Electrical input signals are converted into measurement values with a measuring amplifier. In that regard, components such as amplifiers, analog-digital converters, filters, etc. are utilized, with the aid of which the input signals are amplified, digitized, filtered, etc., in order to obtain utilizable measurement values. The electrical input signals can be supplied by a measuring transducer or several measuring transducers. Correspondingly, a measuring amplifier can comprise one channel or several channels.
The circuits of a measuring amplifier mostly comprise a plurality of individual components, whereby each one of the components has its own tolerance. Especially in a precision measuring amplifier, the tolerances may only be very small. While the original tolerances of the components may still be controlled quite well through strict selection processes, nearly no influence can be had on the drift behavior of the individual components caused by temperature and aging.
Precision measuring amplifiers of the applicant can measure, or convert input signals into measurement values, with a class accuracy of 5 ppm, that is to say 0.0005%, at the boundary of the physical limit. In order to ensure this accuracy and maintain it constant over many years, a device-internal automatic calibration and adjustment or auto-adjustment is necessary. In that regard, the drift of the measuring amplifier is compensated cyclically with an internal reference signal, and in this manner the high class accuracy of the measuring amplifier is maintained constant over its operation time. After the learning or run-up phase of the measuring amplifier, this process involves merely an insignificant re-adjustment.
In earlier-utilized methods, the measurement is cyclically interrupted for a certain time and the zero point and the amplification is corrected with the aid of two reference points (zero value and end value). Thereafter, the measurement can again be continued. However, at this point, one must wait for the filter start-up transient times of filters in the measuring amplifier, which in part are quite considerable. These interruptions of the measuring operation are disturbing for the user and are not acceptable in certain applications. Therefore it is desirable to avoid such interruptions.
A method is disclosed in the EP 2 056 460 B1, with which measurement values of a measuring amplifier can be corrected without interrupting the measuring operation. In that regard, a first amplifier circuit continuously converts an input signal that is permanently supplied to it, that is to say without any interruption, into measurement values. A second amplifier circuit is circuit-connected parallel to the first amplifier circuit. The input signal, a reference signal and a zero or null signal are alternately supplied to it. Measurement values of the second amplifier circuit are corrected based on the reference signal and the zero signal. Thereupon, the differences between the corrected measurement values of the second amplifier circuit and the uncorrected measurement values of the first amplifier circuit are determined at various different levels of the input signal, and based thereon, the zero point error and the amplification error are determined. On the basis of the determined errors, the measurement values of the first amplifier circuit can then be corrected. In this manner, measuring can be performed continuously. The measuring operation does not need be interrupted.
With the method described in the EP 2 056 460 B1, the zero point error and the amplification error can only be optimally determined if the input signal changes significantly. If the change or variation of the input signal is quasi-static, while the measurement value of the first amplifier circuit will be properly corrected in total, however not with respect to the distribution or division between the zero point error and the amplification error. An amplification error can always only be determined if a significant input signal variation regularly arises. When a significant change or variation of the input signal arises, then the measurement value will at first deviate due to the incorrect determination of the zero point error and amplification error. The zero point error and the amplification error are followed-up or re-adjusted and thus corrected after the jump in the input signal and its new determination. However, until then, the measurement value deviates for a certain time, which depends on the implementation.
In the method disclosed in the EP 2 056 460 B1, it additionally gives rise to an error propagation. In this method, the measuring amplifier is indirectly calibrated via an additional amplifier circuit. Due to the two-stage method—first the additional second amplifier circuit is calibrated, and then therefrom the error of the actual first amplifier circuit is derived—errors in each stage are summed together and can thus lead to a larger error of the calibration result.
Furthermore, the method described in the EP 2 056 460 B1 requires that the first amplifier circuit and the second amplifier circuit supply measurement values in parallel during the longest possible time period, in order to be able to determine the zero point error and the amplification error or the correction values for the measurement values of the first amplifier circuit based on the differences of these measurement values at various different input signal levels. Therefore, in measuring amplifiers with several channels, a further identically constructed amplifier circuit is required for each individual amplifier circuit. Thus, for a number N of channels, always 2×N amplifier circuits will be required. This hardware expenditure and complexity is often not practical for a precision measuring amplifier. The construction of a precision measuring amplifier is significantly more complex and costly than the construction of a measuring amplifier for standard requirements. Therefore, due to space and cost reasons, the doubled number of amplifier circuits can only rarely be utilized.