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DI-145 Data Acquisition Starter Kit Developer's Diary

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2/3/2011 Note #12 Behind the scenes

Until now this series of developer notes has dealt only with DI-145 product design issues, from printed circuit boards and IC footprints to individual component issues and firmware. With this installment we’ll look at what happens in the background behind the fast and furious world of new product development engineering.

Of course, products need to be tested and calibrated before they’re delivered to customers. When a product has the expected high volume and low margin of a low cost starter kit it’s crucial that this process be as efficient as possible. To meet this requirement with the DI-145, fixtures need to be designed that allow a calibrator/tester to simply plug the DI-145 into it, press a button, and let a connected computer quickly do the heavy lifting. Here’s what has to happen:

  1. The mechanical aspect of the fixture needs to accommodate the DI-145's form factor, while allowing contact with its analog and digital inputs, the USB interface connector, and the event pushbutton.

  2. The fixture needs to incorporate an interlock mechanism that ensures that it’s impossible to expose personnel to the ±169 VAC that will be present on the analog inputs of the DI-145 during its tolerance test.

  3. The fixture needs to incorporate a precision signal source, digital outputs, and an interface to the controlling computer to properly calibrate the DI-145’s analog channels and test its digital inputs. Analog channel calibration is done electronically by calculating m and b scaling constants per channel and writing them to memory locations inside the DI-145. The instrument’s firmware will access and apply these to raw ADC conversions to report accurate measurements during operation. Also, the instrument’s digital inputs will be exercised to ensure that they operate properly.

The test fixture consists of three components: a frame mounted to a printed circuit board and a clamp to firmly hold the DI-145 in place during calibration and testing. The frame is constructed of wood, since it’s easy to machine and has excellent insulating properties. The printed circuit board is mounted to the wooden fixture and incorporates spring-loaded contact points for each of the DI-145’s inputs. The springs ensure consistent and reliable contact with the instrument during calibration and test.

We won’t know exactly how long the testing and calibration process will take until controller software is merged with the fixture and a DI-145, but the vision is to calibrate and test a DI-145 in mere seconds.

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