Electricity: measuring and testing – Measuring – testing – or sensing electricity – per se – Plural ranges – scales or registration rates
Patent
1986-11-28
1990-01-30
Eisenzopf, Reinhard J.
Electricity: measuring and testing
Measuring, testing, or sensing electricity, per se
Plural ranges, scales or registration rates
324116, 324121R, 324130, 364571, G01R 1320, G01R 1900
Patent
active
048975990
DESCRIPTION:
BRIEF SUMMARY
DESCRIPTION
The invention relates to a signal processing device of the type defined by the preamble to claim 1.
Level adapter circuits are used in signal processing devices in order to provide adaptation to the dynamic range of the signal processing in the device when there are different signal amplitudes.
A device of this kind is known from German patent document DE-A1 21 62 337. In the known device, the ratio of the resistors of a potentiometrically wired operational amplifier is varied by means of various switches. In this circuit, one problem is that the input resistance varies in the different switch positions, and another is that considerable shielding problems arise with high-resistance signal processing. Moreover the input is not free of potential, and compensation voltages cannot readily be supplied for balancing direct voltage.
The invention defined by claim 1 has the object of disclosing a signal processing device of the above type which enables a simple construction, in particular inside a compact measuring instrument (that is, one that is hand-held).
The invention is based on the recognition that with an input amplifier to be operated in the manner of an operational amplifier, the switchover of the input voltage ranges by means of variations can take place only in the range of virtual ground, so that the breakpoints are restricted neither by stray potentials nor by high voltages to be applied.
From the requirements for a high dynamic range, for a simultaneously broad band, and for a high-resistance and reliable input resistor, the result is the construction of a calibration circuit with high-resistance resistors, because this construction has to be equipped with compensating capacitances. To reduce the expense for components and balancing, two divider stages, separated by a preamplifier, are used: one decade divider, and one step divider; the step divider may be wired with relatively low resistance.
To keep the amplification factors as low as possible, a fundamental input division is avoided. To this end, it is advantageous that the input is not delivered to the noninverting input of a dividing amplifier, as is otherwise usually done, but instead a discretely constructed operational amplifier, wired potentiometrically, is used. This has the advantage of being able to operate with an active resistor at the input, without having to accept an attenuation of potential.
The summation point thereby attained at the input of the amplifier is very favorable for other reasons. If correctly designed it is potential-free and at the same time represents a virtual ground; advantageously, an additional regulating voltage can be delivered to this point, which can be used for a direct current shift or for an automatic null calibration. This in turn lessens the requirements for direct voltage stability of the amplifier and with appropriate circuitry lends the opportunity of fully automatic null control and calibration. It is thereby possible to meet the requirements for null stability, measurement accuracy and long-term stability as well as, in combination with the computer, for fully automatic measurement of the alternating voltage and direct voltage component of the signal. One of the decisive advantages of this system, however, is its very advantageous divider.
The virtual ground of the input of the potentiometrically wired operational amplifier makes it possible to meet the demand for high and constant input resistance for all switch stages and to obtain an input current divider. Because the amplifier is "current controlled", the invention provides the opportunity of dividing the input current.
All the divider resistances can be calculated by the rules for parallel connection of resistors, and the input-side resistors are switched over between the virtual ground point of the potentiometrically wired amplifier and the reference potential of the input. As a result of the parallel circuit, the voltage amplification of the amplifier is decreased accordingly. A fixed resistor remains in the input, so that on th
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"A New and Unique Analog-to-Digital Conversion Technique", by McCullough, IEEE Trans. on Ins. & Meas., IM-15, #4, 12/66, pp. 276-283.
Burns W.
CREATEC Gesellschaft fur Elektrotechnik mbH
Eisenzopf Reinhard J.
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