EEVblog #584 - What Effect Does Your Multimeter Input Impedance Have?

[EEVblog]

What effect does your multimeter input impedance have on the circuit you are measuring? Dave shows a practical example of how it can really screw things up if you aren't watching out for it.

[caroper]

Hi Dave,

]Given that you have a Precision High Impedance Voltage source I wondered why you did the Calibration the way you did rather than dialing in a Voltage on the Keathley and then adjusting the POT until the LED extinguished.

Would that have been less accurate or just a different approach to the same task?

Cheers
Chris

[EEVblog]

Would that have been less accurate or just a different approach to the same task?

Just a different approach to the same task.
In practice, few people would have access to a 6 digit voltage source like I have, their meter would generally be the best and most accurate thing in their arsenal, so that's what normally would get used in such a situation.

[caroper]

Thanks Dave,

I realised that the Video was ment to be, and indeed was instructional, I just wondered if there would have been anything wrong in taking the other approach under other circumstances.

Cheers
Chris

[kaushleshchandel]

Very nice!

I am building a Kelvin Varley Divider using precision resistances. But I have seen the readings on Agilent 6.5 Digit meter not show as per the resistance divider set.... Whats happening is that when I calibrate the lowest three KVD strings, its fine... but the moment I go the the fourth & fifth string, the readings start behaving very different than what I would expect.

Ill go and Check if this is due to Impedance setting on Agilent meter.

btw, here is how I am trying to build the KVD.  http://conradhoffman.com/mini_metro_lab.html

And I bought 100 pieces of this from ebay http://www.ebay.com/itm/1x-5K0000-Vishay-VSR-Series-Bulk-Metal-Foil-Precision-Resistors-0-01-5K-/121032384194?pt=LH_DefaultDomain_0&hash=item1c2e179ec2

I have grouped them into 0.0001% range using a Bridge.

[opablo]

I loved this one !... I too say to myself... 10Meg... that's a lot... and never consider the impedance of the circuit under meassurement ja...

You made my mind constantly think in of this:

http://en.wikipedia.org/wiki/Observer_effect_(physics)

In science, the term observer effect refers to changes that the act of observation will make on a phenomenon being observed. ......
The observer effect on a physical process can often be reduced to insignificance by using better instruments or observation techniques.
Historically, the observer effect has been confused with the uncertainty principle.

"The observer effect ... can often be reduced to insignificance by using better instruments." tell that to a quantum physicist and he will start to cry like a baby 

[free_electron]

And it's not only multimeters ! Scope probes have the same effect ! They are either 1 meg or 10 meg input plus they have a few pF capacitance. When looking at ac signals with sufficiently high frequency that comes into play. Probe impedance can collapse depending on frequency !
Good probe makers will give you the bandcurve.

Smart probes (active) have an eeprom on board with the curve in it. The scope reads it and adjusts.

[Dr. Frank]

Nice practical video.

Some additional formula / calculations on the Whiteboard would have made a nice Fundamental Friday video, also.

That would also have shown this well known quantitative rule of thumb:

The introduced error can be calculated by dividing the source resistance of the DUT by  the multimeters resistance, in this case 0.02%. (source resistance in this case is given by those five 10k resistors in parallel)

Et voilá, the reason, why such high precision measurements need high impedance DMMs, would have been quite obvious, also for "young players".

regards from an old geezer - Frank

[juani_c]

Noob question:
¿Is necessary to use a high precision voltage reference if you are going to tweak/calibrate the values whit pots?

[DrMag]

Just thinking... for those of us without a fancy multimeter with multiple input impedances, is it possible to externally accomplish something similar by using a high-impedance op amp in a follower configuration?

[edpalmer42]

I found this video frustrating for two reasons.

First, as has been mentioned, Dave measured the voltage across the pot instead of setting the input voltage and then adjusting the pot to extinguish the LED.  The input voltage is the important parameter so it should be the thing used to make the adjustment.  Doing it this way would automatically include any and all offset voltages and component tolerances.  You don't need a precision voltage source, you use the same meter to set the input voltage that you were going to use to set the pot wiper voltage.

Second, Dave didn't explain how to solve this problem.  If I did have to make the measurement on the pot wiper, I wouldn't use any active circuitry, I'd set up a bridge measurement with an external voltage source and a ten-turn pot to create an infinite-impedance null voltmeter.

Ed

[jippie]

Why does a manufacturer like Agilent provide a 10M input impedance, if they're able to provide a 2G+ input impedance? What's the catch that I'm overlooking?

[c4757p]

Noob question:
¿Is necessary to use a high precision voltage reference if you are going to tweak/calibrate the values whit pots?

Yes, because he designed it so the pots only adjust over a tiny range. The voltage reference has to start off somewhere inside that range.

[robrenz]

Also higher precision reference will usually carry a higher stability.  No point in precisely tweaking a value only to have it drift to a value out of tolerance a week later.

[edpalmer42]

Why does a manufacturer like Agilent provide a 10M input impedance, if they're able to provide a 2G+ input impedance? What's the catch that I'm overlooking?

As Dave mentioned in the video, the high impedance is only on the lower voltage ranges.  Each meter's design is different so you have to check the specs to see where the impedance changes from high to 10M.  Also, if you are using a voltage divider probe to measure voltages higher than the meter's maximum, you want to have an industry standard 10M impedance so that the ratio works out and you definitely don't want the impedance changing if the meter happens to autorange.

Ed

[casinada]

Well, Ideal voltmeters have infinite internal resistance and ideal ampmeters have zero resistance but real devices don't that's why Dave designed a microcurrent device to overcome the burden problems at low currents. Multimeters and scopes have defined input impedances so when you plug probes designed for them they still show the right values. For example High voltage probes for multimeters or scopes. This is a great simplification. The point is to know what you're doing with your test equipment and know the limitations.

[EEVblog]

Also, if you are using a voltage divider probe to measure voltages higher than the meter's maximum, you want to have an industry standard 10M impedance so that the ratio works out and you definitely don't want the impedance changing if the meter happens to autorange.

Yes, and some meter that say they are 10M, can be oddball values like 11M or something that can change with the range, that 10M might just be nominal value. A trap if using external divider probes.

[EEVblog]

¿Is necessary to use a high precision voltage reference if you are going to tweak/calibrate the values whit pots?

No. But higher precision references usually (not always) have a lower tempco. In this case I'm using this reference on another board, so I just re-used it in this design.

[EEVblog]

First, as has been mentioned, Dave measured the voltage across the pot instead of setting the input voltage and then adjusting the pot to extinguish the LED.  The input voltage is the important parameter so it should be the thing used to make the adjustment. Doing it this way would automatically include any and all offset voltages and component tolerances.  You don't need a precision voltage source, you use the same meter to set the input voltage that you were going to use to set the pot wiper voltage.

No, you don't need a precision voltage source, but you do need to rig up suitable small range high resolution adjustment pot or other source on the input to do that. That's extra work. In that case it's just easier to use the high impedance meter directly as I did. Oh, and if it's a high impedance source you've got driving it, beware of any input current...

Second, Dave didn't explain how to solve this problem.  If I did have to make the measurement on the pot wiper, I wouldn't use any active circuitry, I'd set up a bridge measurement with an external voltage source and a ten-turn pot to create an infinite-impedance null voltmeter.

Why? A high impedance voltmeter does the job just fine.

The only point of the video was to show the effect of input impedance on a circuit. But if seems a few people are nit picking that I used a bad example, or should have expanded the video to include different ways to do this adjustment in this instance. But that wasn't the point. Oh well.

[edpalmer42]

The only point of the video was to show the effect of input impedance on a circuit. But if seems a few people are nit picking that I used a bad example, or should have expanded the video to include different ways to do this adjustment in this instance. But that wasn't the point. Oh well.

I hear you Dave.  It's kinda like a different form of 'feature creep'.  Is this a big enough topic to be a 'Fundamental Friday' video?  Maybe expand it to include scope probes and ammeters and cover both the problem and various solutions.

Ed

[AlphZeta]

Of course, I suppose you could always use an extra OPA2376 as voltage followers after the voltage divider so that the impedance of the meter does not mater any more. Especially, OPA2376 is relatively inexpensive. 

[Zucca]

Confused young player here... (It's funny how a master of science can be turned in nothing here at the EEVBlog, experience is everything and I don't have it...)

Second, Dave didn't explain how to solve this problem.  If I did have to make the measurement on the pot wiper, I wouldn't use any active circuitry, I'd set up a bridge measurement with an external voltage source and a ten-turn pot to create an infinite-impedance null voltmeter.

Ed

Man, I want to understand this. Are you talking about a Wheatstone bridge or something similar? If not can someone point me please in the right direction?

Moreover the low impedance in parallel to the pot for the stability jazz is because in every pot the total resistance (across the non wiper terminals) is changing a little by moving the wiper?

[edpalmer42]

Confused young player here... (It's funny how a master of science can be turned in nothing here at the EEVBlog, experience is everything and I don't have it...)

Second, Dave didn't explain how to solve this problem.  If I did have to make the measurement on the pot wiper, I wouldn't use any active circuitry, I'd set up a bridge measurement with an external voltage source and a ten-turn pot to create an infinite-impedance null voltmeter.

Ed

Man, I want to understand this. Are you talking about a Wheatstone bridge or something similar? If not can someone point me please in the right direction?

Moreover the low impedance in parallel to the pot for the stability jazz is because in every pot the total resistance (across the non wiper terminals) is changing a little by moving the wiper?

I stole this diagram from http://www.allaboutcircuits.com/vol_6/chpt_3/13.html



The null meter shown could be any ua or na meter, or it could be any DVM that has 'only' a 10M input impedance.  When you adjust the pot (which can be almost any value) to zero the reading on the meter, you are at a point where no current is flowing, i.e. a point of infinite impedance.  The reading on the voltmeter on the right is now identical to the value at the midpoint of the resistor string but there is no loading on that midpoint.  Do the calculation and you'll find that a simple 10Mohm, 200mv, 3.5 digit meter has a least significant digit that represents a load of only 10 pa.  For Dave's 1V measurement, that is equivalent to 100 Gohms.  And yes, noise and a host of other low-level effects can easily become an issue in measurements like this.

This is a very, very old idea.  I don't know how old, but probably over 100 years.  It dates from the days before vacuum tubes (valves) when the meters had very low sensitivity and tricks like this were needed to make ANY measurements.  Today, it still has value when you're making measurements in high impedance circuits.  Even a '2 Gohm' meter might need some help if the voltage is high enough to exceed the level where the impedance switches from high to low.  Dave's Agilent meter has high impedance on the 10V scale and below, so if he was trying to measure 25V, he might need something like this.

Ed

[Zucca]

Thanks edpalmer42, now it is crystal clear. To me it looks the principle behind that idea is still the Wheatstone bridge, which indeed is much older than, let's say, the NE555.

[T3sl4co1l]

What does mine do?

DMM: Hi-Z (haven't measured typical; 100s M+?) on low range (< 400mV), 10M above
VTVM: 11M, all ranges (1.5 to 1500V in 1.5-5-15-etc. steps)

Since the DMM has autoranging, it can oscillate on a high impedance source.  And it's not just that it looks like it's flipping shit, the oscillation is really there.  A nice demonstration is an electrolytic cap charged to a volt or a few, then clipped to the meter (and nothing else).  It slowly discharges, then recharges due to absorption, and so on.

And yes, VTVM.  And I use it regularly too.

Tim