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Unread 30-01-2010, 23:20
EricVanWyk EricVanWyk is offline
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Re: How Precise and Accurate is the Analog In on the Crio?

Aargh! I was so close to going to sleep after a long day of packing my life into my tiny car! Any insanity in this post is directly attributable to the ratio between my enormous pile of life and my tiny 2 door honda.

Before I get all grumpy with numbers, let me start by saying that that is a really cool idea. I hope to see it at competition.

The analog input module (9201) has 12 bits across -10 to +10V. This is just under 5mV per count ( (10 - -10)/2^12 ). With the right conditions, I'd wager that you could get another 2 (3??) bits of resolution with oversampling. I'd stick it at 4bit oversampling and assume the last bit is garbage. This would put you into the mV per count range.

Considering most strain gauge outputs, I'd give up here and slap a pre-amp on it.

If you were watching me closely, you noticed that I swept a huge pink elephant under the rug about halfway through the last paragraph - what does "with the right conditions" mean? Well, it means a two things:
1) No irregular noise like spikes or pops from your motors.
2) Enough small noise to let over-sampling work.
3) A good understanding of exactly how your sensor behaves.

I'll let someone else chime in on one and three, but I'll rant about two for a moment. Oversampling assumes an additive noise signal that pushes the perceived value around the threshold. I like to imagine it thusly: I have a ruler that is only capable of measuring something to the nearest inch, but I need it to the nearest quarter inch. If I measure it a bunch of times, I'll still only get it to the nearest inch. However, I repeatedly randomly offset the ruler by some amount and average all the measurements. As long as my random offsets average out to zero their impact averages out to zero. BUT, they did allow me to get better resolution, because they "unstuck" my ruler.


PS: Joe Hershberger is way more of an expert on this than I am.

PPS: Don't even try this with a 2009 Analog Breakout. The 2010 Analog Breakout is much cleaner, and stands a chance of getting you what you need.

PPPS: You should be sure to graph your measured value in known steady state while driving. That will tell you everything you need to know about feasibility.
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