EEVblog #929 - Designing A Better Multimeter

[EEVblog]

Dave looks at the current measurement front end of a typical multimeter and figures out how to redesign it for lower burden voltage.
It's not quite as easy as you might think, there are comprises and traps for young players.
And something you probably didn't know about HRC fuses.

µCurrent article: http://www.eevblog.com/files/uCurrentArticle.pdf

[MartinX]

Am I the only one that find the concept of burden voltage to be annoying? Why specify it in this way? Just tell me the shunt resistance, having it presented in mV/µA is much less intuitive to me and it does not make calculating the voltage drop or evaluating the effect on the circuit being measured any easier.

[NivagSwerdna]

What about using the fuse as the current detector?  (and then optically coupling later in the sensor circuit to maintain isolation?)

I imagine fuses do not have constant resistance with temperature but maybe you could compensate for that?

[Brumby]

What about using the fuse as the current detector?

I would SERIOUSLY doubt the accuracy of such an arrangement.  Precision resistance in a fuse is not required for it to provide it's prime function - especially when we are talking about a disposable, clip in field replaceable component.  Not to mention contact resistance between the fuse and holders at each end.

[nctnico]

Another thing that can bite you is the thermo electric effect if you are not careful with the PCB layout and one end of the shunt resistor has a different temperature than the other end. When dealing with signals in the uV range this is a serious thing to consider.

[EEVblog]

I imagine fuses do not have constant resistance with temperature but maybe you could compensate for that?

Massive variation, and yes it does change a huge amount with temperature. Simply not possible for a measurement instrument.

[EEVblog]

Another thing that can bite you is the thermo electric effect if you are not careful with the PCB layout and one end of the shunt resistor has a different temperature than the other end. When dealing with signals in the uV range this is a serious thing to consider.

Meters are essentially a sealed unit and the internals are pretty close to thermal equilibrium, so not an issue in practice.
But yes, the lower you go, the more this is an issue. A big issues in the likes of the 6.5/7.5 digit high end meters.

[nctnico]

Don't say that too quickly. If you run a decent amount of current through a shunt it will get warm and if the legs cool down unequal then you'll get phantom readings. I had some hands on experience with that recently and the ADC used was only 16 bit (2.5uV per LSB).

[digsys]

A possibility -
In past projects where I had similar issues, I made the "front-ends" direct connected and easily replaceable. I then put all the heavy protection after.
"Data' was opto-coupled across. It may be a bit excessive here, but you can achieve absolute accuracy. Replacement input stages could be either plug-in
or simple solder points. Just an idea to throw in the mix.

[Dr. Frank]

This is a very good explanatory video, about the difficulties of current measurements.

As you suggested, I'd like to add a better, more simple circuit, which may be found in mostly any higher grade DMM, in a similar way.
I only omitted the protection circuitry.

You don't need two MUX, and only a FET switch to change the gain.



Current measurements mostly suffer from offset voltages, as the crucial offset across the shunt resistor can't be removed frequently during the measurement, as it's not possible to switch off the current in between. That's a bit different compared to voltage measurements, where the DUT voltage can be switched off from the input amplifier, and an AUTO ZERO phase can be included.

Frank

[G7PSK]

Perhaps we need to go back 70 years and have separate meters for current and voltage at those very low levels, that way you need not worry about some of the compromises that have to be made. A case of the right tool for the right job.

[bktemp]

As you suggested, I'd like to add a better, more simple circuit, which may be found in mostly any higher grade DMM, in a similar way.
I only omitted the protection circuitry.

You don't need two MUX, and only a FET switch to change the gain.

Why is the FET switch better than using a MUX? I see more problems. For example the ON resistance of the FET affects the gain. Therefore you need to make the divider resistors rather high value to reduce the effect of the FET. Using a multiplexer, the ON resistance does not care, because it only switches the feedback voltage into the opamp either directly from the output (1x gain) or divided (10x gain).


The higher resistance of the multimeter fuses is probably because they are super fast acting (FF), therefore they need to dissipate more power to melt in a shorter time.

[rfeecs]

A better multimeter is a sourcemeter, like your Keithley 2400.
Set it to force zero volts and read back current.
No burden voltage.  The source cancels it out.

Of course this comes with its own set of limitations (compliance, stability, etc.).  Not to mention cost.

[Len]

Am I the only one that find the concept of burden voltage to be annoying? Why specify it in this way? Just tell me the shunt resistance, having it presented in mV/µA is much less intuitive to me and it does not make calculating the voltage drop or evaluating the effect on the circuit being measured any easier.

Yeah, specifying the burden resistance is more basic and sometimes more useful, but I also find it handy to know the full-scale burden voltage when I’m making measurements. I don’t often need to do a detailed calculation of the burden voltage, usually I need a general idea of “how bad is it?”

[ez24]

A better multimeter is a sourcemeter, like your Keithley 2400.
Set it to force zero volts and read back current.
No burden voltage.  The source cancels it out.

Of course this comes with its own set of limitations (compliance, stability, etc.).  Not to mention cost.

I think it is not clear that the "better" meter case has already been designed and probably molded, thus the physical size cannot change.  The only way to make better is with a few small components.  Not even a fuse can be changed.  I am surprised the proposed changes can be made at this stage.

Is there an estimate to market yet?  Sounds better every time.  I wonder if Fluke is getting worried, I think this will give them a run for their money.  Is there a model comparison to Fluke yet? 

Also I suggest Dave send out a dozen for beta testing so any "bugs" do not remain a problem forever like the LCD on the 235.

(FYI there are only 13 235's on Amazon)

[Stephan_T]

[...]

The higher resistance of the multimeter fuses is probably because they are super fast acting (FF), therefore they need to dissipate more power to melt in a shorter time.

Yes, power dissipation is a big part of the regular operation of the fuses. They need that power to burn through. FF plays a role in here, but also the HRC voltage. The HRC versions of fuses use sand to extinguish arcing. That also acts as a heat sink by design.
These two are reasons why they need larger voltage (power) than a cheap glass fuse

Another thing that can bite you is the thermo electric effect if you are not careful with the PCB layout and one end of the shunt resistor has a different temperature than the other end. When dealing with signals in the uV range this is a serious thing to consider.

Meters are essentially a sealed unit and the internals are pretty close to thermal equilibrium, so not an issue in practice.
But yes, the lower you go, the more this is an issue. A big issues in the likes of the 6.5/7.5 digit high end meters.

When you run the ampmeter at the top of the protected range, the fuse is at it's maximum power dissipation. Short before burnout. You don't want that anywhere near the sense wires of a microvolt measurement. . The fuse is also a major violation to the thermal symmetry (equilibrium) of your circuit.

In general, the current measurement with a multimeter (CAT-) rated for mains voltages or even higher seems like a (bad) compromise to me.

As a more pragmatic solution to the original problem (current measurement for a low voltage but high current device), I would rather like to have a power supply with external sense wires. This should be much cheaper to achieve than a SMU. 

Measuring milli- or microvolts across an external current shunt (right in the circuit under test) is probably a more precise and clear solution than gilding the Lilly inside the multimeter.

[brutester]

If having 0.1uV offset is a problem in amplifiers, what about MUX parasitic resistance and capacitance? I think they will cause much more issues than the amplifier offset voltage. I like Dr. Frank's approach above - switching on or off the x10 amp.
Some time ago I replaced one of the resistors of a typical amp with CD4052B (dual analog switch) which I was using to change the values of resitor+cap for custom filters. It was low-frequency business so it was actually doing quite good.

[f4eru]

Hello,

A couple of points :

1) don't forget you alsowant to measure AC (RMS ?), so a chopper amp can distort your AC measurement, and RMS calculation -> what is the frequency cutoff of your moldy meter, and your chopper amp ?

2) why not leave the 100Ohm resistor permanently connector to the uA jack ? there's no point in switching it. It also corrects the 1.01R to 0.9999R --> one switch needed only

3) no need of a 3-way mux for sensing. the 10A sense can go through the 1R resistor --> two input mux should be enough

4) diodes can be changed from 4 to 2 diodes if the burden voltage is reduced

[Craftplorer]

Is burden voltage also a problem in lab bench multimeter?
Like in an HP 3478A, Fluke 8840A or even one of the old HP once with lots of range switches?

Please excuse my bad english.

[EEVblog]

This is a very good explanatory video, about the difficulties of current measurements.
As you suggested, I'd like to add a better, more simple circuit, which may be found in mostly any higher grade DMM, in a similar way.
I only omitted the protection circuitry.
You don't need two MUX, and only a FET switch to change the gain.

Yes, I do this lower range sense resistor tap thing in my uCurrent.
I'd already come up with basically the same solution, yes it is better.
And of course this is where you would use the 99R and 0R99 if you wanted to keep nice round values.

[EEVblog]

Is burden voltage also a problem in lab bench multimeter?

Yes, and they usually aren't very good.

[EEVblog]

2) why not leave the 100Ohm resistor permanently connector to the uA jack ? there's no point in switching it. It also corrects the 1.01R to 0.9999R --> one switch needed only

Yes, I do this in the uCurrent.

[NiHaoMike]

Why not use active IV conversion for the uA range? For a mains powered meter, it would also be practical for the low mA ranges.

Use some high current diodes to protect the input and you'll be able to use a bigger fuse with a lower resistance.

The Mooshimeter uses a single 10m shunt and somehow manages a 1uA resolution. The current path is not switched at all.

Am I the only one that find the concept of burden voltage to be annoying? Why specify it in this way? Just tell me the shunt resistance, having it presented in mV/µA is much less intuitive to me and it does not make calculating the voltage drop or evaluating the effect on the circuit being measured any easier.

I prefer the term "insertion resistance".

1) don't forget you alsowant to measure AC (RMS ?), so a chopper amp can distort your AC measurement, and RMS calculation -> what is the frequency cutoff of your moldy meter, and your chopper amp ?

Switch to a higher bandwidth AC coupled preamp. Then usable bandwidth would likely be limited by stray inductance.

[EEVblog]

Probably worth doing a follow-up to this video on practical optimisation of the circuit.

[MobileWill]

I found this great app note with lots of different options.

https://cds.linear.com/docs/en/application-note/an105fa.pdf

I think I want to try MOSFET switching of 2 shunts.

I am looking at using the  LTC2945. http://www.linear.com/product/LTC2945