<span>Background of the invention: The present invention relates to impedance discrimination circuitry for accurately measuring small impedance differences. The invention finds particular application as a pressure transducer for measuring pressure-related impedance variations. It is to be appreciated, however, that the invention may also be applicable to measuring other physical phenomenon which cause impedance variations such as temperature, force, flow, acceleration, chemical concentration, and the like.Heretofore, capacitive pressure transducers have commonly included an oscillator whose frequency varies with pressure. More specifically, a pressure sensitive capacitive element at least in part has controlled the frequency of an oscillator. Commonly, the oscillator has a high frequency and undergoes a relatively small frequency change, e.g. 0.1% to 01.0%, over the range of measured pressures. A demodulation circuit has normally been required to reduce the high frequency to a usable range. This small frequency change has been difficult to interpolate accurately over the time and measured pressure range. Further, frequency variations are caused not only by pressure, but also by time, temperature, and the like. Accordingly, precision capacitive pressure measurements have been difficult to attain, unless the capacitance is large. A pressure transducer whose output varies in proportion to pressure, or more specifically, to a pressure related capacitance variation, is illustrated in U.S. Pat. No. </span><span>3,869,676, issued March 1975 to D. R. Harrison, et al. In the Harrison, et al. patent, a sinusoidal oscillator is connected by like capacitive couplings with opposite terminals of a circularly arranged diode bridge. A reference capacitor and a variable capacitor are connected between the other diode bridge terminals, respectively, and ground. The sine wave from the oscillator causes the diodes to be forward and reverse biased in such a manner that charge moves from a first of the capacitive couplings to the reference capacitor and from the a second of the capacitive couplings to the variable capacitor during a positive-going since wave half cycle. During a negative half cycle, charge flows from the reference capacitor through the diode bridge to the second capacitive coupling and from the variable capacitor to the first capacitive coupling. If the references and the variable capacitors are not equal, this cyclic charge movement causes a charge build up, and therefore a bias voltage on one of the first and second capacitive couplings and a charge deficit on the other. The charge segregation on the first and second capacitive couplings is proportional to the difference in capacitance of the reference and variable capacitors. One of the problems with the Harrison, et al. circuit is that the diodes have a limited linear range. Commonly, a diode saturates at about 600 millivolts forward bias and has a linear acceptable output up to about 300 millivolts.http://www.google.com/patents?vid=USPAT4459856</span>

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