Analog-to-Digital Interfacing

In the real world, changes are continuously analog variations with time. Temperatures rise and fall. Instrument parameters, such as signal output voltage, increase and decrease continuously. In the computer world, changes are discrete steps computers process data using a two-value alphabet, 0 and 1. The next value may or may not be related to the one before it. 

An integrator of a computer usually requires a stronger analog signal from the detector, either 0-1 or 0-10 V, but the output is of the same form. As the signal reaches the A/D board, it is sampled for a specific length of time and 

A Practical User s Guide, Second Edition, by Marvin C. McMaster Copyright 2007 by John Wiley Sons, Inc. 

The frequency of this sampling is defined in samples per second. Most A/D boards can sample at about 30,000 points/sec, while an HPLC signal requires sampling at no more than 10 points/sec. The other controlling variable is the size of the output digital number. A typical A/D board can only process a number up to 65,000 to process the full range of detector outputs, at least a 12-bit board is required and many boards use a 16-bit data path that allows a word size that can handle numbers up to 1 million. 

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McConnell, M., Canales, M., and Lawler, G., A validation protocol for analog-to-digital interfaces in chromatographic data systems, LC-GC, 9, 486, 1991. 

Automatic oxygen titration units are commercially available, e.g., the Metrohm 665 Dosi-mat Oxygen Trtrator, which consists of a remote controlled burette and a UV detector for customer designed bottles. An analog-to-digital interface is required to connect the unit to a computer. 

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. 

A test system, controlled by personal computer (PC), was developed to evaluate the performance of the sensors. A schematic of this system is shown in Figure 3. The signals from the sensors were amplified by a multi-channel electrometer and acquired by a 16 bit analog to digital data acquisition board at a resolution of 0.0145 mV/bit. The test fixture provided the electrical and fluid interface to the sensor substrate. It contained channels which directed the sample, reference and calibrator solutions over the sensors. These channels combined down stream of the sensors to form the liquid junction as shown in Figure 1. Contact probes were used to make electrical connection to the substrate. Fluids were drawn through the test fixture by a peristaltic pump driven by a stepper motor and flow of the different fluids was controlled by the pinch valves. 

Results of on-line, real-time instrumental analyses are posted directly to the database, with or without processing, as soon as the instrument presents the analytical data. All results are available for review and validation immediately after being posted -a feature critical to effective laboratory management. Data is acquired from laboratory instrumentation of virtually any manufacturer or function. Instruments are interfaced to the computer via analog to digital conversion, RS-232C, current loop, IEEE-4888, binary coded decimal (BCD) or bit parallel techniques. 

Analog to digital conversion is performed by an interface operating at up to sixty readings per second in the +/-10 volt input range and is capable of resolving 0.3 microvolts. 

Computer control of the apparatus is obtained by a PC LAB 812 PG interface card that provides the input and output control signals, performs analog-to-digital (A/D) conversion, and clocks the data acquisition rate. The three main parts of the SCC are ... 

B. E. Boser, in RE Analog-to-Digital Converters Sensor and Actuator Interfaces Low-Noise Oscillators, PLLs and Synthesizers, W.M.C. Sansen, R.J. van de Plas-sche, J.H. Huijsing (eds.), Kluwer Dordrecht, 1997. 

The measurement data are not only provided continuously in time, but they are also analog electrical signals in nature. They cannot be used directly by the control program, which requires data in a digital form (e.g., information coded in 16-bit words, for a 16-bit word machine). Therefore, the input interface should contain an analog-to-digital converter (ADC or A/D converter). 

Describe in physical terms the conversion of a signal from analog to digital, and vice versa. Why are these conversions needed in the I/O interface ... 

Process control systems traditionally use an analog signal of 4 to 20 mA DC and so it is recommended that such signals should be interfaced with the MCUs by use of compatible high-speed analog-to-digital converters. 

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