Quote:
Originally Posted by kamocat
Our analog inputs on the cRIO have a range from -10v to 10v.
Our accelerometer has an effective range from 0.6v to 2.4v
That's 9% of the full range, which means you get 9% of the full accuracy.
I was considering ways of rescaling it, and I was thinking a linear regulated 10v supply, a -10v supply, and an LM386 would be a good way to do it.
However, before I go out and buy anything, i wanted to check with you guys. Is there a simpler way to do this? Is there something I'm overlooking?
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I think the easiest thing to do is to use over-sampling. Are you really sure you need to throw hardware at it? If you are sure, there are circuits that remap one voltage range to another. I've done this for the day-job in a couple places; I have a MATLAB script I can send if you are interested.
I looked into doing something like this for the Analog Breakout. I ran into three major problems:
1) Cost.
2) Supplies.
3) Quantization vs Accuracy noise.
Cost:
Since you are one-offing this, I wouldn't worry about it too much. I couldn't design anything useful that was cost-effective in bulk. FIRST has a way different budget than medical products do.
Supplies:
To remap, you need rails that at least cover the range you are aiming for. Creating a clean (-10:10)V rail or even a (0:10)V rail on the robot is difficult, as it requires a boosting switcher of some sort. Boosts are really noisy on the output, and inverters aren't pretty either.
You could nudge the 24V rail up to 28V, filter it, and use a pair of precision LDOs to create the (-10:0:10V). The down-side is that this 0V wouldn't be ground referenced, which would cause serious issues if NI hadn't been so nice to isolate all of their cRIO modules.
Quantization vs Accuracy noise:
Here is the meat of the issue. You are recovering up to 3.4 bits of quantization, but you adding accuracy and high-frequency noise. 9% range on a 12bit ADC leaves 8.6 functional bits.
So the question is now "What is important to you?"
If you are only concerned with relative values, you are in good shape.
If you are concerned with the precise DC value, be careful. If the error exceeds .25%, it is a net loss. This includes all of the errors caused by the op-amp, the resistor dividers, whatever biasing reference you are using, and (possibly) the power supply rails. A lot of this can be compensated for by calibration, but that is a pain in the neck.