensors have expanded beyond traditional fields of use such as temperature, pressure, light, sound or flow measurement, for example into Magnetic, Angular Rate, and Gravity (MARG) sensors .

When a sensor is used, the changes that occur at its environment (the environment which is being monitored by the sensor) affect the output of the sensor through a change in the electronics of that sensor. For example, a voltage source that would change the potential, a current source that would change the current, a resistance change or any combination thereof.

A sensor's sensitivity indicates how much the sensor's output changes when the quantity being measured changes. In some cases, sensors must be biased in order to optimize their performance (e.g. to gain maximum sensitivity or responsivity) . Some sensors require a DC voltage bias for optimal performance, while other sensors require a DC current bias in order to achieve their optimal performance. Typically, a sensor is connected to a signal conditioning circuitry, i.e. an electrical circuit designed to amplify the sensed signal, while minimizing excess gain and maintaining linearity. For sensors that require biasing, obviously, a signal conditioning circuit is required in order to distinguish the sensed signal from the DC bias point.

FIG. 1 illustrates a prior art solution for sensors used to monitor changes in temperature or pressure, whereby the change in the environment of the sensor is reflected in a resistance change, known as the Wheatstone bridge. A Wheatstone bridge is an electrical circuit used to measure an unknown electrical resistance by balancing two contacts of a bridge circuit, one contact of which includes the unknown component.

Another prior art example is illustrated in FIG. 2, where a photo diode sensor is presented. The photo diode is a source for a current and has a DC voltage as bias. Typically, a photodiode is loaded by a serial resistor as a load. In FIG. 2 the photodiode is connected to an operational amplifier based trans impedance amplifier. Yet, in this case the voltage must be a DC voltage and modulation may be affected only if the light source (e.g. a laser) is on/off modulated.

However, sensors are elements that are not designed to be connected directly to a load, thus an amplifier/signal conditioning stage is required in order to enhance the sensed signal's level and to maximize the Signal to Noise Ratio ("SNR"), thereby enabling to achieve more accurate results. The quantity sensed by the sensor may remain at the same value for a long period of time, therefore the bandwidth that would be required, extends usually at the DC range up to the maximum that Is likely to be needed. The output signal is the sensed (input) signal multiplied by the gain of the amplifier used. On the other hand, the noise that would be generated is a combination of the sensor's self-noise and the signal conditioning noise.

Active devices such as a bipolar junction transistor ("BJT") or a field-effect transistor ("FET") that are used in amplifying devices are known in the art as being a source of noise. The maximum noise is in the range close to the value of the DC voltage where the flicker noise is a (1 / f) ⁿ (where n is ³1) noise, associated with the current or the voltage and is usually related to a direct current, as resistance fluctuations are transformed to voltage or current fluctuations via Ohm's law. There is also a flicker noise component in resistors having no DC current conveyed through them, due to temperature fluctuations modulating the resistance. The noise increase due to such flicker noise, varies from one amplifier to another, but is typically considered to be between lKHz to 10MHz.

The present invention seeks to provide a novel solution which enables obtaining an electronic sensing device that comprises an amplifier configured to amplify the input signal (generated in response to a change occurring at the sensor's environment), yet the output signal of the electronic sensing device is characterized in that the amplifier generates a relatively lower noise than the sensing circuitry of the device.

SUMMARY

The disclosure may be summarized by referring to the appended claims .

It is an object of the present invention to provide a differential AC sensor device comprising an amplifier, with essentially a constant bias voltage (or current), without deteriorating the output signal.

It is another object of the invention to provide a method for increasing the level of a signal outputted from a sensor, without adversely affecting the Signal to Noise Ratio (SNR) value due to noise introduced while amplifying that signal.

It is still another object of the invention to provide a sensor device comprising an amplifier, that includes an active bridge .

It is another object of the invention to provide a sensor device comprising an amplifier that operates at a frequency range higher than the flicker noise range.

Other objects of the present disclosure will become apparent from the following description.

According to one embodiment, there is provided an electronic sensing device comprising a sensor circuitry, an amplifier and a modulator / de-modulator, wherein the sensor circuitry is configured to receive an input signal derived from a change that occurred at the electronic sensing device's environment, the modulator is configured to AC modulate the input signal, the amplifier is configured to amplify the modulated signal and the de-modulator is configured to demodulate the modulated amplified signal, thereby obtaining a signal having a higher SNR than the input signal and a noise level which is not higher than that of the input signal.

According to another embodiment, the sensor circuitry comprises an active bridge.

In accordance with another embodiment, the sensor circuitry comprises a polarity current source.

By yet another aspect of the disclosure there is provided a method for obtaining an output signal from an electronic sensing device, comprising:

providing an electronic sensing device comprising a sensor circuitry and configured to receive an input signal derived from a change that occurred at the electronic sensing device's environment ; applying an AC modulation to the input signal as well as to the sensor (e.g. the flicker noise generated by the sensor circuitry) by alternating the bias point;

amplifying the modulated input signal;

de-modulating the amplified modulated signal; and

outputting the demodulated signal which has a higher SNR than the input signal and a noise level which is not higher than the input signal.

In accordance with another embodiment, the step of de modulating the amplified modulated signal comprises subtracting the signal used for modulating the sensor and the input signal from the modulated amplified signal.

According to still another embodiment, the sensor circuitry comprises an active bridge.

In accordance with yet another embodiment, the sensor circuitry comprises a polarity current source.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated herein and constitute a part of this specification, illustrate several embodiments of the disclosure and, together with the description, serve to explain the principles of the embodiments disclosed herein.

FIG. 1 is a schematic representation of a prior art sensor of the type Wheatstone bridge;

FIG. 2 is another schematic representation of a prior art photo diode sensor;

FIG. 3 illustrates a schematic representation of a sensor device construed in accordance with an embodiment of the present invention, that comprises a passive bridge;

FIG. 4 illustrates a portion of an electronic sensing device that comprises an active bridge; FIG . 5 illustrates an electronic sensing device that comprises an active bridge;

FIG . 6 illustrates an electronic sensing device that comprises an active bridge and a polarity current source; and

FIG . 7 exemplifies an embodiment of the invention, for ampl ifying the output signal of an electronic sensing device.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

Some of the specific details and values in the following detailed description refer to certain examples of the disclosure. However, this description is provided only by way of example and is not intended to limit the scope of the invention in any way. As will be appreciated by those skilled in the art, the claimed method and device may be implemented by using other methods that are known in the art per se. In addition, the described embodiments comprise different steps, not all of which are required in all embodiments of the invention. The scope of the invention can be summarized by referring to the appended claims .

The present invention relates to a method and an apparatus for obtaining sensor readings where the signal's SNR is high, thereby achieving more accurate results.

Sensors are some of the most widely used sensors because of their low cost, easy to manufacture, and interface with signal conditioning circuits. Some of the common physical parameters where sensors used widely are temperature, strain, pressure, light intensity, fluid flow or mass flow and humidity. For measuring different physical and chemical parameters by resistive technique, the resistance value can vary from few fractions of an ohm to several ohms to several hundred of ohms. Measurement of resistance changes of ohms to several hundred ohms is not much difficult, however when measuring small resistance changes in the presence of several non-ideal effects such as ambient temperature, electrical noise and input offset voltage, these are likely to substantially reduce the accuracy of the measurement .

In the recent past, an active bridge has been proposed that is useful for direct measurement of in-circuit resistances. Such a system is typically self-balanced, capable of measuring resistors even in a production line. However, the shunt resistances appearing across the unknown resistance affect the accuracy and start loading the Op-Amp. Also, if the resistance change is only a fraction of an ohm, the circuit is less sensitive and effect of offset voltage of the OpAmp, causes significant measurement errors. Thus, in order to improve the sensitivity, a high excitation voltage or large gain of the amplifier or elimination of non-ideal OpAmp offset voltage, are required .

For practical reasons, electronic sensors should preferably operate at a frequency that is higher than frequencies included in the flicker noise range. Specifically, biased sensors may have significant flicker noise. However, that is preferably done when the signal being outputted by the sensing circuitry, is modulated to a high frequency and then after amplifying the signal, it is de-modulated, at which time the output signal is strong enough to achieve a high enough SNR.

Most of the sensors should preferably be biased in order for them to operate properly. Typically, such a bias is obtained by using a DC voltage or a DC current. Still, in order to implement the option of modulation / de-modulation proposed by the present invention, using a DC signal, the bias should be converted to be of the AC type. In some embodiments, the modulation is affected onto the signal being inputted to the sensing circuitry. By AC modulating the sensor bias point, one does not allow the flicker noise to evolve.

As explained above, one of the major advantages of the solution provided by the present invention is that it allows to obtained from the electronic sensor device an output signal which is at a high intensity (e.g. at a maximum level) and at the same time it allows refraining from adding any substantial noise to that output signal (if at all), thereby allowing to obtain an output signal from the electronic sensor device which has a substantially improved SNR.

Now, let us consider FIG. 3 which illustrates the use of a passive bridge. The problem with the implementation of this option is that it leads to the addition of noise to the output signal. In other words, the resistors Rl, R2 and R3 in the passive bridge depicted in FIG. 3, cause to a reduction in the output signal intensity, while adding further noise thereto, resulting typically in a lower SNR value than desired. Output signals resulting from using setups wherein Rl=R3=R4=R and where R2 is either equal to, less than or greater than R, for either one of VO and VO', are shown in the right-hand side of this FIG, which clearly demonstrates the fact that the output signal has a relatively very low SNR value.

FIG. 4 and FIG. 5 illustrate a two-stage sensor device comprising an active bridge. FIG. 4 illustrates a portion of an active bridge, i.e. the first stage of the sensor device whereas FIG. 5 illustrates the full two-stage device. In the active bridge of the illustrated device, the voltage on resistance R2 is always equal to VAC while the signal value is VAC/R2 * (R6+R7 ) , where VAC stands for Volts in an alternating current (AC) circuit. The noise is also smaller due to less resistor noise. The known voltage on the sensor can be also an advantage for a sensor that is sensitive to the working voltage. In order to reduce the AC voltage at the second stage, the modulated input signal is amplified and then demodulated, e.g. by subtracting the modulating signal (i.e. the signal used to modulate the input signal) from the modulated input signal, thereby allowing the user to remain with an amplified input signal, without the noise effects resulting from the input signal's modulation.

The time constant of R12 Cl and R14 C2, is similar to the time constant of the first 2 amplifiers comprised in the first stage (shown above R6 and below R7 in the two FIGs. 4 and 5) . This way, the step of subtracting the modulating signal from the modulated signal is better carried out prior to the stage at which the signal leaves the electronic sensing device as an output signal (e.g. at the edge of the signal's path within the electronic sensing device) .

Signals received while using an active bridge, wherein R2 is either equal to, less than or greater than R, after the first stage (FIG. 4) and after the second stage (FIG. 5), are shown in the right-hand side of the respective figures. As may be seen in the right-hand side part of FIG. 4, the signal leaving the amplification stage of the first step of the procedure is one that comprises the combination of the input signal and the known AC bias, in its amplified form. In the second step of the procedure (in the second stage which is illustrated in FIG. 5, the known AC bias in its amplified form is subtracted from the outcome of the first step, i.e. the combination of the input signal together with the known AC bias in its amplified form, thereby obtaining essentially the amplified input signal itself. The control of the amplitude is achieved by a voltage attenuator, and the bias applied at the first step of the procedure is linear to the bias leaving the attenuator. Consequently, the solution provided herein is independent of the bias that is actually applied, and the output of the device as presented in the right-hand side of FIG. 5, may essentially be identical to the output of a device implementing a passive bridge as presented at the right-hand side of FIG. 3, but with a better signal to noise ratio and with a constant voltage on the sensor .

FIG. 6 demonstrates another example of an embodiment of the present invention, showing that an active bridge solution is also suitable in cases where a polarity current source is used, namely, a source wherein the signal polarity depends on the bias polarity. The two examples presented in FIG. 6 are: using a tunneling diode, and using two photo diodes in series. Once again, the resulting signals received while using such a setup, wherein R2 is either equal to, less than or greater than R, are presented in the right-hand side of FIG. 6.

FIG. 7 exemplifies an embodiment of a method for carrying out the present invention. According to this embodiment, an electronic sensor is provided (step 700) configured to receive an input signal derived from changes that occur at the sensor's environment (step 710) .

The input signal is then modulated (step 720), amplified (step 730) and demodulating the signal by subtracting the modulating signal from the modulated amplified signal (step 740) .

Next, the thus processed signal is outputted from the electronic sensor as an output signal (step 750) having a high SNR.

Other embodiments of the invention will be apparent to those skilled in the art from consideration of the specification and practice of the invention disclosed herein. It is intended that the specification and examples be considered as exemplary only, with a true scope and spirit of the invention being indicated by the following claims.