Building a Smart Laboratory 2015 Type of A/D


Successive approximation


Capability


These are general-purpose devices suitable for a wide range of applications. They have limited resolution, but have amplifiers for low-level signals, and can sequentially access multiple input channels. Their resolutions are up to 18 bits (262,144 steps) and sampling speeds of up to five million samples per second (sps). The higher the resolution, the slower the sampling speed.


Integrating


Good for low speed sampling (<100 sps), high resolution >14 bits, single channel inputs, with good noise rejection. Often used in chromatography.


Sigma-Delta A/D


Up to 24 bits of resolution, single channel input – may not be efficient for multi- channel inputs, low speed, may replace integrating A/Ds.


Flash


Single channel input, 8-bit conversion, approximately 1 billion SPS. Good for very high-speed applications, where low resolution is not a problem. You can digitise electrical noise.


the true value of analogue measurement is never known, just the nearest digital approximation. How close that estimated value is depends on the type of A/D in use. Of the technologies available, this chapter will focus on four of them. Soſtware is needed to use these devices.


Maximum performance will be gained through the use of compiled soſtware, requiring the skills of competent soſtware developers. Most equipment vendors will provide soſtware libraries to support their hardware. For applications where high performance isn’t an issue, graphical icon-based systems can allow the developer to construct a block diagram of the application. Each block has a specified function as well as defined inputs and outputs. Te device functions as a virtual instrument, executing a running program with graphical displays and inputs. Time-critical measurements are done


with the support of programmable interval timers that can be set to notify the system when samples need to be taken. Each time


www.scientific-computing.com/BASL2015 11


the timer interval time expires, the A/D takes measurements from one or more channels. Taking data at precise intervals is essential for most data processing applications, as variations in timing can distort the results. Tis is becoming a problem with modern commercial operating systems. Earlier operating systems, such as DOS


and early versions of Window and the Mac OS, provided basic functions and pretty much kept out of the way when user programs were operating. Current versions, with multi-tasking support, internet access, periodic updates and social media make it difficult to guarantee time- critical functions – particularly at high rates of data collection. Tuning the OS can provide some capability, but most vendors have taken another approach: off-loading time-critical data collection to specialised networked devices that communicate to the host system via serial, USB, and Ethernet connections.


Simple laboratory instruments


As noted above, devices such as analytical balances and pH meters use low-level processing to carry out basic functions that make them easier to work with. Te tare function on a balance avoids a subtraction step and makes it much easier to weigh out a specific quantity of material. Connecting them to an electronic lab notebook (ELN), a laboratory information management system (LIMS), a lab execution system (LES), or a robot, adds computer-controlled sensing capability that can significantly off-load manual work. Accessing that balance through an ELN or LES permits direct insertion of the measurement into the database and avoids the risk of transcription errors. In addition, the informatics soſtware


Fig. 2: Analogue data acquisition Display


Property to be


measured (detector)


Electrical circuit


converting properly to voltage


A/D


Data: Instrumentation


can catch errors and carry out calculations that might be needed in later steps of the procedure. Te connection between the instrument


and computer system may be as simple as an RS-232 connection or USB. Direct Ethernet connections or connections through serial-to- Ethernet converters can offer more flexibility by permitting access to the device from different soſtware systems and users. Te inclusion of smart technologies in instrumentation significantly improves both their utility and the labs’ workflow.


Computerised instrument systems


Tat improvement in workflow becomes more evident as the level of sophistication of the soſtware increases. It is rare to find commercial instrumentation that doesn’t have processing capability either within the instruments’ packaging or, through a connection to an external computer system. Tis offers more flexibility and access to a wider array of resources, including data storage and an improved user-interface. Tis is particularly true for instruments that collect multi- dimensional data such as chromatography, spectroscopy, and thermal analysis. External computer system/soſtware applications that support lab instruments come in different forms: dedicated systems; and multi-user, multi-instrument programs. Dedicated systems are useful for one


particular instrument, or when the instrument requires closely managed command/control such as that required by hyphenated techniques. It may also be a case of the vendor only supplying the soſtware in a dedicated form, which is less costly to develop and support.


Control processor


Communications


Product packaging


Digital I/0 (switches, LEDs, etc.)


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