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Microvolt Display

  1. Dec 26, 2009 #1
    Hi,

    I need a way to display voltages in the range of 1V - 1uV using seven segments, but without using microcontrollers.

    Any ideas are appreciated.

    Thanks in advance.
     
  2. jcsd
  3. Dec 26, 2009 #2

    vk6kro

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    That would be a serious instrument.

    You would need an amplifier with plenty of bandwidth to suit your application and it would have to have switchable precision gain over 6 decades.

    One alternative, if a logarithmic scale would be OK, would be a meter based on the Analog AD8307.

    This is a data sheet for the AD8307:
    http://www.analog.com/static/imported-files/Data_Sheets/AD8307.pdf
    and Google seems to have plenty of references to this chip being used as a power meter.

    Note that it covers the voltage range from about 20 uV upwards, so you would need a preamp if you wanted 1 to 20 uV.

    It has a bandwidth of up to 500 MHz, which is good, but it means you need to be very careful about layout of the circuit. So getting someone else's printed circuit board would be a good idea.
     
  4. Dec 26, 2009 #3
    I'm assuming you want to display up to 1 volt while still viewing the digits flipping around at 1uV?

    Likewise, I assume that you want this instrument to be fairly linear and accurate?

    For this demanding spec, I'd simply purchase a Keithly 6 1/2 digit meter. Trying to build something like this is well beyond the ability of common engineers. At this level of resolution, assumptions about op amps fall apart, very slight temperature imbalances create thermocouple effects, improperly designed ground runs will throw the readings off, and transients from the logic can have a disastrous impact.

    All that said, I've successfully run out to 5 1/4 digits using delta sigma converters without too much trouble. I wouldn't sign up for anything as high as 6-digits though.

    - Mike
     
  5. Dec 26, 2009 #4
    I think I should clarify.

    I want to build a ~3 digit display that can display values as low as ~50uV and as high as ~1V. I guess I would also need a one digit display to show the multiplier.

    ie. |1.||0||3| E |4| uV


    Where each || thing is a seven segment display.
     
  6. Dec 26, 2009 #5
    is there a way i can do a preamp and multiply my microvolt signal by 1000? i know there are op amp amplifiers but can they amplify that much? even if i can, i would still need a way to build meter to measure in the mV range...

    Thanks.
     
  7. Dec 26, 2009 #6

    vk6kro

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    Do you know what maximum frequency you would need to measure?
     
  8. Dec 26, 2009 #7
    the frequency is about 1500hz
     
  9. Dec 27, 2009 #8

    vk6kro

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    And is it just 1500 Hz or do you need all frequencies below that, including DC?

    Maybe it would help if you said what you were trying to do, and what your actual signal is?
     
  10. Dec 27, 2009 #9

    berkeman

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    Staff: Mentor

    Are you familiar with low-noise circuit design considerations? Your specifications as expressed so far are not reasonable, IMO. What exactly are you trying to do?
     
  11. Dec 27, 2009 #10
    I'm trying to build a fluxgate magnetometer with a digital display of three significant digits without using microcontrollers. I need it to be able to measure Earth's magnetic field (uT range) and B fields as strong as those produced by rare-earth magnets (~0.5T).


    To person above: I believe it's just 1500hz.
     
  12. Dec 27, 2009 #11

    Averagesupernova

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    Gold Member

    Is there a requirement for a high input impedance? Like 1M or higher? What is the source impedance?
     
  13. Dec 27, 2009 #12
    You are asking for a dynamic range of 120 dB, from 1 uV to 1 V. This far exceeds the dynamic range of nearly every linear device. A linear ADC that covers 120 dB dynamic range requires ~22 bits. The logarithmic amplifier suggested above (AD8307) has only a ~92 dB dynamic range. I would suggest instead using the AD8304, which has a 160 dB dynamic range. See
    http://www.analog.com/en/fiberoptic/loglimiting-amplifiers/ad8304/products/product.html

    Furthermore, converting a logarithmic output to the desired floating point output is easier.

    Secondly, I would suggest using a dBμV, a dBmV, or even a dBm (the standard) dB scale rather than the floating point output, because it requires only a gain setting after the log chip. (RF engineers normally think in dBm units). See
    http://www.maxim-ic.com/app-notes/index.mvp/id/808
    Bob S
     
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