INSTRUMENTATION AMPLIFIER WITH AUTOMATIC GAIN CONTROL

Abstract: This invention concerns an instrumentation amplifier capable of implemenating automatic control means for the gain of an amplifier (1), which can further be read as the value of the set gain in conjunction with the readout of the amplifier output voltage. The amplifier uses parallel feedback resistors (3) of which only one at a time is connected. The selection is performed on the basis of comparison of the output voltage against a reference voltage level (6, 7). An up/down counter is used to monitor the result of the comparison and to control a multiplexer (2) that connects the resistors (3) over the amplifier (1). The amplifier is best suited to measurements in which multiple low-cost amplifiers must be used and the amplitude of the incoming analog signal varies by several orders of magnitude.

Description: Instrumentation amplifier with automatic gain control The present invention relates to an instrumentation ampli- fier of a type in accordance with the preamble of claim 1. An instrumentation amplifier is by definition a device intended for the measurement of analog signals, often at a low level, from different types of sensors. The amplifier is conventionally constructed using a few operational ampli¬ fiers, but it can also be built around amplifying stages designed around UJT and FET devices . The output signal of the instrumentation amplifier is generally read via an analog-to-digital converter connected to a computer that can convert an output signal range 0...5 V of the amplifier into a corresponding digital signal with a scale of 0...1023. In certain applications the measurement signal level can vary widely, e.g., from 10 millivolts to 5 volts which means a span of three orders of magnitude. If the analog-to-digital converter connected to the computer has a measurement range of 0...5 V, the lower end of the measurement range has insufficient measurement resolution. The converter of the exemplifying case with a resolution of 1:1000 is already insufficient for measurement of millivolt-level signals at any accuracy, although the inherent resolution limit of most analog signals is in the order of 0.1 percent. In the art the practical solution to the problem is the so-called logarithmic amplifier, whose.output signal corresponds logarithmically to the input signal. A drawback to this approach is that the logarithmic amplifier must be implemented using a forward-biased diode, which is a most temperature-sensitive component. The thermal drift of the amplifier must be compensated in one way or another with the help of, e.g., a temperature-dependent resistor also called a thermistor. The practical implementations of a logarithmic amplifier tend to be complicated and costly. The feasibility of applications requiring a multitude of sensors can thus easily be limited by cost factors. It is an object of the present invention to develop a simple and cost-effective instrumentation amplifier, which is capable of measuring the analog output signals of sensors at a sufficient accuracy even if the signal amplitude may vary in a range of several orders of magnitude. The principal advantages of the invention lie in its low cost, which results from the use of basic components. Amplifiers in accordance with the invention avoid the large temperature dependence of logarithmic amplifiers so difficult to compensate. In order to implement the above-described function, the invention is principally characterized by what is stated in the disclosed claims. Figure 1 shows the operating principle and a possible practical implementation of the amplifier at the component level. In accordance with the invention, the instrumentation ampli¬ fier shown in Fig. 1 is implemented by a design which makes the amplifier to perform an automatic gain control until its output signal is always in a range of, e.g., 1...10 V. The amplifier is capable of measuring a voltage span extending from, e.g., one millivolt to ten volts practically at the same accuracy. The amplifier can be constructed using simple basic components, which avoids the large thermal drift so difficult to compensate in a logarithmic amplifier. The amplifier 1 is provided with a feedback network comprising a multiplexer 2 and resistors 3 so that one resistor at a time is connected from the amplifier output to the input. The multiplexer is controlled via control lines 4. The control lines are driven by an up/down counter 5. The output voltage of the amplifier is compared against a 1 V reference voltage with the help of a comparator 6. If the output voltage is below 1 V, the comparator output goes to logic 1 state and the counter is incremented. The output lines 4 of the counter 5 control the multiplexer 2 making it change its channel, whereby a feedback resistor 3 of larger value is selected. Due to the heavy feedback, the output of the amplifier is always held above 1 V. The gain of the amplifier can be read by a computer from the states of the output lines of the counter 5. The above-described simple embodiment of the invention is most elementary; therefore, it should be understood not to limit the scope of the invention that permits variations within the disclosed patent claims. In practical designs the amplifier construction can be implemented with the help of several different component alternatives. For instance, the multiplexer can be replaced by other types of logic circuits.

Claims: WHAT IS CLAIMED IS: 1. An instrumentation amplifier with automatic gain control, said amplifier comprising a feedback network incorporating parallel resistors (3) , of which one at a time is connected to set the amplifier gain through a comparison of the output signal level to a reference voltage (7) so many times with a subsequent change of feedback resistor (3) as is necessary to make the measurement signal from the amplifier to exceed a set reference voltage, and simultaneously with the read operation of the amplified signal, the gain is read from the value of the resistor (3) selected, c h a r a c t e r ¬ i z e d in that the parallel feedback resistors (3) , which define the gain of the amplifier (1) , are connected via a multiplexer (2) or a logic circuit whose control (4) takes place by an up/down counter (5) that further monitors the state of a comparator (6) until it detects the signal to have exceeded the reference voltage level, whereby the comparator stops the counter (5) and latches the value of the feedback resistor, after which the output voltage of the amplifier (1) is read, together with the selected gain, that is, the value of the resistor (3) selected by the state of the output lines of the up/down counter (5) . 2. An amplifier in accordance with claim 1, c h a r a c ¬ t e r i z e d in that, in parallel with the comparator (6) , there is connected another upper limit comparator referenced to the maximum level of the amplifier output voltage and that the second comparator decrements the up/down counter in the opposite direction if the output voltage of the amplifier tends to increase excessively high.

Inventors: OLKKONEN HANNU, PUUMALAINEN PERTTI, OLKKONEN, HANNU, PUUMALAINEN, PERTTI
Applicants: DIGIXRAY LTD OY, OY DIGIXRAY LTD.
Application year: 1990
Main classification: H03G3
IPC codes: H03G3