Amplifiers – With semiconductor amplifying device – Including gain control means
Patent
1998-12-10
2000-08-15
Pascal, Robert
Amplifiers
With semiconductor amplifying device
Including gain control means
330 86, H03G 312
Patent
active
061042455
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The invention relates to an instrumentation amplifier for absolute voltage amplification having a balanced voltage inlet and a balanced voltage outlet, wherein the voltage inlet is coupled with the first inputs of amplification units having a very high open-loop gain, and which is further coupled with a feedback network provided between the outputs and second inputs of the amplification units.
BACKGROUND OF THE INVENTION
An instrumentation amplifier of this kind is known in practice and is used as measuring amplifier for sensors with which process data are measured such as, for instance, pressure, temperature or current strength. A known application is the application of the instrumentation amplifier in the oil industry, where during drilling measuring equipment is inserted in the drill shank and both the measuring instruments and the instrumentation amplifier are exposed to high temperatures. Said temperatures may range from 175-250.degree. C. Under such conditions much is demanded from the specifications and the stability of said specifications of the instrumentation amplifier and consequently from the components incorporated therein. A problem is created by the circumstance that differences in drift between the components of the instrumentation amplifier result in precision and calibration problems; there is another awkward circumstance with the above-mentioned application namely, that it is impossible to carry out maintenance on an instrumentation amplifier inside a drill shank. If such an instrumentation amplifier unexpectedly exhibits problems, the costs involved in replacing it are excessively high in relation to the cost of such an instrumentation amplifier.
SUMMARY OF THE INVENTION
It is therefore the object of the invention to provide an instrumentation amplifier giving a precise voltage amplification at any operating temperature without the need for calibration, wherein the drift of the components occurring in practice has a very greatly reduced effect on the precision and stability of the instrumentation amplifier.
To this end the instrumentation amplifier according to the invention is characterized, in that the feedback network comprises substantially equivalent components, each of which during a measuring cycle cyclically occupies all places in the feedback network by means of selective control of the switches which are electrically coupled with the components, and that the components of the feed-back network thereby form a switch-free current loop. By cyclically exchanging the substantially equivalent components, the instrumentation amplifier according to the invention is provided with an average amplification which depends substantially on the number of components used in the feedback network. In this manner relative insensitivity is obtained with regard to the nominal value pertaining to each separate component, that is to say that the nominal values are very much less sensitive to deviations and are of limited significance for the precision of the amplification provided by means of the instrumentation amplifier. Also, drift of the components has only a very limited influence. The fact that the components of the feedback network thus form a switch-free current loop ensures that the additional resistance caused by such switches has no influence on the precision of the instrumentation amplifier.
In a particular aspect according to the invention the instrumentation amplifier is characterized in that at each nodal point between two directly electrically coupled components six switches are connected, at least one of which is always closed during the measuring cycle, and which switches serve to selectively couple each nodal point separately and in succession with one of the two second inputs of the amplification units, with one of the two outputs of the amplification units, and with a line of the voltage outlet such that with regard to all the nodal points a coupling exists wherein the coupling between the nodal points is non-short-circuiting.
This manner provides a
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Meijer, G.C.M., et al., "New Concepts for Smart Signal Processors and Their Application to PSD Displacement Transducers", Sensors and Actuators, vol. A35, No. 1, Oct. 1, 1992, pp. 23-30.
"Accurate Gain Switching", Electronics World and Wireless World, vol. 97, No. 1674, May 1, 1992, p. 25.
Barrera Joseph
Choe Henry
Myers Jeffrey D.
Pascal Robert
Stichting Voor De Technische Wetenschappen
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