Suggestions for a millivolt DMM

[joeqsmith]

If i would go with option (b) would that be something like a  μcurrent gold + an ordinary multimeter ? if not, would you recommend something prebuilt ?

I think that would depend what you are trying to do.  If you are attempting to read AC riding on some potentially lethal high voltage DC source or something else that is totally safe and you understand what you are doing.    Building a simple amplifier would not pose much of a problem if you are working in low voltage circuits.   Obviously, what I have shown has zero risk for me or the test equipment.    Post some details about what you are trying to do.   

[Electro Detective]

..After watching so many of Dave's harsh and critical DMM reviews,  I'm interested in seeing for myself what he is willing to put his name on. 

After two years, I assume it's as good as it's going to get and this was as good of an excuse as any to finally have a look.

Not only were "Dave's harsh and critical DMM reviews" most welcome from a buyer choice perspective
and a way overdue safety aspect heads up     > no thanks to meter manufacturers on that 
many of these reviews IIRC were made way before any EEVblog meter was planned   

And isn't there the price point to consider too?
DJ isn't flogging a $1000+ bells and whistles meter that falls short on the AC low mV specs as many big dollar meters do anyway

nor do I think the initial design intentions had really low AC mV requirements

Besides, AC millivolts down that low is sub audio territory anyway isn't it?

i.e. buy a dedicated low millivolt AC meter if you really need one, as it's not gonna happen with most 'All In Ones' at any price

[joeqsmith]

..After watching so many of Dave's harsh and critical DMM reviews,  I'm interested in seeing for myself what he is willing to put his name on. 

After two years, I assume it's as good as it's going to get and this was as good of an excuse as any to finally have a look.

Not only were "Dave's harsh and critical DMM reviews" most welcome from a buyer choice perspective
and a way overdue safety aspect heads up     > no thanks to meter manufacturers on that 
many of these reviews IIRC were made way before any EEVblog meter was planned   

And isn't there the price point to consider too?
DJ isn't flogging a $1000+ bells and whistles meter that falls short on the AC low mV specs as many big dollar meters do anyway

nor do I think the initial design intentions had really low AC mV requirements

Besides, AC millivolts down that low is sub audio territory anyway isn't it?

i.e. buy a dedicated low millivolt AC meter if you really need one, as it's not gonna happen with most 'All In Ones' at any price

I wouldn't say that OP is working in audio as there are certainly applications where we are interested in even lower noise levels having nothing to do with audio.   This particular measurement is just one small aspect of what I will be looking at with the 121GW. 

Having looked at the early meter and uncovering several problems, I am very interested in seeing how it has changed.    I was following the firmware updates for a while, but I am really interested in the hardware. 

The 121GW's switch was a weak point with the early version.   While Dave made a video showing a switch life cycle test, when looking at pictures various people had posted, it appeared that they had used various contact designs. 

While I do mention the price of the products, it really has no bearing on how I conduct my tests.   The free meters from Harbor Freight are treated the same as a $900 Gossen for the data I collect.   After testing so many products, I have my favorite brands.   I certainly didn't start out being pro Fluke but the data changed my view.   Point being that I really don't care how the meters perform and the test jigs have no brand bias.   So while people will tell me I am on witch hunt while I am looking at their favorite brand, the data speaks for itself.

Safety wise, I would have no idea, nor do I care.   IMO, the best thing anyone could do would be to check the reports and see how often the company inspects their products.  If you designed a new product, had it certified, then started changing the design, you will normally go through the process again.   You may find companies performing annual tests.   The cost can be significant.  Perhaps  this is why no frills meters like the Fluke 101 that have a very limited feature set, cost $50.  The Aneng meters may cost half that with far more features but if safety is really important, what one would you want to use?

****
Too little coffee.  Fixed some grammar problems.

[Sultanpepper123]

If i would go with option (b) would that be something like a  μcurrent gold + an ordinary multimeter ? if not, would you recommend something prebuilt ?

I think that would depend what you are trying to do.  If you are attempting to read AC riding on some potentially lethal high voltage DC source or something else that is totally safe and you understand what you are doing.    Building a simple amplifier would not pose much of a problem if you are working in low voltage circuits.   Obviously, what I have shown has zero risk for me or the test equipment.    Post some details about what you are trying to do.

Currently I'm working on an Energy utility meter and i'm trying to periodically measure the current transformer's output after the burden resistor where readings start from 119.68uV , so as far as i can tell as long as the burden is there and both CT terminals are connected, this is considered "safe" . I'm also reading mains voltage[230V] which is referenced to neutral ( voltage divided ) to around 140 mV. the concern here is at the CT which will be giving  out very small readings which the microcontroller will read but form time to time i'll need to make sure the metrology front end is reading things correctly, should someone jump in and say that DMM is not useful in this case and an oscilloscope is , i currently have a Siglent SDS1104X-E but i'm sure it wont be able to read those numbers unless introduced to a LNA or Dave's uCurrent ? right now i'm thinkning out loud. Would really appreciate sharing your thoughts     .

Regards
Sultan

[SilverSolder]

If i would go with option (b) would that be something like a  μcurrent gold + an ordinary multimeter ? if not, would you recommend something prebuilt ?

I think that would depend what you are trying to do.  If you are attempting to read AC riding on some potentially lethal high voltage DC source or something else that is totally safe and you understand what you are doing.    Building a simple amplifier would not pose much of a problem if you are working in low voltage circuits.   Obviously, what I have shown has zero risk for me or the test equipment.    Post some details about what you are trying to do.

Currently I'm working on an Energy utility meter and i'm trying to periodically measure the current transformer's output after the burden resistor where readings start from 119.68uV , so as far as i can tell as long as the burden is there and both CT terminals are connected, this is considered "safe" . I'm also reading mains voltage[230V] which is referenced to neutral ( voltage divided ) to around 140 mV. the concern here is at the CT which will be giving  out very small readings which the microcontroller will read but form time to time i'll need to make sure the metrology front end is reading things correctly, should someone jump in and say that DMM is not useful in this case and an oscilloscope is , i currently have a Siglent SDS1104X-E but i'm sure it wont be able to read those numbers unless introduced to a LNA or Dave's uCurrent ? right now i'm thinkning out loud. Would really appreciate sharing your thoughts     .

Regards
Sultan

How many ohms is the burden resistor for the CT,  and how many ohms are the two resistors in the high voltage divider?

Knowing that would help find the most cost effective way of measuring the values you are looking for (including figuring out if a uCurrent would work).

[Sultanpepper123]

If i would go with option (b) would that be something like a  μcurrent gold + an ordinary multimeter ? if not, would you recommend something prebuilt ?

I think that would depend what you are trying to do.  If you are attempting to read AC riding on some potentially lethal high voltage DC source or something else that is totally safe and you understand what you are doing.    Building a simple amplifier would not pose much of a problem if you are working in low voltage circuits.   Obviously, what I have shown has zero risk for me or the test equipment.    Post some details about what you are trying to do.

Currently I'm working on an Energy utility meter and i'm trying to periodically measure the current transformer's output after the burden resistor where readings start from 119.68uV , so as far as i can tell as long as the burden is there and both CT terminals are connected, this is considered "safe" . I'm also reading mains voltage[230V] which is referenced to neutral ( voltage divided ) to around 140 mV. the concern here is at the CT which will be giving  out very small readings which the microcontroller will read but form time to time i'll need to make sure the metrology front end is reading things correctly, should someone jump in and say that DMM is not useful in this case and an oscilloscope is , i currently have a Siglent SDS1104X-E but i'm sure it wont be able to read those numbers unless introduced to a LNA or Dave's uCurrent ? right now i'm thinkning out loud. Would really appreciate sharing your thoughts     .

Regards
Sultan

How many ohms is the burden resistor for the CT,  and how many ohms are the two resistors in the high voltage divider?

Knowing that would help find the most cost effective way of measuring the values you are looking for (including figuring out if a uCurrent would work).

The CT is 1:2500 , starting current is 40 mA till 110 A. with a load resistance rating of 10 ohm CT, my burden resistor is 7.48 ohms . The voltage divider is 10 in series 165K ohm resistors (1.65 M ohm  total) as the top resistor and lower resistor in the V.divider is 1K ohm.

Regards
Sultan

[joeqsmith]

So 40 mA is roughly 120uV and 110A is 330mV.   Sounds like you would really want to just gain up the voltage then.     Even if the sense opens, it seems like it's fairly limited as far as having the amp survive.   

The problem may be that say you use a 1000X amplifier, it would be wanting to put out 330V.    Assuming you are testing steady state and can manually change the gain, you could almost use that amplifier I show the pictures of.    I have 10,100 and 1000X, and using the 10X or 3.3V output wouldn't be a problem with a different battery pack.  Maybe just a couple of back to back diodes to clamp the inputs if you open the sense for some reason.

Another option may be something like Tektronix's ADA400A which has up to a 100X gain. 
http://w140.com/tek_ADA400A.pdf

[SilverSolder]

...and just to comment on the uCurrent, it could be pressed into service as a low level voltmeter in this situation but you would have to put a resistor in series with it to read all of the wide range you are interested in.  Or, you could use it directly as Dave intended, i.e. as a current meter and measure the current through your resistors - but that would require breaking into the circuit on the board so would be for bench testing only.

Personally, since I already own a uCurrent, I would go right ahead and use it - but if you have to go out and buy stuff, a high impedance voltage amplifier as suggested by @joeqsmith (I haven't used that specific model) is more convenient for this specific use case.   

[tautech]

Currently I'm working on an Energy utility meter and i'm trying to periodically measure the current transformer's output after the burden resistor where readings start from 119.68uV , so as far as i can tell as long as the burden is there and both CT terminals are connected, this is considered "safe" . I'm also reading mains voltage[230V] which is referenced to neutral ( voltage divided ) to around 140 mV. the concern here is at the CT which will be giving  out very small readings which the microcontroller will read but form time to time i'll need to make sure the metrology front end is reading things correctly, should someone jump in and say that DMM is not useful in this case and an oscilloscope is , i currently have a Siglent SDS1104X-E but i'm sure it wont be able to read those numbers unless introduced to a LNA or Dave's uCurrent ? right now i'm thinkning out loud. Would really appreciate sharing your thoughts     .

Regards
Sultan

119.68uV will be right down in the channel base noise however if you select a p-p measurement and engage Statistics you might get some usable data. Give it a try. Of course you'll need to have 1x probe and input selected. 

[Sultanpepper123]

So 40 mA is roughly 120uV and 110A is 330mV.   Sounds like you would really want to just gain up the voltage then.     Even if the sense opens, it seems like it's fairly limited as far as having the amp survive.   

The problem may be that say you use a 1000X amplifier, it would be wanting to put out 330V.    Assuming you are testing steady state and can manually change the gain, you could almost use that amplifier I show the pictures of.    I have 10,100 and 1000X, and using the 10X or 3.3V output wouldn't be a problem with a different battery pack.  Maybe just a couple of back to back diodes to clamp the inputs if you open the sense for some reason.

Another option may be something like Tektronix's ADA400A which has up to a 100X gain. 
http://w140.com/tek_ADA400A.pdf

I've never used one of those, looks interesting. Personally option A would be a pretty good exercise. But since this is work related and i'm not going to pay for it  , I'm leaning towards buying a ready made differential preamplifier, the ADA400A looks to have a proprietary connector. Nevertheless, i've realised a new tool type but after searching a bit i've stumbled upon this, based on your experience would this suffice ? Also correct if i'm wrong, we are now going into the track of using an oscilloscope + differential preamplifier , correct ? or does it work on a multimeter ( i currently have a modest UT61E , will that work ? ). Again massive thanks and appreciation for sticking around and the help and sharing your experience and thoughts 

Regards
Sultan

...and just to comment on the uCurrent, it could be pressed into service as a low level voltmeter in this situation but you would have to put a resistor in series with it to read all of the wide range you are interested in.  Or, you could use it directly as Dave intended, i.e. as a current meter and measure the current through your resistors - but that would require breaking into the circuit on the board so would be for bench testing only.

Personally, since I already own a uCurrent, I would go right ahead and use it - but if you have to go out and buy stuff, a high impedance voltage amplifier as suggested by @joeqsmith (I haven't used that specific model) is more convenient for this specific use case.

Would this be considered a step up / suitable for the application ? , Please let me know if there are other alternatives.

Regards
Sultan

[joeqsmith]

I'm not a fan of using ground referenced equipment to look at line signals.   In your case, the current transformer should give you one layer of protection, then you limit the current a fair bit before reaching the shunt so it doesn't seem like a concern but I'll leave that one up to you. 

Tektronix has a power supply that can be used with the probe, allowing it to work with other scopes or meters.    However, I like the amplifier you linked.  I wasn't able to locate a manual for it, but just looking at the description, it looks like it will work for your application.    It should drive your handheld meter or scope.   

[SilverSolder]

The amp you linked looks like it would do the job -  just never feed it more than 400mV on its input in DC mode.

Looks like a pretty cool product, the only thing I thought was a little bit disappointing is the 1% gain accuracy.

[joeqsmith]

Hence my back to back diode comment.   Same applies to this amp.   

I was thinking 1% was pretty decent when you consider the stackup for the transformer, shunt, meter.   If they need 1% total, they may need to do some tricks but I doubt that's the case.   

[maginnovision]

You could characterize it too so that you know what the gain is, it's pretty flat across the range but with multiple ranges 1% is pretty conservative. I have two of them and they work fine as long as you don't use the included supply.

[joeqsmith]

As promised, two brand new, out of the box, 121GWs.   I used the exact same setup as before, compensating for the errors in the attenuators.    It was then rechecked against one of the Fluke 189s.     The office is 20.8 degC. 

It doesn't appear to be all that great which may be why Dave didn't present data for it but it was better than a lot of the meters I looked at.   

I think you get much better results using the amplifier.   Good luck with your testing. 

[Andreas]

i currently have a Siglent SDS1104X-E but i'm sure it wont be able to read those numbers unless introduced to a LNA or Dave's uCurrent ? right now i'm thinkning out loud. Would really appreciate sharing your thoughts     .

for that application I would use a low noise high resolution oscilloscope e.g.:

https://www.picotech.com/oscilloscope/4262/picoscope-4262-specifications

or if it has to be more accurate something like this:
https://www.dewetron.com/products/power-analyzer/dewe2-pa7/

with best regards

Andreas

[BravoV]

Guys, before suggesting anything, consider that the OP is living in Iran, choices are very limited, even OP can afford it.

[Sultanpepper123]

The amp you linked looks like it would do the job -  just never feed it more than 400mV on its input in DC mode.

Looks like a pretty cool product, the only thing I thought was a little bit disappointing is the 1% gain accuracy.

Thank you very much Sir, i think i will go with the amplifier solution.   

Regards
Sultan

...and just to comment on the uCurrent, it could be pressed into service as a low level voltmeter in this situation but you would have to put a resistor in series with it to read all of the wide range you are interested in.  Or, you could use it directly as Dave intended, i.e. as a current meter and measure the current through your resistors - but that would require breaking into the circuit on the board so would be for bench testing only.

Personally, since I already own a uCurrent, I would go right ahead and use it - but if you have to go out and buy stuff, a high impedance voltage amplifier as suggested by @joeqsmith (I haven't used that specific model) is more convenient for this specific use case.   

Thank you very much for sharing your thoughts 

Regards
Sultan

I'm not a fan of using ground referenced equipment to look at line signals.   In your case, the current transformer should give you one layer of protection, then you limit the current a fair bit before reaching the shunt so it doesn't seem like a concern but I'll leave that one up to you. 

Tektronix has a power supply that can be used with the probe, allowing it to work with other scopes or meters.    However, I like the amplifier you linked.  I wasn't able to locate a manual for it, but just looking at the description, it looks like it will work for your application.    It should drive your handheld meter or scope.   

Noted, that gives the amplifier (which is differential input ) an advantage , correct ?

Regards
Sultan

As promised, two brand new, out of the box, 121GWs.   I used the exact same setup as before, compensating for the errors in the attenuators.    It was then rechecked against one of the Fluke 189s.     The office is 20.8 degC. 

It doesn't appear to be all that great which may be why Dave didn't present data for it but it was better than a lot of the meters I looked at.   

I think you get much better results using the amplifier.   Good luck with your testing. 

Very interesting data  . Kudos to you sir for the huge effort and sharing your results with us here and i must say it was really a pleasure. BTW it turns out i've watched a few videos from your youtube channel a few years back while trying to test my boards with , really interesting videos i must say, keep up the good work.

Regards
Sultan

Guys, before suggesting anything, consider that the OP is living in Iran, choices are very limited, even OP can afford it.

Egypt actually  . But i agree with the rest of statement 

Regards

[SilverSolder]

One thing to try is to use the Averaging acquisition mode on your scope (assuming it has it!), and see if it can pick a signal out of the noise floor.  You might be surprised how powerful averaging can be!    Don't use x10 probes, connect the signal straight in with coax cable to give the best chance of picking something up.  Set the averaging as high as you can, and let'er rip!

[Performa01]

One thing to try is to use the Averaging acquisition mode on your scope (assuming it has it!), and see if it can pick a signal out of the noise floor.  You might be surprised how powerful averaging can be!    Don't use x10 probes, connect the signal straight in with coax cable to give the best chance of picking something up.  Set the averaging as high as you can, and let'er rip!

Actually a SDS1004X-E can do this with its true 500µV/div sensitivity, even without averaging.
For best results, the setup should be tweaked a little, but even so you cannot expect a stellar accuracy for a number of reasons:

• The high voltage tolerant split-path input buffer of modern DSOs exhibits a lot of noise at low frequencies. The already weak signal gets divided down by 10-20dB, then amplified back up to its original level by a (MOS)FET OpAmp. The global LF feedback and offset control introduces some additional very low frequency noise.
• One vertical division on the DSO is equivalent to just 25LSB of the 8bit ADC. Signals that are less than one division peak to peak certainly cannot be measured with great accuracy, even if there was no noise.
• A DSO isn't a precision instrument from the outset...

Keeping the above in mind, you'd need to be prepared for errors up to 10% when dealing with microvolt signals.

I've simulated the situation described by the OP. The strong (100mVrms in my example) voltage signal goes into channel 1 and this also needs to be the trigger channel, since even the excellent trigger system in an SDS1004X-E cannot deal with a noisy 100µVrms signal.
Channel 2 is set to 500µV/div and bandwidth limited to 20MHz.
Max. record length is limited to 70kpts in the Acquire menu in order to get the sample rate down and reduce the bandwith even further by this.
Use standard deviation measurement (aka AC-RMS) for estimating the RMS voltage - or, even better, cycle standard deviation measurement, as I've found this to be a bit more accurate in the given situation.

The first screenshot SDS1104X-E_50Hz_100uVrms_Avg16.png illustrates the measurement with 16 times averaging acquisition. Look at the mean value of the measurement statistics.



The 2nd screenshot SDS1104X-E_50Hz_100uVrms_Norm.png shows the same scenario without averaging. Yes, the signal trace doesn't look as pretty as before, but Cycle Stdev measurement accuracy is hardly affected. So it is possible to get a reasonably quick measurement when needed.

[joeqsmith]

I wonder how linear it is.  Some time ago I was attempting to use over sampling with my scope to improve the resolution.  It was really non-linear.  I ended up making a model of the errors and used that to apply a correction.  It worked alright but I doubt I could get down to these levels.   

[Performa01]

I wonder how linear it is.   

A legitimate question - and there is a reason why I've stated that we have to expect up to 10% error...

I've done another test run and this time the measurements were different - the error was higher.

Here are the results:



As can be seen, the error is consistently between +3% and +7%, hence <4% linearity error.

Yet I wondered where the constant error comes from and did a long term observation on the DSO, where I noticed some ultra low frequency interference, with a peak to peak amplitude almost 10% of the measured value.

I checked the 1mV test signal with a Keithley 2015 THD bench meter and sure enough, the reading was unstable there as well, with readings between 950µV and 1.047mV.

Finally I realized what's going on and changed the frequency to 60Hz, thus clearly separating it from the mains frequency. Lo and behold, the DMM reading was suddenly a fairly stable 1.017mV.

The ultra low frequency interference component was just the difference frequency between 50Hz from the AWG and the 50Hz mains, and that difference was just too low to cancel out within a reasonable number of signal periods. On the DSO, I could use the line trigger and the trigger point moved very slowly - slower than the minute hand of a clock...

I can't be bothered to repeat the test with 60Hz right now, but might do it eventually. Anyway, because of this the results of my test run could be taken as some sort of worst case scenario - and serve as a warning at the same time, that measuring mains related signals might include some interference that is hard to detect and cannot be separated.

[joeqsmith]

If we can amplify the signal to get a stable trigger, then we may as well work with the amplified signal.   

I doubt my DSO would allow me to use the average at this low of a signal (because of the trigger).   Maybe I could capture several cycles in a sweep then in software derive a reference to pull each cycle apart, overlay and do the average.  I would still want to model the DSO's errors to get the most out of it.  Seems like a lot of work to with little gains. 

How well does your DSO do if you use the 100uV signal for the trigger and try to average?

[Kleinstein]

For the trigger one does not need to amplify the actual signal, just a signal in phase (e.g. same generator / mains if mains signal) would be good for the trigger.

Just using a longer data window can also be an option, though it includes more higher frequency noise. So ideally one would want a lower than 20 MHz BW limit.

Using a high degree of averaging and a rather short data window (e.g. 1 or 2 periods) nearly turn the DSO into a lock-in amplifier. 

[SilverSolder]

For the trigger one does not need to amplify the actual signal, just a signal in phase (e.g. same generator / mains if mains signal) would be good for the trigger.

Just using a longer data window can also be an option, though it includes more higher frequency noise. So ideally one would want a lower than 20 MHz BW limit.

Using a high degree of averaging and a rather short data window (e.g. 1 or 2 periods) nearly turn the DSO into a lock-in amplifier. 

I use my scope as a null detector in a bridge, synced to the generator, and using a uCurrent to amplify the current into a ratio transformer "variable tap".  I am getting 0.1ppm resolution by looking at the phase relationship over 1 period - the ability to average the noise out of a noisy AC waveform is definitely a killer feature of a DSO, I can't think of any other instrument that could do this.