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DI-145 Data Acquisition Starter Kit Developer's Diary

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1/4/2011 Note #8 Revision B Update: Crosstalk

Happy New Year!

Catching up on DI-145 developments early in the New Year, the new Revision B artwork arrived just before Christmas and the techs were able to assemble two working prototypes just before the break. One enterprising engineer took a look at it over the holiday and made the following determinations.

The malfunctioning channel four was not due to a soldering problem with the mismatched Atmel processor footprint as suspected (like I said, our techs are good). The problem is in the Atmel processor code, which is being debugged as I write this.

In the meantime, another Revision B enhancement was a beefier ground plane between analog channels. That change has had the desired effect of lowering the noise threshold of the design and minimizing crosstalk. With the first prototype, engineers found some lingering crosstalk between channels if they are left open, a source of continuous confusion for starter kit customers. As it always goes, “I connect a signal to channel one and I see it on channel two. My unit is defective.” Actually, it probably isn’t.

If channel two is left open in this example, what should you see? Not zero volts. That can only be established by shorting a channel’s input to ground. So, while seeing a duplicate of a channel connected to a signal source on adjacent channels that are enabled but left open may be peculiar, it’s no cause for alarm and it’s easily resolved by either disabling open channels through software or connecting them to a low impedance signal source or ground. Yet, this event is a leading subject of technical support questions from starter kit customers, which makes it a target for resolution in the new DI-145 design.

DI-145 Data Acquisition Starter Kit PCP Rev B
Figure A – A comparison of DI-145 revision A and B artwork. Revision B (right) added a ground pour in the areas indicated by “1” and “2” to bolster performance (click for a larger image.)

A major differentiating feature of the DI-145 from its predecessor is an amplifier per channel, which is required to resolve the crosstalk problem. Of course, the higher the amplifiers’ input impedance, the better. So far, barring difficulties that have yet to be seen, the DI-145 design sports 2 MΩ total differential input impedance (1 MΩ for each differential input to ground). However, this high impedance made each channel more susceptible to adjacent channel activity than anticipated, since AC signals can and do couple between channels using the parasitic capacitance of the printed circuit board. The solution to this problem was a ground pour between channels (see the area labeled “1” in Figure A). For good measure, a ground pour was also applied to the USB interface area of the PCB (labeled “2” in Figure A) to minimize the chance of spurious noise that results in USB interface disruption.

The next step is a thorough shake-down of all the DI-145’s features. Please stay tuned.

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