Author Topic: EEVblog 121GW Multimeter Issues  (Read 180253 times)

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Online Dr. Frank

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Re: EEVblog 121GW Multimeter Issues
« Reply #575 on: August 22, 2018, 08:23:29 am »
UEi have reported back on the Mohm range drift issue, and the problem is the 1N4007 diodes D7 and D8, they are going to be replaced by a single TVS in future builds.
I don't have any further details, but I presume there is some sort of temperature/voltage dependency related issue with those diodes that can manifest itself under certain circumstances.
They are sending me an updated meter with the new part fitted for testing.

I assumed a temperature dependent leakage current in Ohm mode, due to protection diodes:
https://www.eevblog.com/forum/testgear/eevblog-121gw-discussion-thread/msg1728887/#msg1728887

but only now I know where these are located!

It's clear, that 1N4007 has high leakage currents on reverse voltages, not suitable for such a circuit. These create non-linearity over voltage, and the strong temperature dependence of this leakage current creates that huge 0.64%/K gain change in 50MOhm range.

Edit: I'd replace these 1N4007 by reversed npn transistors, as these usually have very low leakage current, as already being used similarly  in the 121GW, i.e. Q1, Q2 (n.a.), Q3, Q5. n-JFETs. Used as diodes, these have low leakage, but also high breakdown voltage (which the E-B diode of npns don't have), as 15V may be present there.

Frank
« Last Edit: August 22, 2018, 11:23:17 am by Dr. Frank »
 

Offline M4x

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Re: EEVblog 121GW Multimeter Issues
« Reply #576 on: August 22, 2018, 11:49:43 am »
I have one of the kickstarter meters and would be happy to check some things out on it. But only be able to get to it on a weekend.



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+1.

I've one of the first available meters ordered directly from Dave. I've standard components, a linear laboratory power supply, a Gossen Metrahit One Plus, different test leads and a Logic analyzer from Saleae (can record analog signals) at hand.

Just tell me what you would like to see.
 

Online zucca

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Re: EEVblog 121GW Multimeter Issues
« Reply #577 on: August 22, 2018, 12:29:19 pm »
I'd replace these 1N4007 by reversed npn transistors, as these usually have very low leakage current, as already being used similarly  in the 121GW, i.e. Q1, Q2 (n.a.), Q3, Q5. n-JFETs. Used as diodes, these have low leakage, but also high breakdown voltage (which the E-B diode of npns don't have), as 15V may be present there.

Cool idea, and then recal the meter isn't it? Keep us posted.
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Offline bicycleguy

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Re: EEVblog 121GW Multimeter Issues
« Reply #578 on: August 22, 2018, 05:05:49 pm »
One problem is that hundreds of meters have been shipped, but very few people are commenting in this thread or reporting their experiences with their meters. As Joe said, it is almost as if everyone has just put them on a shelf in their boxes for posterity.

So trying to find a pattern is very difficult with such a small sample size.
I take a little offense to that thought.

Most people have no time to look at every little problem, most of which, like this, are in the noise.  Thank you for looking at this problem though, but don't expect everyone to care about it.  I have used the meter nearly everyday since its arrival and found and posted a few issues I've had.  If I was measuring sub 3ohm resistances I would be more concerned, but I'm not.  I also think that maybe a different instrument might be better suited for that task.

I also have to comment that if the software was truly open source (or hacked to be) as I thought when I purchased the meter I might be more inclined to get into the discussion.

Edit: I think I new going in the software wasn't open source, but assumed it would be revealed shortly.  It's getting there: https://www.eevblog.com/forum/testgear/eevblog-121gw-multimeter-firmware-details/msg1407514/#msg1407514
« Last Edit: August 23, 2018, 04:11:10 pm by bicycleguy »
 

Offline JS

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Re: EEVblog 121GW Multimeter Issues
« Reply #579 on: August 22, 2018, 05:20:43 pm »


...
As for logging, if the spikes were real I would want to see them but they are not.   I don't see a problem like this with even the cheapest logging meters I have when sampling at the same rates.    I really can't think of a time where I have needed a handheld meter to record faster than a second.  In most cases, even that is fast.   If I am need to see faster changing signals, I am using something else to collect the data.  For me, what ever I collect, I want the data to be accurate first and foremost.   If the meter is not the right tool for the job because of it's speed, I will use another tool.
  I think that slower samples in 121gw are single samples stored every once in a while but from the set of fast samples, not filtered to get better, more stable readings, so the spikes would still br there but you wouldn't have way to know that is a spike without the samples around it.
I saw 13 spikes in 15 minutes at 5Sa/s, with the spikes taking at least 3 samples, thats 1% of the samples corrupted.
So if you were to sample once every minute  you would catch a spike every two hours but you probably wouldn't be able to tell thats a spike.

If Im logging Im going to the pc and can filter myself, so I rather use the raw data than a unknown filter. For the display I want a steady reading, so if it can log something to the SD and show other to the display would be fine, but BT logs the display, so would make the app a bit worse. Also, when logging slow I do like to be filtered, as I don't have that much data around each point. The apikes I don't want them anywere, the signal might be changing fast and would be harder to filter. It might be something in the chipset that flags this samples.

JS

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Offline IanB

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Re: EEVblog 121GW Multimeter Issues
« Reply #580 on: August 22, 2018, 06:30:23 pm »
I saw 13 spikes in 15 minutes at 5Sa/s, with the spikes taking at least 3 samples, thats 1% of the samples corrupted.

I saw a spike when measuring DC mV directly (the green trace below just after 100 s). At the time I thought it might have been some external influence. It didn't occur to me that the meter itself might have introduced the artifact. That's interesting.

I'm not an EE--what am I doing here?
 

Offline JS

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Re: EEVblog 121GW Multimeter Issues
« Reply #581 on: August 22, 2018, 10:41:37 pm »
I saw 13 spikes in 15 minutes at 5Sa/s, with the spikes taking at least 3 samples, thats 1% of the samples corrupted.

I saw a spike when measuring DC mV directly (the green trace below just after 100 s). At the time I thought it might have been some external influence. It didn't occur to me that the meter itself might have introduced the artifact. That's interesting.


Thanks! I should check the mV range then, it could help to narrow the cause, IE if it's present there we know for sure the problem is not on the resistor current source but on the ADC side...

JS

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Offline JS

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Re: EEVblog 121GW Multimeter Issues
« Reply #582 on: August 23, 2018, 02:44:55 am »
Hand proximity tests using various configurations with firmware 1.0. 

https://youtu.be/s7QVwFXmxgQ

  You are seeing something similar to what I did, I haven't tested with any ESD protection as I don't even have grounding here, I have a stick half way into earth (need a bigger hammer for the other half) as I'm moving and haven't settled yet.
  I found something interesting that might have to do with this, if I snap the right side of the rubber protector the reading jumps, the left side doesn't do that...

  I will run some logging in mV range now, to look for correlations between the noise I see in the ohms range and there, as usual, the logs without perturbations, no touching buttons.

JS
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Offline JS

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Re: EEVblog 121GW Multimeter Issues
« Reply #583 on: August 23, 2018, 07:34:10 am »
New measurements tonight, this is getting serious...

  I ran some short captures in the mV range, 200 samples or so, I saw a few glitches but interestingly right at samples 61, 121 and 181 at V1.26, even more intresting, in V1.00 there are smaller glitches one or two samples later. With the open leads, right at the start is easy to see the error introduced by me taking the finger out of the meter after pressing mem, in both versions, V1.00 smaller and slower as expected.


  Now at the freq plots, V1.26 shows a flat noise response in both tests, while V1.00 shows a low pass response.


  If we compare the two versions, doing V1.00/V1.26 we see a similar frequency behavior with the leads open or closed.


  Then I decided to make a longer sample, to see if I can catch the pole, left it running for about 11 minutes and cropped some samples at each end, so we can see the response down to a few mHz, for this I think I over did it leaving long leads open far away and ended with some strange noise with a clear frequency at around 1Hz, and the overall noise of the V1.26 was much higher so the ref plot doesn't start at 0dB. Also, I tweaked a freq plot a bit as I don't really know the sampling rate, but with the long test I estimated a slower sampling rate than the 200ms specified in the manual, that's why the response looks a bit shifted.
  Even at a few mHz the response doesn't flattens out, going down by 10dB/dec up to a few 100mHz where it steeps down even further and returns back up, I guess due to the measurement being so noisy. I don't have a more controlled LF noise source to generate for this measurements but should be easy enough for the next test using a zener and a divider to work in the mV range, I guess I'd need to run it even longer to catch the pole. Also, with a more controlled noise source I'd like to run some intercorrelation calculations between the samples to put a number in the glitches being the same between both versions.


  To finish up, the glitches and button press sensitivity is present in both versions, V1.00 clearly has some filtering going on, checking the schematic and datasheet of the HY3131 there are many analog selectable LPF available at the chipset, first C21 with the internal resistors and then C16 to C26 and likely some of the surrounding resistors, plus a configurable digital comb filter after the ADC, to then go the the µC where they could be doing more processing of the signal... We need an open source code right now!!!

JS
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Online EEVblog

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Re: EEVblog 121GW Multimeter Issues
« Reply #584 on: August 23, 2018, 08:11:33 am »
Thanks for this update.  Good to hear they are working on it.  What ever their reason for this change, it could have an added advantage if they pick the parts right.  U9 is the first thing to get damaged in my testing.  I am using a TVS rather than the 4007s to protect it and swapped the HEF4053 for a CD4053.  It held up pretty good after that.

Yes, that is what future meters will have. The TVS is the same as used in the other protection path.
 
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Re: EEVblog 121GW Multimeter Issues
« Reply #585 on: August 23, 2018, 08:14:27 am »
I also have to comment that if the software was truly open source (or hacked to be) as I thought when I purchased the meter I might be more inclined to get into the discussion.

Sorry anyone thought the meter would be open source firmware, that was never promised.
The schematic is available, the PC software is fully open source, and the protocols are open.
 
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Offline firewalker

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Re: EEVblog 121GW Multimeter Issues
« Reply #586 on: August 23, 2018, 08:32:29 am »
Is any of these problem a show stopper? I believe not for the vast majority of users. If you need a feature pact meter the 121GW is great. I would say it's still away from a meter that you can trust all the digits though.

The bugs like not defaulting to the default scale is more "serious" imho.

Maybe after all the hardware changes someone could purchase only a bare populated pcb to replace the old revision!  :scared: :scared: :scared:

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Offline JS

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Re: EEVblog 121GW Multimeter Issues
« Reply #587 on: August 23, 2018, 08:53:19 am »
Show stopper not at all, but I have two reasons to have this quality time with my meter, first getting to know it's strengths and limitations, take the most out of it, know when it's suitable or not. Then if I can help polish it a little bit firther great, I don't think this is a project without any more effort into it, I still see tweaks being made, I wish I had my meter much earlier and have a pinch into helping for any upgrade, but I'm not demanding anything, just putting some more processed info and MHO into this.

I always understood that the firmware wasn't going to be open source, I'm not demanding that the UEI original firmware gets releases. The display lib already is public, the schematic is public, the datasheets are available, all it's left is getting to wright some code and some people is already doing so. I'm not great at that, not my highest strenght, there are better people with that around here, at least to start building up something more complete but I would totally jump in to help in the signal processing for instance. I'd probably start mocking around with the app sooner, where some of that can be done and still useful to add functionality in the phone display that's not on the main display already.

Thanks Dave for all the info about this meter, there is no other brand new one around that has this much info available...

JS

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Online EEVblog

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Re: EEVblog 121GW Multimeter Issues
« Reply #588 on: August 23, 2018, 11:21:23 am »
The bugs like not defaulting to the default scale is more "serious" imho.

What bug is that?  :-//
 

Offline firewalker

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Re: EEVblog 121GW Multimeter Issues
« Reply #589 on: August 23, 2018, 11:47:08 am »
The bugs like not defaulting to the default scale is more "serious" imho.

What bug is that?  :-//

Select a position. E.g. V DC. Press Mode to change it in AC. Turn off the meter and on. Select again V. It remembers the AC mode selected.

Alexander.
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Re: EEVblog 121GW Multimeter Issues
« Reply #590 on: August 23, 2018, 11:54:49 am »
Select a position. E.g. V DC. Press Mode to change it in AC. Turn off the meter and on. Select again V. It remembers the AC mode selected.

That's a feature. Some like it, some don't. The Brymen does exactly the same thing.
I can't see how this can possibly be a "serious" issue?
 

Offline firewalker

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Re: EEVblog 121GW Multimeter Issues
« Reply #591 on: August 23, 2018, 12:00:04 pm »
One scenario is that you try to measure a high DC or AC voltage and the meter is in the other mode. It will show 0...

Could it be implemented as a user option?

The manual clearly states that the DC is the default mode.



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Re: EEVblog 121GW Multimeter Issues
« Reply #592 on: August 23, 2018, 12:03:03 pm »
One scenario is that you try to measure a high DC or AC voltage and the meter is in the other mode. It will show 0...

It's a multimeter, it does multiple things, you should check your range before measurement.
Look, many people love this feature.
Yes, maybe it can be an option, but that's way down our list of issues to address.
We'll fix the manual.
 
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Offline firewalker

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Re: EEVblog 121GW Multimeter Issues
« Reply #593 on: August 23, 2018, 12:05:47 pm »
We'll fix the manual.

Bug solved! :D :D :D

Alexander.
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Offline chronos42

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Re: EEVblog 121GW Multimeter Issues
« Reply #594 on: August 23, 2018, 03:04:03 pm »
Hi,

I think I found another design issue in the meter circuit.
I wanted to find out whether the meter can be used with lithium batteries instead of standard cells or not.
Problem with lithium cells ist the higher voltage. 4 lithium cells have up to 7.7V when they come out of the package. After a short time the voltage goes back to approximate 6.6V and stays a long time at this value.
Problem or not?
I could not find any hint to this question, so I started to explore the schematics.
First it looked good, all internal voltages came from low drop regulators.
+4V, VDD(3.3V) analog supplies and VDD_P  (3.3V) for the 15V booster and the CPU.
So, no problem at all, none of the voltages can go to high.
But wait, what is that?
The voltage reference, ZD1, a ADR3412 reference regulator is connected directly to B+ via R94, a 0 ohm resistor. B+ is the battery voltage. And this is a problem in my opinion. According to the datasheet the input voltage range of the ADR3412 is 2.3V..5.5V, the absolute maximum is 6V.
That means even with alkaline batteries the voltage is above the input range and with new alkaline batteries also above the absolute maximum. With lithium cells it is way off from the allowed input voltage.
Populate R12 instead of R94 will supply the reference with +4V, so no problems in this configuration. But R94 is populated, not only in the schematics. I have checked this in my meter and actually R94 is populated. I will remove R94 and place it in the R12 position.I cannot understand why the reference is connected to B+, that makes no sense for me. Is this really a design flaw or did I overlook something?
« Last Edit: August 23, 2018, 06:15:04 pm by chronos42 »
 

Offline JS

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Re: EEVblog 121GW Multimeter Issues
« Reply #595 on: August 23, 2018, 04:20:56 pm »
Hi,

I think I found another design issue in the meter circuit.
I wanted to find out whether the meter can be used with lithium batteries instead of standard cells or not.
Problem with lithium cells ist the higher voltage. 4 lithium cells have up to 7.7V when they come out of the package. After a short time the voltage goes back to approximate 7 V and stays a long time at this value.
Problem or not?
I could not find any hint to this question, so I started to explore the schematics.
First it looked good, all internal voltages came from low drop regulators.
+4V, VDD(3.3V) analog supplies and VDD_P  (3.3V) for the 15V booster and the CPU.
So, no problem at all, none of the voltages can go to high.
But wait, what is that?
The voltage reference, ZD1, a ADR3412 reference regulator is connected directly to B+ via R94, a 0 ohm resistor. B+ is the battery voltage. And this is a problem in my opinion. According to the datasheet the input voltage range of the ADR3412 is 2.3V..5.5V, the absolute maximum is 6V.
That means even with alkaline batteries the voltage is above the input range and with new alkaline batteries also above the absolute maximum. With lithium cells it is way off from the allowed input voltage.
Populate R12 instead of R94 will supply the reference with +4V, so no problems in this configuration. But R94 is populated, not only in the schematics. I have checked this in my meter and actually R94 is populated. I will remove R94 and place it in the R12 position.I cannot understand why the reference is connected to B+, that makes no sense for me. Is this really a design flaw or did I overlook something?
I think a lower rail would be even better, as the voltage at 4V reg would change with batteries running low, and to avoid the up to 50ppm/V line regulation coef, typ is lower but gets worse when it's warm. Using a 3.3V rail would make sense but a quiet one would be needed, as a LDO running there will stay there even with the batteries at 0.9V or 0.8V each.

The footprint is not there, so the 4V would still be much more stable than the batteries so... And the meter should stay within cal at 4V so you could do the swap and still kind of trust factory calibration, but meh...

JS

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Offline bicycleguy

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Re: EEVblog 121GW Multimeter Issues
« Reply #596 on: August 23, 2018, 04:22:52 pm »
I also have to comment that if the software was truly open source (or hacked to be) as I thought when I purchased the meter I might be more inclined to get into the discussion.

Sorry anyone thought the meter would be open source firmware, that was never promised.
The schematic is available, the PC software is fully open source, and the protocols are open.

Sorry for my comment, I've edited it.  I first thought it was open source firmware, but by the time I ordered I knew what I was getting and don't regret the purchase .. love it and look forward to improving it.
 

Offline Seppy

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Re: EEVblog 121GW Multimeter Issues
« Reply #597 on: August 24, 2018, 12:26:34 am »
 

Offline JS

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Re: EEVblog 121GW Multimeter Issues
« Reply #598 on: August 24, 2018, 12:50:12 am »
I had a glitch today, V1.26 in the 50Ω manual range, leads open showed OFL as expected, with shorted leads, 22Ω or even 1kΩ resistor connected showed 0.000Ω (not GΩ, MΩ or kΩ, I looked at the screen like 10 times to be sure)

Happened just once and I've not been able to reproduce it yet.

JS
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Online EEVblog

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Re: EEVblog 121GW Multimeter Issues
« Reply #599 on: August 24, 2018, 12:52:29 am »
Hi,

I think I found another design issue in the meter circuit.
I wanted to find out whether the meter can be used with lithium batteries instead of standard cells or not.
Problem with lithium cells ist the higher voltage. 4 lithium cells have up to 7.7V when they come out of the package. After a short time the voltage goes back to approximate 6.6V and stays a long time at this value.
Problem or not?
I could not find any hint to this question, so I started to explore the schematics.
First it looked good, all internal voltages came from low drop regulators.
+4V, VDD(3.3V) analog supplies and VDD_P  (3.3V) for the 15V booster and the CPU.
So, no problem at all, none of the voltages can go to high.
But wait, what is that?
The voltage reference, ZD1, a ADR3412 reference regulator is connected directly to B+ via R94, a 0 ohm resistor. B+ is the battery voltage. And this is a problem in my opinion. According to the datasheet the input voltage range of the ADR3412 is 2.3V..5.5V, the absolute maximum is 6V.
That means even with alkaline batteries the voltage is above the input range and with new alkaline batteries also above the absolute maximum. With lithium cells it is way off from the allowed input voltage.
Populate R12 instead of R94 will supply the reference with +4V, so no problems in this configuration. But R94 is populated, not only in the schematics. I have checked this in my meter and actually R94 is populated. I will remove R94 and place it in the R12 position.I cannot understand why the reference is connected to B+, that makes no sense for me. Is this really a design flaw or did I overlook something?

The voltage reference is connected to AGND, not AVSS which is the battery negative. The full battery voltage is not across the reference, as AGND is a referenced voltage rail lifted above ground.
If you actually measure the voltage across the reference I think you'll find it's fine.
 


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