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ADC, Accelerometer and capacitors — Parallax Forums

ADC, Accelerometer and capacitors

JavalinJavalin Posts: 892
edited 2007-10-04 15:08 in Propeller 1
Hello all,

Getting confused with the above componants - mainly with the wiring up of.

I have·a LIS tripple axis accelerometer which provides 0-5v for each axis, connected to a MCP3208 8 channel 12-bit SPI ADC chip.

I have seen various schematics (i.e. http://www.sparkfun.com/datasheets/Sensors/ADXL3xx-Breakout.pdf·- from http://www.sparkfun.com/commerce/product_info.php?products_id=692·product) - which show a low value cap to ground.

Also seen on·the ADC manual last night - its got some schematics on the sheet showing the same low voltage cap to ground - and much talk about sampling and corner frequency's.

(MCP3208 manual here - http://www.microchip.com/stellent/idcplg?IdcService=SS_GET_PAGE&nodeId=1335&dDocName=en010534)

Can anybody explain this to me?

Gratefull as always for help!
James

Comments

  • LeonLeon Posts: 7,620
    edited 2007-10-04 09:49
    The capacitors on the ADXL outputs set the bandwidth (0.5 Hz to 2.5 kHz). See the data sheet.

    Leon

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  • JavalinJavalin Posts: 892
    edited 2007-10-04 09:55
    Yes I see that - but I dont understand what it means, and what the implications are....

    J
  • LeonLeon Posts: 7,620
    edited 2007-10-04 12:16
    If you are measuring vibration, the signal will have a certain bandwidth. You need to be able to adjust the accelerometer bandwidth to suit the signal bandwidth, to minimise noise.

    Leon

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  • JavalinJavalin Posts: 892
    edited 2007-10-04 12:41
    what if I am measuring acceleration....?

    J
  • LeonLeon Posts: 7,620
    edited 2007-10-04 13:25
    You still need to set an appropriate bandwidth. There is a trade-off between noise and bandwidth that you have to optimise for a particular situation.

    Leon

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  • lnielsenlnielsen Posts: 72
    edited 2007-10-04 14:49
    You most likely want a lower bandwidth for acceleration. How often are you planning to poll the unit for readings? With a high bandwidth, the reading is going to change very rapidly based on any vibration the unit is sensing. You would need to poll the unit several times and average the reading to get the acceleration. If you don't care about vibration then a lower bandwidth, slightly higher than your polling rate, would be best.

    If you are breadboarding - I suggest you write a little program to poll and display as quickly as possible and then try various capacitor values to see how this affects the readings.

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  • JavalinJavalin Posts: 892
    edited 2007-10-04 15:08
    Hi Inielsen,

    upto 20htz probably - maybe oversampling (averaging)·x2 or x3 .... its an ASM routine so can be sampled very quickly.

    Done a bit of reading up and it makes more sense now - a good link is

    http://www2.egr.uh.edu/~glover/applets/Sampling/Sampling.html

    Currently its a SMD chip on a PCB - with no cap's.

    I see SparkFun have a 0.1uf cap on their's - is this likely to be a "standard" ish value for removing high-freq signals?

    J
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