Instrumentation amplifier accuracy

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. 

In the single-beam instrument, photometric accuracy to a great extent is dependent on the precise linearity of the electronic amplifiers, as well as on the stability of these amplifiers between the time that the 100 % level is established, the time that the zero line is checked, and the time during which the spectrum is obtained. Another factor is the presence of atmospheric absorption bands or solvent bands in the vicinity of the band undergoing a quantitative measurement. This is one of the inherent limitations in the accuracy of single-beam systems which is subject to some control by the analyst. 

The flow capacity of the transducer can be increased bv adding a booster relav like the one shown in Fig, 8-7.3/ , The flow capacity of the booster relav is nominally fiftv to one hundred times that of the nozzle amplifier shown in Fig, 8-7.3 3 and makes the combined trans-diicer/booster suitably responsive to operate pneumatic actuators. This type of transducer is stable into all sizes of load volumes and produces measured accuracy (see Instrument Society of America [ISA]-S5l, 1-1979, Process Instrumentation Terminology for the definition of measured accuracy) of 0,5 percent to 1,0 percent of span. 

Electronic instrumentation is available for the measurement of D.C. and A.C. voltage, current and power as well as impedance. Such instruments usually have higher sensitivities, operating frequencies and input impedance than is normally found in the electromechanical instrumentation described above. However, they may need to incorporate amplifiers and they invariably need power to operate the final display. Hence, an independent power source is needed. Both mains and battery units are available. The accuracy of measurement is very dependent on the amplifier, and bandwidth and adequate gain are important qualities. 

Unlike the photoplate, the Faraday detector (or Faraday cup) is still very much in use today. The main reasons for its lasting popularity are accuracy, reliability, and mgged construction. The simplest form of Faraday detector is a metal (conductive) cup that collects charged particles and is electrically connected to an instrument that measures the produced current (Fig. 2.21b). Faraday cups are not particularly sensitive and the signal produced must in most applications be significantly amplified. An important application for Faraday detectors is precise measurements of ratios of stable isotopes [278]. See, for example, Section 2.2.7 and Chapter 11 for examples of applications and methods in which Faraday detectors are utilized. 

This technique is of high accuracy and is meant to be used in precision measurement instrumentation, for it is inherently insensitive to the DC-offset and the AC-noise in the sinusoidal signal which can be substantially reduced by a great variety of electronic devices ranging from various electronic analogue filters, and digital filters to the most effective lock-in amplifiers. 

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. 

Since this is a book concerned primarily with applications, no further details are given concerning instrumentation. The reader is referred to Alpert et al. (1970), in which are discussed an optical diagram of a double-beam spectrophotometer operating variables (resolution, photometric accuracy) components of infrared spectrophotometers (sources, types of photometers, dispersing elements, detectors, amplifiers, and recorders) special operating features, such as optimization of scan time and available instruments and their specifications. The books by Martin (1966), Conn and Avery (1960), and Potts (1963), and the chapter by Herscher (1966) are also recommended for details on some of these topics. 

The second block of the circuit is a high accuracy instrumentation operational amplifier that makes both sensors output signals Vout 1, Vout 2 difference. Two potentiometers enable respectively offset and gain adjustment. The DC offset adjustment generates an output signal equal to OV when the system detects the zero in the XY position of the platform. The maximum D(3 voltage adjustment which is proportional to the platform movement is made by means of the gain adjustment. 

The latroscan FID scanner unit consists of a hydrogen flame jet and an ion collector. Following the sample application and solvent development, a set of ten rods are scanned and burnt, the ions are captured by the collector electrode and the signal is amplified in a similar way as in gas chromatography (GC). The more recent versions of the instrument (Mark IV and V) have an improved detector system (Oshima and Ackman, 1991) in which the collector and a circular electrode are close to the rod being scanned. These improvements minimize losses of ions and increase the sensitivity of the instrument. The reproducibility and accuracy of the TLC/FID method relies strongly on the standardization of all the operational parameters. Thus it will be useful and necessary to consider the various factors that affect the response of the flame ionization detector of the latroscan system. 

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