Input Impedance of the Voltmeter

• posted

Hi,

I have a following multimeter

Specification Sheet:

Voltmeter:

dp/B0073C9ELO

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 voltmete r with 10Mohm input impedance and 1mV resolution. The Voltmeter while measu ring 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?

Electrodes _______________________ __|____ Voltmeter ______ |_______________________

melissa

• posted

About how much voltage do you expect?

As far as bias current goes, turn on the meter, set it to its 600 mv range, don't connect it to anything, and note the indicated voltage.

If it's less than 100 millivolts (which it sure should be) then the meter's internal bias current is below 10 na.

That's not the same as the current it will load your electrodes by.

My two handheld Fluke meters both read 0.2 mv open-circuit, which corresponds to 20 pa internal bias current.

```--
John Larkin         Highland Technology, Inc

jlarkin at highlandtechnology dot com ```
• posted

Frequency/dp/B0073C9ELO

You're not trying to measure current. You're supposed to use a voltmeter that won't shove more than that much current into your circuit.

Ideal voltmeters present an open circuit to the thing you're measuring. Real things aren't ideal. In a voltmeter we measure this divergence from ideal by treating it as a current source with a shunt resistance. Ideally, the current source will be zero, and the shunt resistance will be infinite.

See JL's test. You should really measure the difference in voltage from leads shorted to leads open in the most sensitive range -- with a bit of math, that'll tell you both the voltmeters offset voltage (another divergence from ideal) and bias current.

```--
Tim Wescott
Wescott Design Services ```
• posted

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 readings. 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 happen. 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 limitations. 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.

• posted

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

• posted

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.

• posted

Hi,

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 the pair of electrodes connected together via gel. The standard says that I ca n use function generator with a series resistor.

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

1. 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 ou tput with an oscilloscope.

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

melissa

• posted

1) Initial circuit: +----+ TP1 TP2 TP3 | AC |--*-/\/-*-/\/-*--+ +----+ Rdmm R | | = gel (internal impedance unk) | | +--------------------+ | GND 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 400Hz). 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 supply. 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).

• posted

I am little confused. The Standard says that keep the circuit as shown in f igure1 as it is and replace the 1Mohm ouput resistor with scope and measure the output. But you are saying that the scope can not measure the DC offse t output noise because of coupling capacitor. What should I use? Should I r emove the coupling capacitor?

Are there other instruments available that can measure this voltage? The st andard also says that the ouput voltage of the circuit ( figure1) should no t exceed 150u peak to peak over 5 min. The mesauring instrument should have frequency response range of 0.01 to 1000 Hz and a minimum impedance of 10M Ohm. According to my knowledge oscilloscope do have these characteristics b ut will I be able to see 150uV peak to peak?

melissa

• posted

how about a VTVM?

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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 schematic.

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).

• posted

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.

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I skimmed over the rambling post and it sounded like they wanted a volt meter.

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Well, it DOES help to READ...

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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.

melissa

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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.
• posted

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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