Analysis of frequency characteristics of correction of additive displacements in DC voltage calibrators

: pp. 3-9
Lviv Polytechnic National University
Lviv Polytechnic National University, Ukraine
Lviv Polytechnic National University
Lviv Politecnic National University, Department of Metrology, Standardization and Certification

It is known that in order to ensure the metrological reliability of measurement industrial means, operative control of their metrological parameters in operating conditions and in real time is necessary to ensure metrological measurement. For this purpose, it is expedient to use portable, many-bit coded-controlled measures of electrical quantities, in particular DC voltage calibrators. Recently, the method of inverting switching is used to correct the error additive component in the measuring instruments. As a result, the output signal of the DC voltage calibrator, in addition to the constant, has a variable component of the frequency modulation (frequency of the clock generator).

Usually this component amplitude does not exceed the values of tens of millivolts, and for its weakening, use active low frequencies filters (ALFF).

DC voltage calibrator with automatic additive errors correction by the method of measuring current inversion the actually convert the alternating voltage, while they may cause specific errors due to the frequency dependence of the transmission coefficients and the weakening of the operating amplifier (OA) common-mode component and the voltage calibrator transmission coefficient. In order to minimize installation time of the reproducible voltage values, with the instantaneous changes to the code-guided divider control code, it is necessary to select the active filters correctly cutoff frequency. In order to reduce the voltage component at the output of the voltage calibrator, it is necessary to use filters passive or active.

At high transmission ratios and at low frequencies, the transmission filter characteristic should be slightly dependent on the OA parameters. To check this assumption, we will consider the frequency dependence of the OP transmission coefficient and establish the time dependence of the output voltage ALFF. The results of mathematical modeling with the help of the software package Mathcad 2000 Professional are shown, which show that in the field of low frequencies (1 kHz to 5 kHz) and with the qualities not higher 1/2 parameters of modern OP practically do not affect the filter characteristics. In this case, the OA should have as wide a bandwidth as it is covered by a differentiating feedback.

The presented graphs of transients show that broadband serial OA will not be saturated with pulsed input voltages, and the output voltage setting time of the ALFF practically does not depend on the used OA parameter. Consequently, the cutoff ALFF frequency, used in the correction path of the additive error component codon measures for a modern element base, can reach the unit’s values of kilohertz.

To significantly reduce the ALFF setting time, choose the highest filter possible cutoff frequency and use a links cascade combination, the relationship between the parameters of which elements are given by the coefficient α. It should take into account the mutual bypass of the filter links. In the ALFF basis, filters suitable for microelectronic execution with the dozens installation time of microseconds with an error can be implemented not exceeding 0.01 %.

Two-stage ALLF was analyzed, to eliminate the saturation of OA, which used a negative feedback circuit on two-anode stabilizers. Output resistance ALLF and its coefficient of weakening the variable component of the input signals is determined by the passage resistors values of resistance. Therefore, two-stage ALLF a mathematical study of the efficiency in the Electronic Workbench software environment has confirmed theoretical assumptions and provided practical criteria for choosing the values of these resistors.

An analysis a DC voltage calibrator with the correction error additive components of the frequency properties by the method of double switching inverting and the averaging of the output signal by the active low pass filter are carried out.

The two-stage active low-pass filter scheme for a DC voltage calibrator with automatic additive errors correction of based on commutation inverting is provided, which provides the necessary weakening coefficient of the component variable pulsations, whose amplitude is determined by the equivalent error additive component of the calibrator and switching interferences with its minimal output resistance. Theoretical analysis is conducted efficiency confirms of the active filters low frequency used in DC voltage calibrators with the additive component error correction.

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