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Guide to using WEBENCH Sensor AFE Designer

31 Jan 2013  | Shrikrishna Srinivasan

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The operating sensor temperature is specified along with device error. Of course, these parameters are specific to the sensor AFE being used. You can change the register settings by clicking on the corresponding block in the block diagram, which opens up a window with the current register settings as shown in figure 3. You could also use the register map which lists all the registers at once (figure 4).

Figure 4: Here's a list of all of the registers at once.

When gain = 1X, and sample rate = 26.8 SPS:

Then noise = 2.90µVrms, ENOB = 21.5 bits, device error @ 22°C = 0.385%

When gain = 8X, and sample rate = 26.8 SPS:

Then noise = 1.08µVrms, ENOB = 20 bits, device error @ 22°C = 0.018%

Selecting multiple input channels is supported by the software since the LMP90100 offers this feature. Adding a new sensor automatically selects its input pins with the goal of maximising the number of sensors that can be selected. This can be achieved using a single-ended configuration whenever possible. Typically, every sensor attached to the sensor AFE requires a different signal path configuration. Since each input channel can be independently configured, gain, sample rate and sensor diagnostics can be set for each sensor connected independent of the others. Next, let's add a pressure sensor channel to the sensor AFE. As with the RTD, let's start with the default configuration.

When gain = 1X, sample rate = 214.65 SPS:

Then noise = 8.25µVrms, ENOB = 20 bits, device error @ 1.45 SPI = 0.797%

When gain = 8X, sample rate = 214.65 SPS:

Then noise = 2.96µVrms, ENOB = 18.5 bits, device error @ 1.45 PSI = 0.0599%

The tool also has a help bar to walk you through the device configuration process. The performance data displayed belongs to the currently selected channel. A different channel can be selected by clicking on another sensor in the list of selected sensors (listed below the SELECT/REMOVE sensor buttons).

Once the registers are configured, you can save the register map as an XML file by clicking on the SAVE button. The downloadable offline tool can be downloaded and the evaluation board/SPIO-4 USB interface board can be purchased from the page presented when you click on the CHECK-OUT button. In addition to all the capabilities of the online design tool, the offline tool also has the ability to communicate with the evaluation board through the SPIO-4 interface board and can be used to evaluate the sensor AFE device on the bench. The saved configuration register map from the online design tool can be uploaded into the offline tool to pre-configure the device, thus saving time and effort.

Chip evaluation
Data capture and static evaluations are achieved by connecting the SPIO-4 digital controller board (SPIO-4 board) to a PC via USB, and running the offline sensor AFE software. The data capture board generates the SPI signals to communicate with the LMP90100. The board driver and instructions to download the driver are included in the software folder. Once the configuration file is uploaded into the offline tool, it's essential to write all the registers to the board to ensure it's set to the desired configuration. The stage is now set for real-time data capture!

Data can be captured by clicking on the RUN button in the measurements tab as shown in figure 5. There are two modes of data capture supported by the tool. Single-shot-mode allows you to specify the amount of data you want to capture. Alternatively, you can use the run continuously mode, which acts like an oscilloscope. In other words, it continuously captures data until the process is manually stopped. Output format can be viewed in counts or in the sensor's output voltage, or in units of data being measured. Status flags are displayed as well. Finally, data can be exported to a csv file for further analysis. For more details, please refer to the software user guide in the software folder.

Figure 5: Capturing data is as easy as clicking RUN.

Summary
With products optimised for specific types of sensors, this sensor AFE platform delivers an alternative to long hardware development cycles. Unique register settings can be specified for each sensor signal path. Configuration trade-offs can be considered based on performance, power, and background calibration. The combination of sensor AFE ICs, WEBENCH Sensor AFE Designer software, and hardware development platform creates a design environment that enables sensor signal path design. Overall design time is touted to be dramatically reduced, which allows customers to get to market faster with a solution.

About the author
Shrikrishna Srinivasan is a software engineer for TI's precision analogue group where he is responsible for developing software, such as sensor AFEs using JAVA and FLASH development platforms. Shrikrishna received his MSEE from Santa Clara University, California.

To download the PDF version of this article, click here.


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