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creating a RC filter.. I don't understand what Freescale is saying — Parallax Forums

creating a RC filter.. I don't understand what Freescale is saying

evergreenevergreen Posts: 43
edited 2010-04-17 21:01 in BASIC Stamp
Hey guys,

I have a pressure sensor, MPXA4115A, that I would like to filter (by hardware and software). This thread will be about filtering via hardware.

My A/D converter of choice will be the LTC1298 (12-bit, sold by Parallax, lots of documents and sample coding)

According to Freescale's tips (in this document, http://www.freescale.com/files/sensors/doc/app_note/AN1646.pdf), they have two examples of filter circuits that could be build (Figures 2 and 3 on page 2).

One paragraph on page 2 says (in deciding which of the 2 ciruits to use),
"Some A/D’s will not work well with the source impedance of
a single pole RC filter. Please consult your A/D converter
technical data sheet if input impedance is a concern. In
applications where the A/D converter is sensitive to high
source impedance, a buffer should be used."

So my question is, with the LTC1298, which circuit should I use?

Thanks

Comments

  • Beau SchwabeBeau Schwabe Posts: 6,568
    edited 2006-04-16 19:19
    evergreen,

    Looks like you don't need a 'buffer" in your application.

    Reference: (Page 17 figure 9)
    www.linear.com/pc/downloadDocument.do?navId=H0,C1,C1155,C1001,C1158,P1445,D2760

    ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
    Beau Schwabe

    IC Layout Engineer
    Parallax, Inc.
  • evergreenevergreen Posts: 43
    edited 2006-04-16 21:48
    Okay, sweet. Thank you very much.

    One more question though,

    In Figure 2 on page 2 of http://www.freescale.com/files/sensors/doc/app_note/AN1646.pdf

    What are those 4 arrows supposed to connect to? I'm guessing the left 2 are for 5v, the next one is for ground, and the fourth one is also for 5v?

    Thanks
    Matt
  • Beau SchwabeBeau Schwabe Posts: 6,568
    edited 2006-04-17 02:31
    I believe all 4 of the arrows pointing down go to ground. The one arrow pointing up goes to +5V

    ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
    Beau Schwabe

    IC Layout Engineer
    Parallax, Inc.
  • dexfdexf Posts: 7
    edited 2010-04-17 21:01
    evergreen,

    I used the 4115 sensor in an altimeter project. Take a look at:

    http://www.parallax.com/tabid/818/Default.aspx

    This sensor is quite straightforward to use. The altimeter was looking at very small changes
    in pressure so there is a bunch of gain that you probably won't need. Pins 4,5 and 6 are
    no connects. Don't hook anything to them. The RC filtering is to smooth out noise that is
    inherent in the sensor. But you can get the·idea from this design.

    Hope this helps.

    Dex

    Post Edited (dexf) : 4/17/2010 9:27:08 PM GMT
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