Patent Description:
In present vacuum gauge (e.g., WYDE Gauge™) analog front end electronic designs, as shown in <FIG>, the pressure reading is a direct result of the signal amplitude appearing on the bridge node of the transformer circuit's secondary winding. However, due to the proportional nature between the voltage amplitude and applied pressure, at low pressures, signal amplification is often a necessity, as shown in <FIG>. At or close to vacuum pressure, signal amplification often times needs to be very high before being presented to an analog-to-digital converter ("ADC") for digitization. This high level of signal amplification creates the problem of an unpredictable phase relationship between the reference wave and the amplified signal wave itself along with unwanted noise injection and amplifier nonlinearity. Consequently, it is desirable to have the best signal quality at vacuum without too much signal gain.

The signal amplitude that appears on the bridge voltage is a function of the transformer's primary side signal amplitude as well as the change in sensor capacitance in relation to the reference capacitance due to pressure change. Prior art solutions to these problems include:.

Along with the distortion of the variable gain block that could be introduced along the signal paths, there are other opportunities for noise to be injected from places such as the power supply and ground before presenting to the ADC.

<CIT> relates to a capacitance manometer having stress relief for fixed electrode. An improved capacitance manometer includes a thin, electrically conductive diaphragm fixedly mounted to a housing. The diaphragm separates a first chamber that is subjected to a reference pressure from a second chamber that is subjected to a pressure that is to be measured relative to the reference pressure. The diaphragm flexes in response to a pressure differential between the two chambers. The diaphragm comprises one electrode of a variable capacitor. A second electrode of the variable capacitor is provided by a fixed electrically conductive area mounted on a ceramic disc or other fixed electrode support.

In the following, each of the described methods, apparatuses, embodiments, examples, and aspects, which do not fully correspond to the invention as defined in the claims is thus not according to the invention and is, as well as the whole following description, present for illustration purposes only or to highlight specific aspects or features of the claims. Embodiments not falling under the scope of the claims should be interpreted as examples useful for understanding the invention.

What is needed is an improvement to the existing sensor analog front end circuit that can be found in the present WYDE Gauge unit's analog boards. Embodiments herein provide a performance enhancement with less circuit BOM and thus less power consumption while achieving better manufacturability. Embodiments are specifically targeted towards the challenging low pressure SNR requirements. Embodiments offer many advantages over the prior art in areas such as manufacturability, simplicity, and saving of circuit components while maintaining equal or better performance compared to its predecessors. In addition, embodiments are invaluable for low pressure gauges at or below one Torr (<NUM> Torr) full scale. As opposed to the prior art, embodiments provide the best signal quality at vacuum.

Embodiments of a bridge voltage inversion circuit overcome the disadvantages of the prior art and provide the advantages described above. These and other advantages may be achieved by, for example, a bridge voltage inversion circuit for a pressure gauge that includes a transformer including a primary winding and a secondary winding that outputs a bridge voltage, a reference capacitor connected to a first side of the secondary winding of the transformer, and a sensor capacitor connected to a second side of the secondary winding of the transformer. The sensor capacitor senses and responds to a pressure. A capacitance of the sensor capacitor is at a minimum when the pressure is at vacuum. The reference capacitor and sensor capacitor are selected so that the capacitance of the sensor capacitor at vacuum is less than a capacitance of the reference capacitor. The bridge voltage is at a maximum amplitude when the pressure is at vacuum, and a fold-over-pressure at which the bridge voltage is at the minimum amplitude is greater than a full-scale pressure.

The bridge voltage inversion circuit may further includes a circuit that outputs a reference signal that drives the transformer and a gain setting block for adjusting a signal gain before the transformer. The bridge voltage inversion circuit may further include an analog multiplexer that receives and multiplexes the reference signal and the bridge voltage signal and outputs the multiplexed signals to an analog-to-digital converter. The bridge voltage inversion circuit may further include a buffer that receives and buffers the bridge voltage. The bridge voltage inversion circuit may further include a power OPAMP driver that amplifies the reference signal. The amplified reference signal may be supplied to the primary winding of the transformer. The bridge voltage inversion circuit may further include an amplifier connected to the secondary winding of the transformer that receives and amplifies the bridge voltage signal. The sensor capacitor may be a diaphragm capacitor. The capacitance of the reference capacitor may be ten percent (<NUM>%) greater than the capacitance of the sensor capacitor at full scale.

These and other advantages may be achieved by, for example, a bridge voltage inversion circuit for a pressure gauge that includes a transformer including a primary winding and a secondary winding that outputs a bridge voltage, a first sensor capacitor connected to a first side of the secondary winding of the transformer, and a second sensor capacitor connected to a second side of the secondary winding of the transformer. The first and second sensor capacitors sense and respond to a pressure. A capacitance of the second sensor capacitor is at a minimum when the pressure is at vacuum. The first sensor capacitor and the second sensor capacitor are selected so that the capacitance of the second sensor capacitor at vacuum is less than a capacitance of the first sensor capacitor at vacuum. The bridge voltage is at a maximum amplitude when the pressure is at vacuum, and a fold-over pressure at which the bridge voltage is at the minimum amplitude is greater than a full-scale pressure. The first sensor capacitor and the second sensor capacitor may be diaphragm capacitors.

These and other advantages may be achieved by, for example, a pressure gauge sensor that includes a transformer including a primary winding and a secondary winding that outputs a bridge voltage, a first capacitor with one end at which pressure is applied and another end connected to a first side of the secondary winding of the transformer, and a second sensor capacitor with one end at which the pressure is applied and another end connected to a second side of the secondary winding of the transformer. The first and second sensor capacitors sense and respond to a pressure. A capacitance of the second sensor capacitor is at a minimum when the pressure is at vacuum. The first sensor capacitor and the second sensor capacitor are selected so that the capacitance of the second sensor capacitor at vacuum is less than a capacitance of the first sensor capacitor at vacuum. The bridge voltage is at a maximum amplitude when the pressure is at vacuum, and a fold-over pressure at which the bridge voltage is at the minimum amplitude is greater than a full-scale pressure.

The drawing figures depict one or more implementations in accord with the present concepts, by way of example only, not by way of limitations.

In this section, some embodiments of the invention will be described more fully with reference to the accompanying drawings, in which preferred embodiments of the invention are shown. This invention, however, may be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will convey the scope of the invention to those skilled in the art. Like numbers refer to like elements throughout, and prime notation is used to indicate similar elements in alternative embodiments.

Embodiments of a bridge voltage inversion circuit for a vacuum pressure gauge/sensor are described herein. Embodiments of the bridge voltage inversion circuit provide the best signal quality at vacuum. As noted above, pressure readings are proportional to the signal amplitude of the bridge voltage; the mismatch of a reference capacitor current versus a sensor capacitor current provides the pressure reading. The reference capacitor is of fixed capacitance. When pressure changes, the sensor capacitor bridge voltage changes proportional to the change in pressure. At vacuum pressure, the amplitude of the bridge voltage is lost due to the presence of noise, and it is difficult to rely on the mismatch between the reference capacitor and the sensor capacitor. It is most desirable, however, to have the best signal at vacuum. Embodiments achieve this desire by inverting the signal so that the largest, rather than the smallest, capacitance difference (ΔC) is present at vacuum pressure.

Presently, there is an industry wide push towards lower full scale (FS) pressure gauges (e.g., ≤ <NUM> Torr). This undoubtedly presents a new level of challenge to sensor designers who are familiar with the inner workings of higher pressure gauges (e.g., ≥ <NUM> Torr). To answer this challenge, without changing the basic fundamental sensing electrical architecture, embodiments of a bridge voltage inversion circuit provide a sensor circuit with inverted amplitude bridge voltage (aka "INA").

With reference now to <FIG> shown is an embodiment of a vacuum pressure gauge front-end <NUM> that includes a bridge circuit <NUM> that inverts the bridge voltage. In vacuum pressure gauge front end circuits, including prior art circuits such as shown in <FIG>, sensor electronics consists of a transformer circuit design based on a coaxial cable primary and secondary winding. This transformer circuit design forms the so-called "bridge circuit" <NUM>. The term "bridge" in this context means that there are inductance-capacitance resonance circuits on either side of the transformer <NUM>. The transformer <NUM> includes a primary winding <NUM> and a secondary winding <NUM>. The bridge voltage is output from the secondary winding. On one side of the bridge circuit <NUM>, there is reference capacitor <NUM> to ground and one half the inductance from the secondary winding of the transformer <NUM>. On the other side of the bridge circuit <NUM>, there is the sensor capacitor <NUM> (e.g., a sensor capacitor for which capacitance increases as pressure increases) to ground and the other half of the inductance from the secondary winding <NUM> of the transformer <NUM>. The reference capacitor <NUM> is connected to a first side of the secondary winding <NUM> of the transformer <NUM>, and the sensor capacitor <NUM> is connected to a second side of the secondary winding <NUM> of the transformer <NUM>. The sensor capacitor <NUM> may be a diaphragm capacitor, but can be any capacitor that changes capacitance based on pressure applied to the capacitor. The reference capacitor <NUM> is a fixed capacitor having a constant capacitance. If the resonance circuits from each side of the bridge circuit <NUM> are equal in inductance (L) and capacitance (C) values, then bridge voltage is zero. Whenever there is a mismatch in capacitance values, a sinewave with amplitude proportional to the capacitance mismatch amount would be generated.

In other words, the transduction between pressure and amplitude is generated through the order of pressure, sensor capacitance (Csensor), capacitance difference (ΔC), and bridge voltage. In prior art circuits, such as shown in <FIG>, the capacitance values of the reference capacitor and sensor capacitor are chosen so that the bridge voltage (Vamplitude) is proportional to pressure. For example, the capacitance of the reference capacitor may be Cref and the capacitance of the sensor capacitor at vacuum may be Csensor. As a result, as pressure increases, a positive ΔC= Cref - Csensor will result. At vacuum, Cref ~ Csensor, the capacitance difference (ΔC) is the highest. The term "full scale pressure" here refers to the maximum pressure where the Gauge's accuracy specification is still guaranteed. For example, many gauges on the market today have full scale in the ranges of <NUM> Torr or <NUM> Torr.

With continuing reference to <FIG>, in embodiments of vacuum pressure gauge front-end <NUM> with bridge circuit <NUM> that inverts the bridge voltage, capacitance values of the reference capacitor <NUM> and sensor capacitor <NUM> are chosen so that Vamplitude is inversely proportional to the pressure. For example, the capacitance of the reference capacitor may be ten percent (<NUM>%) greater than the capacitance of the sensor capacitor at full scale. Alternatively, the capacitance (Cref) of the reference capacitor <NUM> may be fifty percent (<NUM>%) greater than the capacitance (Csensor) of the sensor capacitor <NUM> at vacuum. For example, the capacitance (Cref) of the reference capacitor <NUM> may be around <NUM> pF, and the capacitance (Csensor) of the sensor capacitor <NUM> at vacuum may be around <NUM> pF. The capacitance of the sensor capacitor <NUM> increases as the pressure at the sensor increases. Accordingly, in embodiments of vacuum pressure gauge front-end <NUM> with bridge circuit <NUM>, the capacitance difference (ΔC) would be the most negative at vacuum, and the capacitance difference (ΔC) would approach zero (<NUM>) at full scale pressure.

A sine wave is used to drive embodiments of vacuum pressure gauge front-end <NUM>. In embodiments, the signal spectral purity needs to be guaranteed in order for an on-board DSP engine to perform digital algorithms, for example, an algorithm to digitally filter out "common" noise between Vref and Vsig. The sinewave signal, which is generated by a signal generator <NUM>, is passed through a gain setting block <NUM> which may be used to adjust the signal gain. The signal then is passed through a power OpAmp Driver <NUM> to drive transformer <NUM>. The power OPAMP driver <NUM> amplifies the signal, and the amplified signal is supplied to the primary winding <NUM> of the transformer <NUM>. The signal is also passed to the output of the vacuum pressure gauge front-end <NUM> as Vref.

With continued reference to <FIG>, the output of bridge circuit <NUM>, i.e., the bridge voltage, may be amplified by an amplifier (a power OpAmp driver) <NUM> and alternatingly passed as Vsig through an analog-to-digital convertor (ADC) <NUM> with Vref to the on-board DSP engine (not shown). An analog multiplexer <NUM> receives the reference sine wave and the bridge voltage signal, multiplexes the reference sine wave and the bridge voltage signal, and outputs the multiplexed signals to an analog-to-digital converter. In embodiments shown, winding of transformer <NUM> consists of a shielded cable (e.g., a coaxial cable), where the center core carries the bridge voltage signal Vamplitude and the shield is driven by a unity gain buffer <NUM> with a replica of the center core signal. This is done to minimize the leakage current between the center conductor and the shield through the capacitances that created this current leakage path. In addition, the shield also prevents external EMI sources from interfering with the signal integrity at the winding center conductor.

In the case of prior art vacuum pressure gauge/sensor, with bridge circuit such as shown in <FIG>, additional gain/phase adjust and OPAMP circuits are required for tuning the amplitude and phase of the bridge voltage. The need for tuning the amplitude and phase arise from the fact that at vacuum, where the bridge voltage amplitude is the smallest, additional signal gain is required for proper analog-to-digital (AD) conversion. Unfortunately, whenever the signal gain is increased significantly, there is always a risk that the phase relationship between Vref and Vsig is altered. As the phase relationship between Vref and Vsig become unpredictable, the digital algorithm between Vref and Vsig cannot work properly. With the present embodiment of a bridge voltage inversion circuit shown in <FIG>, these problems are avoided at the most critical pressure levels (i.e., at or near vacuum)-and the additional gain/phase adjust and variable gain circuits shown in <FIG> may be omitted.

With reference now to <FIG>, shown are the sine waves of Vref (Reference Wave) and Vsig (clean, large signal wave). The clean, large signal wave shown in <FIG> compares quite favorably to the noisy, small signal wave shown in <FIG>hat is produced by the prior art bridge circuit shown in <FIG>.

With reference now to <FIG>, shown is a voltage-pressure graph showing voltage amplitude to applied pressure relationship of the embodiment of a bridge voltage inversion circuit shown in <FIG>. Compared to voltage-pressure graph of <FIG>, we see how bridge signal voltage Vamplitude is at maximum (Vmax) at vacuum, slowly decreasing to a minimum (as pressure causes Csensor to increase until Cref is around Csensor which implies ΔC approaches <NUM>). Further pressure increase as Csensor becomes greater than Cref causes ΔC to increase again until Vmax is approached before the sensor capacitor <NUM> shorts (the capacitor plates touch), as shown as Pmax in <FIG>. As illustrated by <FIG>, the bridge voltage is generated from an absolute value of the capacitance difference ΔC between Cref and Csensor. Consequently, for Cref and Csensor, ΔC will be Cref - Csensor at vacuum and will decrease to ~<NUM> as Csensor approaches Cref at P_fo (fold-over pressure). When Csensor increases above Cref, ΔC will increase again as Csensor continues to increase as described herein.

As described herein, embodiments of the bridge voltage inversion circuit bring several added benefits beyond higher performance at or near vacuum pressure:.

In the INA configuration of the bridge voltage inversion circuit <NUM>, a gain setting block <NUM> is used to adjust the transformer <NUM> primary amplitudes. The purpose of this gain setting block <NUM> is to allow extra control of the bridge voltage amplitude in the event of pressure sensor short due to over pressure. Another benefit of using gain setting block <NUM> for shortage protection is to maintain the same amount of power delivered to the reference capacitor <NUM> in the event of a over pressure condition.

With reference now to <FIG>, embodiments of bridge voltage inversion circuit <NUM> may be used with multi-electrode sensors. As shown in <FIG>, bridge voltage inversion circuit <NUM> is used with two sensor capacitors <NUM> and <NUM> rather than a single sensor capacitor and a reference capacitor. The sinewave signal, which is generated by a signal generator <NUM>, is passed through a gain setting block <NUM> which may be used to adjust the signal gain. The signal then is passed through a power OpAmp Driver <NUM> to drive transformer <NUM>. The power OPAMP driver <NUM> amplifies the signal, and the amplified signal is supplied to the primary winding <NUM> of the transformer <NUM>. This transformer <NUM> circuit design forms the bridge circuit <NUM>. The transformer <NUM> includes a primary winding <NUM> and a secondary winding <NUM>. A first sensor capacitor <NUM> and a second sensor capacitor <NUM> are part of the multi-electrode sensor and can both vary when pressure is changed. The first sensor capacitor <NUM> and the second capacity <NUM> may be diaphragm capacitors, but can be any capacitors that change capacitance based on pressure applied to the capacitors. The first sensor capacitor <NUM> is connected to a first side of the secondary winding <NUM> of the transformer <NUM>, and the second sensor capacitor <NUM> is connected to a second side of the secondary winding <NUM> of the transformer <NUM>. In the embodiments of the bridge voltage inversion circuit <NUM>, the capacitance Csen1 of the first capacitor <NUM> at vacuum is greater than the capacitance Csen2 of the second capacitor <NUM> at vacuum. Bridge voltage inversion circuit <NUM> may be used, because the condition Csen1 > Csen2 always holds true up to Pmax. A multi-electrode of the vacuum gauge sensor is a sensor construction by which external reference capacitors is not used. All sensor related capacitances are integrated in the sensor construction itself. The output of bridge circuit <NUM>, i.e., the bridge voltage, may be amplified by an amplifier (a power OpAmp driver) <NUM> and alternatingly passed to a buffer <NUM> that receives and buffers the bridge voltage output from the transformer <NUM>. The output bridge signal from the buffer <NUM> is passed to an analog-to-digital convertor (ADC) <NUM> and further to the on-board DSP engine (not shown).

With reference now to <FIG>, shown are structures of an exemplary sensor that employs the bridge voltage inversion circuit of the claimed invention. <FIG> sows a side view of the exemplary sensor, and <FIG> shows a front view of the sensor. The sensor <NUM> includes an inner capacitor <NUM> and an outer capacitor <NUM>. Consequently, the inverted amplitude (INA) approach not only can be applied to the sensor capacitor/reference capacitor sensor construction, as shown in <FIG>, the concept can be extended to sensors with integrated multiple electrodes, as shown in <FIG>. More specifically, in the case of inner and outer electrode with concentric circle construction (other geometry variants possible), as shown in <FIG>, the capacitance between the inner electrode <NUM> (sensor capacitor <NUM>) and outer electrode <NUM> (sensor capacitor <NUM>) with respect to a common ground plane often times cannot be perfectly matched during the manufacturing process. INA's way of intentional mismatch lends itself very well for this type of sensor electrode construction. In the exemplary sensor shown in <FIG>, the inner capacitor <NUM> may correspond to the first capacitor <NUM> shown in <FIG>, and the outer capacitor <NUM> may correspond to the second capacitor <NUM>. The capacitance difference between the two sensor sides is a design choice based on many other factors, such as the tension of the diaphragm, spacing between diaphragm and electrode(s), sizes and shapes of each electrodes, etc. Therefore generally speaking, the larger the difference between the capacitances for both electrodes, the larger the signal amplitude we would likely have under vacuum conditions.

Claim 1:
A bridge voltage inversion circuit (<NUM>) for a pressure gauge, comprising:
a transformer (<NUM>) including a primary winding (<NUM>) and a secondary winding (<NUM>) that outputs a bridge voltage;
a reference capacitor (<NUM>) connected to a first side of the secondary winding (<NUM>) of the transformer (<NUM>); and
a sensor capacitor (<NUM>) connected to a second side of the secondary winding (<NUM>) of the transformer (<NUM>), characterized by:
wherein the reference capacitor has a fixed constant capacitance, wherein the sensor capacitor senses and responds to a pressure and is configured so that capacitance of the sensor capacity increases as pressure increases, and a capacitance of the sensor capacitor is at a minimum when the pressure is at vacuum, wherein the capacitance of the sensor capacitor at vacuum is less than the constant capacitance of the reference capacitor, wherein a bridge voltage is generated based on a capacitance mismatch between the reference capacitor and the sensor capacitor and the bridge voltage is at a maximum amplitude when the pressure is at vacuum, and wherein the reference capacitor and the sensor capacitor are configured so that a fold-over-pressure (P fo) at which the bridge voltage is at the minimum amplitude is greater than a full-scale pressure (P fs).