Instrumentation amplifier nonlinearity

The most important thing to do is to scrutinize more specifications than the resolution of the AID converter that is used on the DAQ board. For DC-class measurements, you should at least consider the settling time of the instrumentation amplifier, differential non-linearity (DNL), relative accuracy, integral nonlinearity (INL), and noise. If the manufacturer of the board you are considering does not supply you with each of these specifications in the data sheets, you can ask the vendor to provide them or you can run tests yourself to determine these specifications of your DAQ board. 

Such single-mode lasers, often pulse amplified by dye laser amplifiers pumped by injection-locked Nd YAG lasers, are used in nonlinear Raman techniques by which an instrumental resolution better than 0.001 cm is achieved (Esherick and Owyoung (1982), Schrotter et al. (1988a)). 

A comprehensive overview of frequency-domain DOT techniques is given in [88]. Particular instraments are described in [166, 347, 410]. It is commonly believed that modulation techniques are less expensive and achieve shorter acquisition times, whereas TCSPC delivers a better absolute accuracy of optical tissue properties. It must be doubted that this general statement is correct for any particular instrument. Certainly, relatively inexpensive frequency-domain instruments can be built by using sine-wave-modulated LEDs, standard avalanche photodiodes, and radio or cellphone receiver chips. Instruments of this type usually have a considerable amplitude-phase crosstalk". Amplitude-phase crosstalk is a dependence of the measured phase on the amplitude of the signal. It results from nonlinearity in the detectors, amplifiers, and mixers, and from synchronous signal pickup [6]. This makes it difficult to obtain absolute optical tissue properties. A carefully designed system [382] reached a systematic phase error of 0.5° at 100 MHz. A system that compensates the amplitude-phase crosstalk via a reference channel reached an RMS phase error of 0.2° at 100 MHz [370]. These phase errors correspond to a time shift of 14 ps and 5.5 ps RMS, respectively. 

Instrumental Factors. Unsatisfactory performance of an instrument may be caused by fluctuations in the power-supply voltage, an unstable light-source, or a nonlinear response of the detector-amplifier system. A double-beam system helps to minimize deviations due to these factors. In addition, the following instrumental sources of possible deviations should be understood ... 

Most transducers converting chemical concentration into an electrical signal have a nonlinear response for example, electrode potential and optical transmission are not directly proportional to concentration. In general, this nonlinearity is easily and simply corrected in equilibrium analytical measurements. However, it is considerably more difficult to instrumentally correct the response-versus-concentration function in reaction-rate methods, and often the correction itself can introduce significant errors in the analytical results. For example, the simple nonlinear feedback elements employed in log-response operational-amplifier circuits are not sufficiently accurate in transforming transmittance into absorbance to be used for many analytical purposes. 

The applications of Fourier analysis are extensive in the electronics and instrumentation industry. One typical application, the computation of total harmonic distortion (THD), is described herein. This application provides a measure of the nonlinear distortion, which is introduced to a pure sinusoidal signal when it passes through a system of interest, perhaps an amplifier. The root-mean-square (rms) total harmonic distortion (THDrms) is defined as the ratio of the rms value of the sum of the harmonics, not including the fundamental, to the rms value of the fundamental. 

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