What is the use of an instrumentation amplifier?

　　1. What is the function of an instrumentation amplifier?

　　Instrumentation amplifiers are sometimes misunderstood; not all amplifiers used for instruments are instrumentation amplifiers, and all instrumentation amplifiers are not only used for instruments. The most important function of an instrumentation amplifier is to detect weak signals in noisy environments. Since noise voltage usually manifests as a common-mode signal while useful signals are differential signals, the high common-mode rejection ratio (CMR) of the instrumentation amplifier can extract and amplify useful signals from noise. Additionally, in practical applications of instrumentation amplifiers, the signal sources typically have output impedances of several kilohms (kΩ) or even higher, requiring the amplifier to have very high input impedance (usually reaching GΩ level). Instrumentation amplifiers not only meet this requirement but also ensure that the input impedances of both input terminals are equal. The operating frequency of instrumentation amplifiers generally ranges from DC to about 1 MHz. At higher frequencies, the effect of input capacitance becomes more significant than that of input resistance; at this point, differential amplifiers are usually used to handle high-speed signals, which increases speed but reduces input impedance.

　　2. How to prevent overvoltage at the input of an instrumentation amplifier?

　　Design engineers need external resistors to prevent excessive drive current from causing overvoltage through internal ESD clamping diodes. The value of the current-limiting resistor depends on the noise level of the instrumentation amplifier, power supply voltage, and required overvoltage protection. Recommended values are provided in the data sheet for the instrumentation amplifier.

　　Using external current-limiting resistors can increase noise; therefore, another method is to use external high-current clamping diodes to significantly reduce resistance values. It must be noted that most ordinary diodes have significant leakage currents, which can cause large offset errors at the output of the instrumentation amplifier. Since this leakage current increases exponentially with temperature, design engineers should avoid using ordinary diodes in high-impedance signal source applications.

　　3. How does RFI filtering work?

　　Long wires between sensors and instrumentation amplifiers are susceptible to radio frequency interference (RFI). After RF rectification by the instrumentation amplifier, it will manifest as a DC output offset error. A solution to filter out RF interference before it reaches the instrumentation amplifier is shown in the diagram. Components R1a and C1a form a low-pass filter for the non-inverting input terminal; similarly, components R1b and C1b form a low-pass filter for the inverting input terminal.

　　It is important that the cutoff frequencies of these two low-pass filters match well; otherwise, common-mode signals may be converted into differential signals. C2 should be at least ten times C1; at high frequencies, this requirement is somewhat relaxed due to both inputs being 'shorted'. Nevertheless, matching C1a and C1b is crucial. They should be selected as ±5% C0G film capacitors. The differential bandwidth of this filter is {1/2πR(2C2+C1)}, while its common-mode bandwidth is [1/2πR1C1)].

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