Classifications

• • H—ELECTRICITY
  • H03—ELECTRONIC CIRCUITRY
  • H03G—CONTROL OF AMPLIFICATION
  • H03G3/00—Gain control in amplifiers or frequency changers
  • H03G3/20—Automatic control
  • H03G3/30—Automatic control in amplifiers having semiconductor devices
  • H03G3/3005—Automatic control in amplifiers having semiconductor devices in amplifiers suitable for low-frequencies, e.g. audio amplifiers
  • H03G3/3026—Automatic control in amplifiers having semiconductor devices in amplifiers suitable for low-frequencies, e.g. audio amplifiers the gain being discontinuously variable, e.g. controlled by switching

Definitions

• the present invention relates to an instrumentation ampli- bomb 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 ⁇ bombs, 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.
• the measurement signal level can vary widely, e.g., from 10 millivolts to 5 volts which means a span of three orders of magnitude.
• 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.
• 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.
• 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.
• Figure 1 shows the operating principle and a possible practical implementation of the amplifier at the component level.
• the instrumentation ampli ⁇ bomb 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.

Landscapes

• Engineering & Computer Science (AREA)
• Multimedia (AREA)
• Amplifiers (AREA)
• Indication And Recording Devices For Special Purposes And Tariff Metering Devices (AREA)
• Analogue/Digital Conversion (AREA)
• Control Of Amplification And Gain Control (AREA)

Applications Claiming Priority (2)

Publications (1)

Family

ID=8529071

Family Applications (1)

Country Status (5)

Cited By (3)

Citations (4)

• 1989
  • 1989-09-28 FI FI894591A patent/FI87861C/fi not_active IP Right Cessation
• 1990
  • 1990-09-26 CA CA 2066705 patent/CA2066705A1/en not_active Abandoned
  • 1990-09-26 JP JP51327790A patent/JPH05500590A/ja active Pending
  • 1990-09-26 WO PCT/FI1990/000226 patent/WO1991005406A1/en not_active Application Discontinuation
  • 1990-09-26 EP EP19900914180 patent/EP0494198A1/en not_active Withdrawn

Patent Citations (4)

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