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Very low ref voltage

Discussion in 'Electronic Design' started by Marco Trapanese, Jun 21, 2012.

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  1. Hello,

    to test and calibrate several acquisition boards I'd need a very low
    reference voltages.

    I'd prefer a good thermal and short-term stability rather than a very
    accurate output voltage.

    My goal is to build a circuit with these outputs:

    - 100 uV
    - 1 mV
    - 10 mV
    - 100 mV

    also a 10 uV output will be very appreciated.
    The output current could be quite small, less than 1 mA.

    Obviously, I'm looking for the lowest noise and good short-term
    stability - I want the output doesn't change more than 1/1000 in a
    couple of hours.

    Do you think it's possible to make such a circuit? Any idea how to get
    those low voltages?

    Thanks
    Marco
     
  2. Guest

    What kind of circuit topology are you thinking about ?

    At least in any unbalanced circuits, the ground noise could swamp the
    reference voltage.
     
  3. 1mA is actually quite a bit of current at 10uV, that implies a load
    resistance of 10 milliohm.

    Most DC-accurate data acquisition boards have relatively high
    impedance inputs- gigohms, megohms.
    1/1000 of 10uV is 10nV. That will be extremely difficult because of
    thermal EMFs. 0.1% + (say) 5uV might not be very hard.
    Can't you just use a voltage divider off of an ordinary reference?
     
  4. Il 21/06/2012 15:35, Spehro Pefhany ha scritto:

    I'm sorry, I mean at F.S. And of course the requested current will be
    negligible in most applications.


    In this case what kind of resistors should I use?

    Thanks
    Marco
     
  5. Il 21/06/2012 16:48, George Herold ha scritto:

    The input stages are op-amps and INA as already pointed out by the other
    friend. Sensors have an impedance about 350 ohm typically (e.g. strain
    gauges).

    If I use a 1.25V as standard ref, to have 100 uV I may use 1.5M over 120
    ohm. Not bad.


    Well, I think I over-estimated that :)

    Marco
     
  6. For 0.1%-ish lab stability, just about any precision metal film
    resistor will do.

    Keep the lower end resistor to something like <10 ohms and you should
    be okay.

    If you want to get fancy you can use low-value resistors with 4
    terminations, but that gets expensive.

    Here's a 1206 0.1% 10 ohm resistor +/-25ppm/°C for 44 cents.

    http://www.digikey.ca/product-detail/en/CRT1206-BY-10R0ELF
     
  7. Il 23/06/2012 22:54, whit3rd ha scritto:


    Thanks for the hint.
    Marco
     
  8. Uwe Hercksen

    Uwe Hercksen Guest

    Hello,

    if the two resistors are on the same isolating base material, the
    thermal match is even better.

    Bye
     
  9. Guest

    One way to get a stable and accurate voltage divider is to get a large
    number of XX kohm resistors, hopefully from the same batch.

    To get a 10:1 voltage divider, put nine of these resistors in series
    and one to ground.

    Likewise for current measurement, put multiple equal values in
    parallel and a single one through the actual current sensor.

    This will compensate for the temperature coefficient as well as
    inaccurate XX values.
     
  10. Guest

    A better way, in CMOS is to use gate leakage (tunneling current). The gate
    oxide thickness is held to very tight tolerances as are the gates themselves.
    It turns out to be a very good high value divider. Ratios other than 1/2 can
    be had by varying the gate size but it's not as good as a 1:1 divider. Of
    course you can't load it much, either. ;-)
     
  11. Jamie

    Jamie Guest

    Have used Mosfets for voltage references due to their T coefficiency
    being +, and - on the Vg(th).

    For the better part, it seems to work. But, they don't seem to be
    very fast and you have to keep them with in a window. At least the Mfets
    I used were like that and they do operate in a cold environment.

    I didn't use an RF mosfet and there was a reason for that, I just
    can't remember why?

    Jamie
     
  12. SoothSayer

    SoothSayer Guest

    Creating too low a voltage reference introduces problems one must keep
    an eye out for.

    Can't be considered 'stable' or even 'referenceable' once it gets below
    a certain point.

    Pretty much why the industry has them where they are. They have
    already found the most stable, best value to provide.
     
  13. SoothSayer

    SoothSayer Guest

    Abating the 1/f drops the noise floor and get you a lot of benefit.

    It is important to excise as much as possible.
     
  14. Il 26/06/2012 05:16, SoothSayer ha scritto:

    What is this "certain point", approximately?

    Marco
     
  15. Il 26/06/2012 15:16, George Herold ha scritto:

    So you're saying I can't get my low refs...
    What about the ideas proposed by other friends?

    Marco
     
  16. A 1-volt reference is pretty useless for calibrating thermocouple
    instrumentation.

    The most accurate and stable voltage reference that I know of is only
    about 500-700uV, which is inconvenient in some ways, but worth working
    with in some cases.

    Hey, I can now generate ratios of AC voltages on my bench with a
    digitally selected resolution of 0.01ppm. Happy happy, joy joy.
     
  17. The best reference that I know of that is commercially available is
    thousands of Josephson junctions in series that are pumped (?) by
    ~70GHz microwaves. The guts needs to be cold (4K) to work.
     
  18. tm

    tm Guest

    But you can't find one on ebay. :)
     
  19. Il 26/06/2012 16:51, George Herold ha scritto:

    Don't worry, thank you anyway.

    Marco
     
  20. From what I've read, it sounds like the JJs nowadays are a lot better.
    Apparently (http://arxiv.org/abs/0708.0904) but all the stuff I'm
    doing is LTc. There's supply issues with He these days too.
     
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