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Input Impedance of the Voltmeter

Discussion in 'Electronic Design' started by [email protected], Aug 8, 2013.

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  1. Guest


    I have a following multimeter

    Specification Sheet:


    I am trying to measure the DC offset voltage of Electrodes. The process is to connect two electrodes together with gel between them and use a voltmeter with 10Mohm input impedance and 1mV resolution. The Voltmeter while measuring should apply less than 10nA of bias current to the electrodes.

    I do not understand how can I measure 10nA of current during this test. or How do I know that this meter will apply less than 10nA of bias current?


    __|____ Voltmeter

  2. Robert Baer

    Robert Baer Guest

    Firstly, a DMM cannot and will not "apply" any bias.
    Any voltage read by the meter would be from potential differences
    between the electrodes in the gel; the DC offset voltage of Electrodes.
    I suggest you start with the DMM set to 6V full scale; that is the
    equivalent of 0.6uA or 600nA full scale.
    The most sensitive scale, 600mV will allow 60nA full scale and thus
    allow reasonable resolution and accuracy.
    Make the reading(s) with the meter connected one way, and the same
    readings with the meter connected the other way; log and document all
    You might see +12uA one way and -8uA the other way for an apparent
    error of 4uA; use two electrodes of exactly the same PH and usage
    history (as internal chemistry changes during use due to ion migration).
    Using the above "example" for real-life readings, this test setup
    with equal electrodes should then give +2uA one way and -2uA the other
    way and thus a consistent experience.
    Well, that would put the lie to the ASS-u-ME-mption that "a DMM
    cannot and will not apply any bias".
    There are some DMMs that _DO_ apply a bias to determine if test leads
    are correctly used for a given scale (but they are what i call "fancy").
    The datasheet specs seem to never give a direct clue that this can
    One of my DMMs is a Protek 6500 that uses the equivalent of 40nA for
    sensing meter probe use/misuse; that makes for a reading of +50nA on the
    500uA scale with the leads plugged in and open - obviously ZERO current
    is flowing from external source(s).

    So..make some measurement in KNOWN conditions and document them.
    For example, for your actual needs, set the meter to the 600mV scale
    and NO leads plugged in; make a few dozen readings and log them. Better
    be zero, darn close or average of zero with max + and max - readings
    darn close to zero.
    Next short the leads and repeat the measurements; you may have to
    twist the leads together or get the meter away from AC wiring.

    That gives you a "base" of confidence and a better knowledge of meter
    If you are an "electron pusher", use a fresh carbon-zinc AA cell and
    build a voltage divider; say 100K in series with 1K and read voltage
    across the 1K both polarities - should be close to 15.3mV which is a
    reasonable reference point.
    Again, make a log; draw the circuit with values.

    If you want to get more picky, use the meter to measure the resistors
    and the battery; use those values in the diagram; this makes for a
    "calibration" scheme.
    Know your equipment; draw the FULL circuit (eeach electrode as a
    half-cell of X mV and resistance), gel resistance (even if initially
    unknown; it ain't zero nor infinite), meter resistance, etc.

    Have fun; you will wind up knowing more than the majority of us
    electron pushers.
  3. Robert Baer

    Robert Baer Guest

    ---thus that tends to imply the the internal op-amp input bias current
    is n the region of 20pA.
  4. Robert Baer

    Robert Baer Guest

    Other considerations..
    1) Is it necessary to have 10nA flow thru the test setup?
    2) Does the current measurement require a specified load, eg zero
    ohms AKA short circuit?
    3) Does the test procedure _require_ a current measurement, or is it
    OK to measure voltage (that is what using the DMM on the 600mV scale
    going to do).
    Step on your eggs carefully as they may be alligator eggs, not duck eggs.
  5. Guest


    I also need to do some of following tests

    1. Impedance (AC):
    The AMMI standard suggests that I should apply 100uA pk-pk sine wave to thepair of electrodes connected together via gel. The standard says that I can use function generator with a series resistor.

    Question: If I place an oscilloscope across the electrodes , then I can seethe voltage drop acorss the electrode using oscilloscop probe 1 and also voltage drop across the series resistor using probe 2. But how will I plot the impedance of electrodes? Can I plot the impedance on the scope? Is oscilloscope be a good way to go or should I buy an expensive impedance meter?

    2. Offset instability and internal noise:

    The standard recommends the following circuit

    Figure 1 can be used to calculate the internal noise and instability.

    Question: The standard says that I can replace the 1MOhm resistor at the output with an oscilloscope.

    3. Figure 2 : says that I need 200V to test the ciruit. How can I generate 200Volts DC.

  6. Robert Baer

    Robert Baer Guest

    1) Initial circuit:

    +----+ TP1 TP2 TP3
    | AC |--*-/\/-*-/\/-*--+
    +----+ Rdmm R |
    | = gel (internal impedance unk)
    | |
    Adjust AC signal frequency as needed (make dern sure it is a sine
    wave); Use your DMM in series TP1,TP2) and start with R = 1Meg and set
    DMM to the 600uA AC scale.
    Slowly increase generator output from zero to get 100uA pk-pk
    (meaning RMS reading times 2.828); if 1Meg is too much, try 100K.
    Be advised the DMM does not have a good frequency response; reality
    is 10KC max - 1KC is likely to give reasonable accuracy (yes, it states
    Simple ohms law will give you the gel impedance/resistance and
    applied voltage.

    2) i am going to assume that first resistor is the gel resistance
    "broken out" for clarity. Knowing the value from 1) above, one can
    calculate the RC filter time constant which will attenuate any noise by
    6dB (or more) above that frequency - so noise "measurement" depends on
    that RC time constant.
    Likewise,the capacitor to the load (YES! most scopes have a 1Meg
    input impedance but watch for a 10X probe presenting 10Megs).
    "offset instability" is a puzzler - NO DC offset can be seen or
    measured at the load (by a scope) because of that coupling capacitor;
    review specs carefully.

    3) You would need to make or buy a DC power supply for that; before
    getting a fixed 200V supply, review all possible needs for a "high
    voltage" supply; you may find that a 0-500V supply would be required,
    and if you will be doing multi-channel electrophoresis it may need to
    have adjustable current limiting.
    Turns out (for other purposes) i had bought a Hoeffer Scientific
    supply adjustable 0-500V and 0-400mA which suggests that the gel tray
    needs active and controlled cooling (now things are getting expensive).
    Fortunately, your circuit does not require continuous "large"
    currents and you could (for this experiment) get away with a 1mA or less
    That Hoefer is a switcher and noisy as all hell and i am being polite.

    Best i could find with a quick search is Jameco 2172172 or 2172172 at
    $50 plus shipping; adjustable from 100V to 200V with 350mA capability.

    I suggest that you refine the schematics, particularly #2 and give
    values and suggested parts/components (that op amp fer instance).
  7. Guest

    I am little confused. The Standard says that keep the circuit as shown in figure1 as it is and replace the 1Mohm ouput resistor with scope and measurethe output. But you are saying that the scope can not measure the DC offset output noise because of coupling capacitor. What should I use? Should I remove the coupling capacitor?

    Are there other instruments available that can measure this voltage? The standard also says that the ouput voltage of the circuit ( figure1) should not exceed 150u peak to peak over 5 min. The mesauring instrument should havefrequency response range of 0.01 to 1000 Hz and a minimum impedance of 10MOhm. According to my knowledge oscilloscope do have these characteristics but will I be able to see 150uV peak to peak?

  8. how about a VTVM?
  9. Robert Baer

    Robert Baer Guest

    OOPS! For some dumb reason, your query did not get copied.
    I am ASS-u-ME-ing that the right-most (load) resistor is supposed to
    be 1Meg; most scopes with a X1 probe will present the required 1Meg.
    BUT be aware that a X1 probe will NOT do that.

    Note the capacitor coupling the source (gel,R,C) to the scope/load
    cannot pass DC and CANNOT not measure the DC offset!
    The "standard" may require that a DC offset be measured, but that
    circuit can not do it.
    Now you can delete that "output" series capacitor and make measurements.
    Yes, this modified circuit does not conform to the "standard"; too
    darn bad, not good to march off a cliff.
    Here is where FULL DOCUMENTATION becomes virtually mandatory,
    especially in a report for peer review, class requirement, thesis, etc.
    PLEASE give VALUES for all circuits; use ExpressSCH to generate good
    looking circuits (or LTspice‎ which can give DC and AC values).

    Then you say "The measuring instrument should have frequency response
    range of 0.01 to 1000 Hz and a minimum impedance of 10MOhm" which
    appears to contradict the 1Meg previously mentioned.
    An AC voltmeter can do this low level noise measurement; shielding is
    highly recommended; beware of the AC voltmeter input resistance/impedance.
    Note the AC meter will, in effect, act like it is AC coupled as per

    A scope can have a 10X probe and so give the 10Meg load, BUT a
    special plugin would be needed for the sensitivity.
    A Tektronix 7A22 Differential Amplifier plug-in goes down to 10uV -
    making the sensitivity 100uV/div with a 10X probe.
    That makes the measurement subject to large error due to poor
    resolution, making the use of an AC voltmeter a better choice (assuming
    you get one that can do the job).

    I am tempted to say that the "standard", at least as represented, is
    INCOMPLETE and thus not too useful.
    I would ask for FULL and COMPLETE citation to avoid contradictions
    avoid mis-understandings, allow useful results, and allow repeatable
    results (WRT others).
  10. Robert Baer

    Robert Baer Guest

    Cite ONE that can do the specified job; you will not find any.
    Better for her to use the BabyBird (GooGull), search "low voltage
    measurements" and get at least the first PDF cited.
    You might learn something also, instead of shooting off in the dark
    about something that does not even have a chance of working.
  11. I skimmed over the rambling post and it sounded like they wanted a volt
  12. Robert Baer

    Robert Baer Guest

    Well, it DOES help to READ...
  13. Guest

    The Jameco supply that you suggested is a constant current supply. It will provide 350 mA no matter what is the load. Am I right? I am confused that then why data sheet does not say that 200 V is a compliance voltage.

  14. Robert Baer

    Robert Baer Guest

    Oops! yes, that Jameco supply is constant current; adjusted from
    210mA to 350mA with voltage range from 100V to 200V.
    May not be what you need.
    Did more searching; found MPJA has their 8529 PS, 175-210V. 0.12A
    Linear Power Supply IHB200-0.12 at $52.61.
    Hell to find that voltage..if you need more current, it is far worse.
  15. Robert Baer

    Robert Baer Guest

    The BabyBird (GooGull AKA Google) helped me here..the B&K 9184 specs
    are 0-100V,2A/100-200V,1A DC and should be more than adequate for any
    needs; Mouser 615-9184. Pricey at $1,385.
    Wasted an hour and got no further.
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