Analog input

Analog Input and Outputs Analog inputs are generally divided into two categories ... 

Multiplexer. This permits one A/D converter to service multiple analog inputs. The number of inputs to a multiplexer is usually between 8 and 256. 

Chromatographs can perform a total composition analysis for a sample. It is possible but inconvenient to provide an analog input for each component. Furthermore, it is often desirable to capture other information, such as the time that the analysis was made (normally the time the sample was injected). 

Load cells are capable of resolutions of 1 part in 100,000. A/D converters for analog inputs cannot even approach such resolutions. 

The resolution of the analog I/O channels of the controller vaiy somewhat, with 12-bit and 14-bit conversions quite common. Sample rates for the majority of the constant sample rate controllers range from I to 10 samples/second. Hard-wired single-pole, low-pass filters are installed on the analog inputs to the controller to protect the sampler from aliasing errors. 

Static performance measurements related to positioner/ac tuator operation are conformity, measured accuracy, hysteresis, dead baud, repeatability, and locked stem-pressure gain. Definitions and standardized test procedures for determining these measurements can be found in ISA-S75.13-1989, Method of Evaluating the Performance of Positioners with Analog Input Signals and Pneumatic Output . 

Analog inputs and outputs in data-based (DDC) controllers are usually standardized and record the signals from sensors or transducers as 0(2)-10 VDC or 0(4)-20 mA. The inputs module can also be standardized for resistances, as Pt 100 DIN, Pt 1000 or Ni 1000 DIN (Pr = platinum, Ni = nickel). 

Carpenter, G. A., and Grossberg, S., ART2 self-organization of stable category recognition codes for analog input patterns, Appl. Opt. 26, 4919 (1987a). 

Pressure measurements are made with four differential pressure transducers which produce a 0- to 5-VDC output signal from an unregulated 24-VDC Input. The range of three of the transducers 1s 0 to 500 psi and the other 1s 0 to 320 psl. The signal from these transducers 1s connected to an analog Input card 1n the HP 3497. 

Interface. The interface includes a 10-bit analog-to-digital converter, a ten-channel multiplexer, and eight contact sense lines. The converter will operate at rates up to 20,000 conversions per second with a precision of 1/1000 for analog inputs with either 10 or 100 mv full scale values. The contact sense lines are used to communicate elution dumps and sample injections by signaling the computer on a circuit closure. The interface, with eight contact sense lines and four active A/D channels, is capable of handling data from four GPC instruments simultaneously. 

Fig. 5.6. A block diagram of an optical coherence tomography/Raman spectroscopy system C, circulator RSOD, rapid scanning optical delay BP, 785 bandpass BSO, beam shaping optics DM1, dichroic mirror at 990 nm DM2, dichroic mirror at 800-950 nm LP, long pass at 808 nm GP, galvanometer pair BD, balanced detector BPF, electronic band-pass filter AI-AO DAQ, analog input-output data acquisition (reprinted with permission from [34]. Copyright 2008 Optical Society of America)...

A DSP system begins at the conversion from the analog input and ends at the conversion from the output of the processing system to the analog output as shown in the figure... 

The A/D board can reside either in the detector, an interface box, or in the computer. If the board is in the computer, it will have analog input terminals similar to a strip chart recorder or it will be connected to an interface box that will have this type of connector. If the A/D card is in the detector, the detector will have some type of digital interface port and a cable to connect it to the computer. In the personal computer world, once the signal is digitized outside the computer, it will be sent to the computer over one of three types of communication cables parallel, serial, or GPIB. 

Primary process measurement from an analog input (AI) block to the primary PID... 

Word-at-a-time conversion. This technique (flash converters) converts an analog input signal in a very short step (5 to 200 nsec) into digital form by comparing the input signal with all possible values at a time. Therefore a high number of comparators (2" 1, for a resolution of n bits) is needed to produce the result. As the number of comparators increases exponentially, this technique is limited to moderate resolution (typically 8 bit). 

Figure 9.12 Firedamp-proof PC workstation for installation in coat mines. The main components are fitted into a flameproof- d - Group I housing. The interfaces for PC keyboard and mouse are intrinsically safe as well as interfaces for 4-20 mA analogous inputs, RS 485 and FSK-PROFIBUS. An optical-electrical interface (Ethernet 10/100 Mbit) is available additionally.

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