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Info on mV/V signal processing?

Discussion in 'Electronic Design' started by Glenn Ashmore, Feb 2, 2004.

  1. Anyone know of a site where can I read up on processing mV/V signals
    from various sensors?


    --
    Glenn Ashmore

    I'm building a 45' cutter in strip/composite. Watch my progress (or lack
    there of) at: http://www.rutuonline.com
    Shameless Commercial Division: http://www.spade-anchor-us.com
     
  2. Glenn Ashmore wrote...
    Strain gages? I had good results with the scheme
    of using moderate-level (e.g. 1V) ac excitation,
    ac-amplifying and phase-detecting the resulting
    bridge-imbalance ac signal. Or alternately, you
    can integrate with one dc excitation polarity
    and then flip the polarity and deintegrate. :>)

    Both approaches are insensitive to dc drift and
    to most interfering ac signals, despite the small
    signal size. Now, to find a commercial vendor
    who uses them?

    Thanks,
    - Win

    whill_at_picovolt-dot-com
     
  3. Jim Thompson

    Jim Thompson Guest

    I did some AC designs for Talos Systems eons ago, but they seem to
    have vanished. Does anyone know who may have acquired them?

    ...Jim Thompson
     
  4. Actually mostly strain gauge based pressure gauges. Your answer
    reinforces my opinion that I need to do a lot more reading. :)

    --
    Glenn Ashmore

    I'm building a 45' cutter in strip/composite. Watch my progress (or lack
    there of) at: http://www.rutuonline.com
    Shameless Commercial Division: http://www.spade-anchor-us.com
     
  5. Glenn Ashmore wrote...
    Hmm, there are no doubt lots of places where you could glue
    strain gages and measure stress on your boat's structure, but
    yes pressure sensors are also a common use of strain gages.
    But what pressure are you planning on measuring? Hopefully
    not the increasing pressure as your boat... ahh, strike that,
    bad joke.

    Thanks,
    - Win

    whill_at_picovolt-dot-com
     
  6. Tim Wescott

    Tim Wescott Guest

    The basic idea is that your strain gauge gives an output that's proportional
    to the input, even to negative voltages. You could excite it with DC and
    look at the result, but (a) amplifiers that have low DC offset are spendy,
    and (b) if you have long wire runs it's really easy to pick up some DC
    offset from ground loops and whatnot.

    So if you put an _AC_ excitation into your strain gauge you'll get AC out.
    You can AC couple your amplifier (i.e. build it to ignore the average
    voltage) and amplify the heck out of the strain gauge signal, then rectify
    (or demodulate) the signal and look at it in DC. Since the voltage is now
    big you don't have nearly the trouble with those pesky offsets.

    I've had very good success with this in mixed-signal systems by driving the
    excitation from a microprocessor (or DSP) and sampling the result with an
    ADC and doing the demodulation in software. It is much easier to get the
    offsets arbitrarily small in this case. You can also do an all-analog
    design, of course.
     
  7. Some day I would like to wire up RUTU like Larry Ellison does his boats
    with strain gauges on all the shrouds, stays and lines routed to a
    processor network that automatically trims the sails but I am basically
    a "cockpit potato". All I want to do is make sure that the watermaker
    pressure is about 800psi without putting down my beer. :)

    I picked up a couple of 12V excitation 1000 psi pressure transducers on
    eBay. One is 4-40 mA output and the other is 0-5V. What I want to do
    is rig one up to a cheap LCD display in the cockpit.

    You guys pointed me at the LM2907 which I have put to good use
    translating pulses from the proximity sensor on the pump drive and the
    cheap GEMS flow meters. I am hoping you can help me here with a simple
    solution before Sperhro talks me into anothe PIC. I really need to get
    back to work on the rest of the boat. :)


    --
    Glenn Ashmore

    I'm building a 45' cutter in strip/composite. Watch my progress (or lack
    there of) at: http://www.rutuonline.com
    Shameless Commercial Division: http://www.spade-anchor-us.com
     
  8. Hahaha. ;-)

    What's the mV needed for here? 4-20 (right? 4-40 is a little screw) mA
    is easy to interface to, and 0-5V is even easier. The signal
    conditioning is built into that kind of transducer, so it's all high
    level stuff which any old op-amp can do. What power do you have
    available? What's the input voltage range on the xducer power
    (especially minimum voltage for rated accuracy)?

    Best regards,
    Spehro Pefhany
     
  9. Analog Devices and Burr Brown both have neat little devices that will
    interface directly to strain guages and many other small-signal sensors.
    Couldn't speil them off the top of my head but just try searching for 24bit
    sigma-delta ADCs. The parts I am thinking of come with built in amps and can
    be configured for AC and DC excitation + a whole lot more (i should be a
    salesman).
     
  10. GPG

    GPG Guest

    All I want to do is make sure that the watermaker
    Plenty of cheap LCD voltmeter kits around for V out(easier)
    Current loop 4 to 40? Stndard is 4 to 20, with power and measurement
    from the same 2 wires
     
  11. There's a common-mode voltage range problem with typical LCD voltmeter
    modules unless he uses an isolated supply (such as a 9V battery) for
    the meter supply. Other than that, it's duck soup.
    Yes. mA, that is.

    Best regards,
    Spehro Pefhany
     
  12. Sitting here looking at these two transducers and realized that the 1-5V
    model would be a lot easier to use so I started playing with it. The
    LCD meter takes 0 to 200 mV so I figured a 40:1 voltage splitter would
    display the pressure correctly. Breadboarded it up and with the bench
    supply it works great but then I realized that the transducer output is
    1 to 5 not 0 to 5. Zero PSI is not zero volts. How do I get rid of
    that 1V offset before it gets to the divider???


    Spehro Pefhany
    --
    Glenn Ashmore

    I'm building a 45' cutter in strip/composite. Watch my progress (or lack
    there of) at: http://www.rutuonline.com
    Shameless Commercial Division: http://www.spade-anchor-us.com
     
  13. Glenn Ashmore wrote...
    You can raise up the other terminal of the differential
    200mV meter with a second "offset" divider.

    Thanks,
    - Win

    whill_at_picovolt-dot-com
     
  14. Dumb stick time. I shoulda thought of that.
    --
    Glenn Ashmore

    I'm building a 45' cutter in strip/composite. Watch my progress (or lack
    there of) at: http://www.rutuonline.com
    Shameless Commercial Division: http://www.spade-anchor-us.com
     
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