Download A Designer's Guide to Instrumentation Amplifiers, 2nd by Charles Kitchin, Lew Counts PDF

By Charles Kitchin, Lew Counts

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Another less serious but still significant error source is the so-called thermocouple effect, sometimes referred to as thermal EMF. This occurs when two different conductors, such as copper and metal film, are tied together. 2 mV/C may be produced. An example of these effects is shown in Figure 5-12. A final error source occurs when there is a thermal gradient across the external gain resistor. Something as simple as mounting a resistor on end to conserve board space will invariably produce a temperature gradient across the resistor.

5-7 Since device specifications on different data sheets often refer to different types of errors, it is very easy for the unwary designer to make an inaccurate comparison between products. Any (or several) of four basic error categories may be listed: input errors, output errors, total error RTI, and total error RTO. Here follows an attempt to list, and hopefully simplify, an otherwise complicated set of definitions. Input errors are those contributed by the amplifier’s input stage alone; output errors are those due to the output section.

Figure 5-11. A Typical Discrete 3-Op Amp In-Amp Using Large Value, Low TC Feedback Resistors Even when resistors with matched temperature coefficients (TC) are used, gain errors that vary with input signal level can still occur. , higher power) resistors will reduce these effects, but accurate, low TC power resistors are expensive and hard to find. 5-6 When a discrete 3-op amp in-amp is used, as shown in Figure 5-11, these errors will be reduced.

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