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Sensor signal conditioning need not be difficult (Part 2 of 2)

In Part 1 of this two-part article, we looked at the basic operation of the Wheatstone bridge-based pressure sensor and processing circuitry for interpreting the bridge sensor output, including offset and gain correction. In Part 2 , we will introduce and discuss a new on-line tool that can be used to design the signal conditioning circuitry for these types of sensors.

While the approach to sensor interface design certainly seems straightforward enough, those who have designed or attempted to design these circuits know that it is not as simple as it might seem. However, this task just became a lot easier with an on-line tool for designing the conditioning circuitry for various sensors.

WEBENCH® Sensor Designer is a simple, easy to use tool that assists in the selection of amplifiers and ADCs and in the design of the final circuit for amplifying and digitizing the output of pressure sensors, load cells, thermocouples, thermopiles and photo sensors, together with estimated performance parameters. Circuit boards are also available to confirm the functioning of the final circuit. It provides circuit solutions for common sensing needs, allowing the rapid progression of the design from concept, to development of a Bill of Materials (BOM) through evaluation and on to prototyping.

The user may start with a sensor from a list of those provided by market leaders, or may specify his/her own sensor by its parameters. The WEBENCH Sensor Designer tool provides an estimate of signal-path performance, BOM, budgetary costs, and links to reference boards for selected sensing applications to test and validate the solution.

The basic procedure for using this tool begins with the user selecting or defining the sensor, after which the tool will develop a circuit for conditioning and digitizing the sensor output, producing a schematic, BOM and estimated performance parameters. The user examines the estimated performance parameters and either accepts the circuit or can choose another sensor or define a custom sensor. He/she can also choose other amplifiers and/or another ADC, then again examine the estimated performance parameters. Once the user determines that the estimated performance is within his/her needs, the design can be checked out with a reference board that is readily available. In most cases, the reference board can be easily modified to match the WEBENCH design.

A walk through the design process will indicate the simplicity and ease of use of this valuable tool. Figure 3 shows the appearance of the screen after the tool is opened at http://www.national.com/analog/webench/sensors/.



Figure 3. The Sensor Designer assists in designing circuitry for three basic sensor types.

(Click on image to enlarge)

The first step is to select the type of sensor to be used: Bridge Pressure Sensor, Thermocouple, or Photo Sensor. The Bridge Pressure Sensor Designer can be used for load cell circuit design and the Thermocouple Designer can be used for thermopile circuit design. For purpose of illustration we will use the Bridge Pressure Sensor Designer.

Selecting the “Pressure Sensor” button at the above-mentioned URL and clicking on “Start Design,” or clicking on the green “Pressure Sensor” button brings us to a page with a number of pressure sensors. As previously mentioned, if the sensor is not there, the user may create a custom sensor from one closest to the desired sensor. The desired sensor is then selected, after which the “Create Design” button is pushed, revealing a page similar to that of Figure 4 .



Figure 4: Design Summary of Sensor Webench® design

(Click on image to enlarge)

Clicking anywhere in the Operating Values, Performance area will provide detailed information on the design, including voltages, power requirements and error analysis. The System Values area allows the user to change some of the operating values, including ADC reference voltage, ADC resolution, resistor tolerances, filter bandwidth, operating temperature range, and the sensor used.

Clicking anywhere on the Schematic area or on the BOM area will reveal a circuit schematic, similar to that shown in Figure 5 , and a circuit BOM.



Figure 5. Circuit schematic as shown in the WEBENCH Sensor Path design for bridge sensors.
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(Click on image to enlarge)

The BOM may be sorted, in ascending or descending order, by clicking on the name of the column by which the BOM items are to be sorted. Similarly, clicking on the Build It, Documentation area will open a panel with links to design tips, information on the products in the design (including datasheets) and available reference boards. From here the user can order a blank board to be assembled with components in the generated BOM. At any time, changing anything in one of these panels will change the design, and all of the other panels are immediately updated.

Once the WEBENCH Sensor Designer indicates the desired performance, the initial phase of the design is completed and the verification stage can begin. National Semiconductor has reference designs available that can greatly help with this effort. Figure 6 is an illustration of a fully assembled Differential Sensor Board that is suitable for pressure sensors, load cells, and other bridge-based sensors.



Figure 6: Reference designs such as this Differential Pressure Sensor Board greatly ease the verification process.
(Click on image to enlarge)

The reference board designs closely match those of the WEBENCH Sensor Designer and are designed for use with version 2 of National Semiconductor's Universal Serial Interface Board, (USI-2). While the USI-2 board greatly simplifies data capture and uploads conversion results to a computer (via USB), it is not a requirement to use the USI-2 board with the reference boards.
When a reference board is used with the USI-2 board and the provided sensor software, offset and full-scale errors of the entire sensor and interface circuit can be calibrated, if the user so chooses. Figure 7 shows the complete system connected to a laptop computer.



Figure 7 The reference designs are small, easy-to-use boards.

(Click on image to enlarge)

This article examined the basic operation of a Wheatstone Bridge-based pressure sensor and its processing circuitry. And a new online sensor design tool, called WEBENCH Sensor Designer, showed how to simplify design by quickly moving the user through the concept, simulation and prototyping stages of sensor design, including the signal conditioning circuitry. Configuring a complete sensor signal-path design can now be as easy as making a few keystrokes with your PC.

About the Author
Nicholas “Nick” Gray is a Staff Applications Engineer with National Semiconductor's Data Conversion Products Group. Nick's career includes video circuit design and over 30 years as an applications engineer, mostly for data converter products. He received his BSEE degree from Gonzaga University, Spokane, Washington in 1965 and has done graduate work at California State University, San Jose, California.

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