EEVblog #929 - Designing A Better Multimeter

[joeqsmith]

What about two fuses in parallel with half the amps?

It doesn't work! You can buy parallelled fuses for very high currents but these are stuck together in the factory and there are many things to consider to make sure the current is shared equally so it doesn't blow by accident.

I like Photonicinduction's channel.   Note the internal construction of this fuse.  It does not seem like this would ever work on a smaller scale with multiple fuses but I have never tired it. 

https://youtu.be/El7gp64PYnE?t=49

[Huluvu]

in case of low voltage and high current, when burden voltage really plays a role, I simply use an chunky 50A/50mV external 4 wire shunt and measure the voltage drop.

[rfeecs]

One more try to make the voltage burden zero. This time including the fuse burden.
What do you think?

You are getting closer and closer to an SMU.
Your circuit is a feedback ammeter, commonly used for low current measurements:
http://www.tek.com/sites/tek.com/files/media/document/resources/LowCurtMsmntsAppNote.pdf

Apparently the interest here is more to make a small tweek to the basic multimeter to improve burden voltage.

One issue with the feedback ammeter is your op-amp has to be capable of sinking/sourcing the current that you are trying to measure.  Also it has to be stable and not oscillate with whatever load you are presenting to it.

[Kleinstein]

The Feedback circuit (TIA) is only practical for low currents, that is about <1 mA for a battery powered meter and maybe 50 mA for a mains powered one. A few >(e.g. HP3457) high grade mains powered DMM actually use this for something like a 20 mA range. It might be attractive for the low current ranges (as this could allow even lower currents) and reduce leakage through the protection circuit.

The main problem here is the relatively high drop in the 500 mA range as here we have the fuse and quiet some drop on the shunt that add up.

The 1st. possible improvement is a really good (e.g. low offset and low noise) amplifier for the shunt voltage so one could use a lower drop on the shunt. Gain could be something like time 2 or times 20 - so always some amplification. Using just one switch as initially shown by Dr. Frank can he a slight advantage, as any extra switch / MUX can add thermal EMF. Whether switching the gain is using a FET or CMOS MUX is not that important, both ways are good enough and cheap. However the possible improvement is limited, maybe a factor of 2. Really low noise amps also need more current.

A 2nd possible improvement would be a slightly larger fuse. The problem could be getting such multimeter grade fuses at 1 or 2 A. The protection diodes (and maybe the shunts) also need to be a larger.

A 3rd way would be adding an extra shunt, so that one does not have to use them for a 1:100 range. The downside are extra costs for switching and the extra shunt(s) and maybe odd ranges. Even only an extra shunt for the 500 mA would already avoid the worst case.

[Chupacabras]

How to make that multiplexing mentioned in the end of the video?
Is it better to use some mosfets? Or is there any specialized IC?

[kvresto]

In the video Dave spoke of the sense wire across the 0R01 shunt resistor in the Amps range, and said he wont go into the details of how it would be layed out on the board. It seems to me that it would have to be close to the amp, but can someone elaborate and explain how to best layout this track?, pros, cons ect?

[Kleinstein]

The sense wire connection to the amplifier should be in such a way that there is no large area between this and the signal lines to the amplifier. It does not have to be extra short. The line resistance should not be that critical as the resistors to set the gain are relatively high in a battery operated circuit. The main concern would be inductive coupled external signals. One should also avoid vias as they could introduce minute thermal EMF. Ideally the layout around a 4 wire shunt is symmetric, so get equal temperature at the terminals. However space may not allow a fully symmetric layout and it is more like avoiding large temperature differences.

For the MUX one could use CMOS chips (e.g. DG4...), as the voltage is low and super low leakages is not an absolute priority. There are quite a lot of such chips to chose from. For just a single switch (especially one towards ground and only low voltage) one might use a separate MOSFET.

[tatus1969]

How to make that multiplexing mentioned in the end of the video?
Is it better to use some mosfets? Or is there any specialized IC?

that should all be done by the big rotary switch, no need for additional electronics.

[f4eru]

>>that should all be done by the big rotary switch, no need for additional electronics.
No. That's so '70s.
Today, every moldy meter has autoranging.

[tatus1969]

>>that should all be done by the big rotary switch, no need for additional electronics.
No. That's so '70s.
Today, every moldy meter has autoranging.

good point...

[Kleinstein]

Switching the shunts is usually still done mechanically - auto ranging with switching shunts can cause some upsets. Changing amplification is definitely better done under software control as auto-ranging.

Depending on the form of the 10 mOhms shunt, one may not even need to switch the sense voltage. Just have the shunts in series - especially also having the 100 Ohms one also in series is not problem. If at all having the 1 Ohms and 10 mOhms in series could be a slight problem as there will be some coper in the effective shunt part.  In this case only changing the amplification would be left to an electronic switch/FET.

[David Hess]

>>that should all be done by the big rotary switch, no need for additional electronics.
No. That's so '70s.
Today, every moldy meter has autoranging.

Automatic ranging is not a killer feature.  For those who prefer manual ranging, it is still available.  B&K for instance has 10 different models with manual ranging to complement their automatic ranging multimeters.

[EEVblog]

PART 2:

[riyadh144]

I have been thinking why don't we use a current mirror with an output ratio small enough? I am sure we can find Mosfets with low Rds, and so we can sense the current with that.

[Dr. Frank]

Using the (parasitic) reverse diodes of the MOSFETs for protection purposes will affect the precision of the sensitive ranges, due to their leakage currents. These lower ranges resolve nA, maybe pA currents.

Especially, if you apply a reverse current, in the "negative" direction of the DMMs input jacks, these 50mV burden voltage already create relevant forward currents in these MOSFET diodes, and if you combine two of such MOSFETs in parallel, this would be the case for either direction.

More common precision current protection schemes use either the low leakage of the B-E diode of a bipolar transistor, like in the HP3458A, which includes power dissipation protection by software, or they use at least two diodes of a rectifier bridge in series, combined with an OpAmp to reduce these diode leakage currents to near zero, like in most of HPs 6 1/2 digits DMMs.






Daves last circuit with one shunt for each decade can be used in DMMs with electronic gain calibration only, as the shunts for the different ranges have the sequence 0.01, 0.11, 1.11, 11.11 and 111.11, not a decade sequence.
Therefore this can't be used for an external front end, like the µcurrent.


Frank

[Psi]

Regarding the mosfet idea.

What happens when John uses his meter to measure 20A but at the full rated voltage of 600/1000V.

Unless i've missed something you're going to need some pretty HUGE mosfets to do 20A @ 1kv?
And high voltage mosfets usually have terrible on-resistance

Apologies if i missed something in the video, i did skip through a little. 


Looking at $8 for one that can do 50A at 600V with 70mR RDS
http://www.digikey.co.nz/product-detail/en/alpha-omega-semiconductor-inc/AOK53S60L/AOK53S60L-ND/3973573

Or $36 to get better RDS of 17mR  (150A/600V)
http://www.digikey.co.nz/product-detail/en/ixys/IXFB150N65X2/IXFB150N65X2-ND/5629500

[bktemp]

Regarding the mosfet idea.

What happens when John uses his meter for 20A but at the full rated voltage of 600/1000V.

Unless i've missed something you're going to need some pretty HUGE mosfets to do that?
And high voltage mosfets usually have terrible on-resistance

The voltage doesn't care, because ou are measuring current. The voltage drop across the current shunt is limited by the back to back diodes (mosfet's body diodes) to around +/-1V for a short time until autoranging kicks in and connects the 10mOhm shunt, limiting the voltage furthur until the fuse blows when the current is too high.
You can use low voltage (<10V) mosfets.

[Psi]

Ah, got ya

That's pretty neat!
Works by the fact that the two body diodes means the "off" fets don't conduct until ~1V and the current always takes the part of least resistance through the "on" fet.

[EEVblog]

Using the (parasitic) reverse diodes of the MOSFETs for protection purposes will affect the precision of the sensitive ranges, due to their leakage currents. These lower ranges resolve nA, maybe pA currents.

Gossen and Tek/Fluke get away with it just fine for 10nA resolution.

More common precision current protection schemes use either the low leakage of the B-E diode of a bipolar transistor, like in the HP3458A, which includes power dissipation protection by software, or they use at least two diodes of a rectifier bridge in series, combined with an OpAmp to reduce these diode leakage currents to near zero, like in most of HPs 6 1/2 digits DMMs.

Bench meters also usually have poor CAT rating.

Daves last circuit with one shunt for each decade can be used in DMMs with electronic gain calibration only, as the shunts for the different ranges have the sequence 0.01, 0.11, 1.11, 11.11 and 111.11, not a decade sequence.
Therefore this can't be used for an external front end, like the µcurrent.

Of course, just change the value if you wanted that.

[bktemp]

Using the (parasitic) reverse diodes of the MOSFETs for protection purposes will affect the precision of the sensitive ranges, due to their leakage currents. These lower ranges resolve nA, maybe pA currents.

Gossen and Tek/Fluke get away with it just fine for 10nA resolution.

I did some quick measurments: The leakage current of most low voltage power mosfets is <5nA (10pA-5nA depending on mosfet and polarity) at room temperature and +/-50mV. If you heat it up, the current goes much higher (>1uA).
Most datasheets specify <1uA at room temperature, so maybe you need to verify each mosfet or at least some batches at maybe 40°C or so before using it for such an application.

[tszaboo]

What happens when John uses his meter to measure 20A but at the full rated voltage of 600/1000V.

How does that work? You have 1000V over the current terminals?
You got that Dave is talking about handheld DMM, that is why space, and price is important. Two cables connected only, and it is floating.

I'm not particularly fan of the multiplexer reduction. Probably it is better for the BOM, but MUXes are not expensive. A DG409, you would typically use for this, 2x4:1 is 80 cents. I like routing those analog signals close to each other, have symmetrical filtering, symmetrical tap from the shunt, and so on. Also, your last suggestion is with 5 shunts. A nice 0R1 or 1R shunt is expensive. Recently, I had to design a circuit to measure 500mA with 6.5 digit multimeter accuracy. The most expensive part was the shunt.
Also, probably the biggest issue I'm having with your setup: I can measure -100mA with my Fluke 87. Does this rely on the fact that the FET backwards will not conduct with a voltage of ~100mV? Surrely it will conduct some and it will be hugely temperature dependent. As will the FET conduct when it is open, up to few hundred microamps. These new, switching, low cost FETs will be especially bad for it, as all characteristic they care about is Rds and maybe gate charge. I worked with these FET switches for current measurement, they are all but ideal.
So, i encourage you to actually build the circuit. But then again, maybe it is fine for 3-4 digit.

[Brutte]

I have been thinking why don't we use a current mirror with an output ratio small enough? I am sure we can find Mosfets with low Rds, and so we can sense the current with that.

The current mirror is just primitive gain stage (A type amplifier) controlled in feedback to mirror the current of an input stage. So that is a simplified variation of an active circuit mentioned. The temperature dependence of both uni and bipolar mirrors can be matched maybe to 1% with the tongue at right angle. Dave is looking for something with 50000 counts so that is way too far from the goal.

For measuring 10A currents of a 3.3V power supply I think a simple and practical option is to use a remote shunt (R01) and a 50000 count voltmeter set to a 100mV full scale range. A kind of custom leads with shunt at the end maybe.
With standard leads, even 1mm diameter copper leads (2*0.5m) give over 0R02 resistance and 200mV drop at 10A so that won't work well with 3V3 supply.

[Dr. Frank]

I did some quick measurments: The leakage current of most low voltage power mosfets is <5nA (10pA-5nA depending on mosfet and polarity) at room temperature and +/-50mV. If you heat it up, the current goes much higher (>1uA).
Most datasheets specify <1uA at room temperature, so maybe you need to verify each mosfet or at least some batches at maybe 40°C or so before using it for such an application.

You need to measure the leakage of the protection/reverse diode, when the FET is explicitly switched off, and also the FORWARD current at 50mV bias, when the FET is explicitly switched off.

The FETs own leakage is also interesting, but has to be separated.

Either forward or leakage (reverse) current might be as high as many 100nA, up into the µA range, at higher temperatures.

Frank

[EEVblog]

Also, probably the biggest issue I'm having with your setup: I can measure -100mA with my Fluke 87. Does this rely on the fact that the FET backwards will not conduct with a voltage of ~100mV? Surrely it will conduct some and it will be hugely temperature dependent. As will the FET conduct when it is open, up to few hundred microamps. These new, switching, low cost FETs will be especially bad for it, as all characteristic they care about is Rds and maybe gate charge. I worked with these FET switches for current measurement, they are all but ideal.
So, i encourage you to actually build the circuit. But then again, maybe it is fine for 3-4 digit.

Both Gossen and Tek/Fluke got away with this MOSFET switching solution just fine in their high end meters with at east 10nA resolution.
Yes device selection will be important, but it works.

[bktemp]

You need to measure the leakage of the protection/reverse diode, when the FET is explicitly switched off, and also the FORWARD current at 50mV bias, when the FET is explicitly switched off.

I shorted source and gate together and measured the drain-source current at +50mV / -50mV and also at +100mV / -100mV. I didn't control the temperature, because I only wanted a rough estimate of the typical D-S leakage current for some mosfets at room temperature.
I also tried measuring gate-source current at +/-10V, but it is too low for the capabilities of my measuring equipment (1pA resolution).
At least at room temperature, both the forward and reverse leakage current is negligible for nA resolution.

The temperature is probably the biggest problem, maybe I do some measurements at different temperatures, because the leakage current probably increases exponentially:
Let's assume the mosfet used has 10mOhms RDson. Because there are two in parallel we have 5mOhms total. At 10A the mosfets will dissipate 0.5W total. The datasheet gives max. 50K/W for a mosfet in SO8 package. 0.25W per mosfet will rise the junction temperature to 12.5°C above pcb temperature (probably more, because they are close together). The 10mOhm shunt dissipates 1W and increases the temperature of the pcb near the mosfets probably to more than 10°C above ambient, giving a mosfet junction temperature of about 50°C. If you are trying to measure nAs immediately after measuring 10A the measurement will probably be not accurate.