System of basis functions for the measurement circuit of a sensor with temporally separated channels
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SYSTEM OF BASIS FUNCTIONS FOR THE MEASUREMENT CIRCUIT OF A SENSOR WITH TEMPORALLY SEPARATED CHANNELS V. P. Arbuzov and M. A. Mishina
UDC 621.317.733
The requirements for a system of basis functions that provides temporal separation of the channels in sensor measurement circuits are formulated. A system of basis functions for temporal separation of these channels is developed in accordance with these specifications. Keywords: system of basis functions, measurement circuit, temporal separation of channels.
One of the major components of the error in converters from passive (R, L, C) to active (U or I) electrical parameters based on operational amplifiers is the error owing to the finite gain of the operational amplifier. This error is a function of several parameters, so that correcting it requires some amount of design excess which will permit “two channel” operation, by either spatial or temporal separation of the channels, or a combination of these. This last variant is typical of test methods for enhancement of accuracy [1], which can be implemented, as can the spatial separation of channels, through the use of supplementary quantities for conversion; but this makes the object of the measurements (a parametric converter in sensors) more complicated. As opposed to the methods just mentioned, techniques for temporal separation of the channels of measurement circuits for sensors [2] can, in theory, fully correct the error without making the object of measurement (the parametric converter in a sensor) more complicated. Including a capacitive parametric converter in a design, along with the working capacitor of an additional reference capacitor and conversion into the output signal of a measurement circuit of a relative informative parameter (C0 /Cx, Cx /C0), makes it possible to eliminate the influence of the uninformative components of the probe input signal. However, the measurement circuit also brings as error into the result of a measurement; this can be corrected by separate conversion of the capacitances of working and reference capacitors of the probe and the use of a logometric correction. Figure 1 shows a block diagram of a measurement circuit with temporal separation of the channels for conversion of the capacitances of a probe. The signals U1(t) and U2(t) are used for conversion of the capacitances of the capacitors C0 and Cx into an active electrical quantity, i.e., the output signal UAC(t) of the of the active converter, which consists of the sum of two voltages, of which one is proportional to C0 and the other, to Cx: UAC(t) = U1(t)λ1 + U2(t)λ2,
(1)
where λ1 = ƒ(C0, ξ1, ξ2, ..., ξn), λ2 = ƒ(Cx, ξ1, ξ2, ..., ξn) are coefficients that depend on the capacitances of the reference and working capacitors, respectively, and on the parameters of the active converter, ξi, which are multiplicatively coupled to the capacitances of the capacitors in the parametric converter of the sensor. The channel selector unit separates the output signal of the active converter into two components U1 and U2, of which one d
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