Author Topic: Will this 2V signal damage my bench multimeter over time?  (Read 5508 times)

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Offline AmarukTopic starter

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Re: Will this 2V signal damage my bench multimeter over time?
« Reply #25 on: December 31, 2021, 05:55:19 pm »
Here's a challenge  - does any other Bench Meter on the planet do this when set to a manual range (excepting when a range is exceeded of course, which isn't the case here) ?
Eloso

Eloso, I did a measurement along the lines of your experiment above. In my case I used my Keithley 236 SMU as a voltage source as it can measure very low currents while putting out a voltage. I also set it to 4-wire sense and joined force and sense leads at the Siglent SDM3045X terminals (see image below). This is what I found out:

For voltages just below 2.2V, and before the relay activates, the current drawn from the Keithley 236 power supply is about 100nA. For voltages just above 2.2V, and after the relay has activated, the current drawn from the Keithley power supply is about 250nA.

Below is an image of my Siglent SDM3045X in action when I presented it with a 2.3V from the Keithley power supply and sensing at the input terminals. Impressive!

And to answer your question: My Keysight 34461A DMM does not have this type of relay in the manual voltage range.

Finally, I should just also add that the cabling from my Keithley include some in-expensive coax cables from the power supply to a breakout box with 4mm banana connectors so the isolation is not as good as it could be if I had used proper triax cables.
« Last Edit: December 31, 2021, 06:02:58 pm by Amaruk »
 

Offline eloso

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Re: Will this 2V signal damage my bench multimeter over time?
« Reply #26 on: December 31, 2021, 06:17:42 pm »
Quote
Eloso, I did a measurement along the lines of your experiment above. In my case I used my Keithley 236 SMU as a voltage source as it can measure very low currents while putting out a voltage. I also set it to 4-wire sense and joined force and sense leads at the Siglent SDM3045X terminals (see image below). This is what I found out:

For voltages just below 2.2V, and before the relay activates, the current drawn from the Keithley 236 power supply is about 100nA. For voltages just above 2.2V, and after the relay has activated, the current drawn from the Keithley power supply is about 250nA.


Well that should be a lot more accurate than anything I could do - thanks.   Doing the sums seems to indicate that at less than 2.2V the DC input resistance of the Siglent is 9.5Mohm and above 2.2V it is 22Mohm.

Surely, surely thats a bug ?   I know we all have to be cogniscent at all times of the impact of our instruments on the DUT but it is particlarly unhelpful for this impact to change across a definite threshold.  What if you were to ever measure voltage across a very high impedance sensor, tracking it against time ? Oh, that's a nice jump in my figures, just where it crosses 2.2V  ::)

Regards


Eloso
 

Online Kleinstein

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Re: Will this 2V signal damage my bench multimeter over time?
« Reply #27 on: December 31, 2021, 06:30:12 pm »
This really looks like something is totally messed up. A bit like getting wrong firmware (e.g. from the 3055) so that at 2 V another 10 M resistors / divider is added in parallel to 10 M already there below 2 V.

Is the test with the 600 mV range impedance set to 10 M or high Z ?  There is a chance this may make a difference.

Given the relatively small price difference between the SDM3045 and SDM3055, the 3045 does not look that atractive anyway.
 

Online bdunham7

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Re: Will this 2V signal damage my bench multimeter over time?
« Reply #28 on: December 31, 2021, 07:14:25 pm »
For voltages just below 2.2V, and before the relay activates, the current drawn from the Keithley 236 power supply is about 100nA. For voltages just above 2.2V, and after the relay has activated, the current drawn from the Keithley power supply is about 250nA.


That's not kosher.  It also appears not to meet the published specification for DC input impedance which is stated to be 10M +/- 2%.  And is a non-trivial problem--I can think of multiple situations where this would make the meter completely unusable for certain tasks.
A 3.5 digit 4.5 digit 5 digit 5.5 digit 6.5 digit 7.5 digit DMM is good enough for most people.
 

Offline AmarukTopic starter

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Re: Will this 2V signal damage my bench multimeter over time?
« Reply #29 on: December 31, 2021, 08:24:35 pm »
That's not kosher.
And you are absolutely right. My data was actually wrong. This time I hooked up my 34461A in series as a current meter and used my second Keithley 236 power supply as a voltage supply using its 2-wire sensing mode. And now it all checks out just fine-both before the relay activates (1.9V) and after (2.3V). As you can see in the statistics in the images below, the input impedance is steady at around 10Mohm both at 1.9V and at 2.3V. Thus, the relay does not seem to affect input impedance. That is good news!! Sorry for the confusion.

I should also add that my Keithley's do not have valid calibrations but both my Siglent DMM and my Keysight DMM do. That is another reason this second set of data is better - the readings come from calibrated instruments. But I am still not sure what happened in my first set of measurements. A good example of why it is good to re-check measurements that do not make sense.
« Last Edit: December 31, 2021, 08:44:48 pm by Amaruk »
 
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Offline eloso

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Re: Will this 2V signal damage my bench multimeter over time?
« Reply #30 on: January 01, 2022, 11:01:51 am »


Quote
As you can see in the statistics in the images below, the input impedance is steady at around 10Mohm both at 1.9V and at 2.3V. Thus, the relay does not seem to affect input impedance.


Thanks Amaruk.  I too have revisited the measurement.

Still using the voltage across a 1Mohm resistor to calculate current flow into the Siglent when measuring 1.8V and 3.6V.  i.e. either side of the relay "click".

What is different this time is that I am using a different millivoltmeter. I measured the input resistance of this millivoltmeter with the Siglent to be a suprisingly low 800Kohm which obviously loads the 1Mohm current sensing resistor quite a bit.

Because I have learned never to trust my math  :-\   I created an LTSpice model of all of this  and the results  - both before the relay click and after the relay click, the LTSpice numbers all add up to 10Mohm input of the Siglent, matching the mV readings taken on the meter.

So no impedance change on the Siglent when the relay clicks.


I don't know what was going on with the VC8145 but on mV it was giving some very strange results. I did use a short twisted measuring probe set but maybe it was still some noise getting in.


If my latest measurement was the only one, I would probably accept it with nagging doubts, but it ties in perfectly with results obtained by Amaruk which seem beyond doubt anyway.   

So definitely no funny impedance change problem.

When that relay clicks, my VC8145 does give some totally different indication of impedance  so whether it is a bug or not, it is definitely altering something. Based on  our latest measurements I am at a loss to understand exactly what. 

The fact remains that on a manual range or auto range, if I want to measure a voltage between 2.2V and 6 volts , I have to put up with a relay click, and a delay every time I apply the probe to the DUT.  Doesn't make me proud to own, nor happy to use, this meter.

Regards

Eloso

 
 

Offline AmarukTopic starter

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Re: Will this 2V signal damage my bench multimeter over time?
« Reply #31 on: January 01, 2022, 03:22:58 pm »

The fact remains that on a manual range or auto range, if I want to measure a voltage between 2.2V and 6 volts , I have to put up with a relay click, and a delay every time I apply the probe to the DUT.  Doesn't make me proud to own, nor happy to use, this meter.

Exactly!

You can even make the case that there is no manual 6V or 6mA DC ranges on this multimeter...

Sure, the spec sheet states that those ranges exist (see image below) and the meter allows you to select those ranges. But in reality they do not exist.

The point of a manual range is to obtain extra measurement speed (see this answer from Keysight: https://edadocs.software.keysight.com/kkbopen/what-is-the-effect-of-choosing-auto-or-manual-range-580285257.html). Thus, when a manual range is selected the meter should measure and not go between ranges. That defeats the purpose...

Hopefully it is just a firmware issue and it will get resolved quickly.

For your reference, I experienced a "severe" firmware bug with a brand new Keysight power supply not that long ago. It took their support team less then 48 hours to acknowledge and release a new firmware that had a fix in it!! (Link to thread: https://www.eevblog.com/forum/testgear/keysight-edu36311a-current-reading-bug/msg3840200/#msg3840200)

To have a firmware bug in a product is no big deal as it is very difficult to check for everything when you code. It all comes down to customer service AFTER the purchase. How quickly is something fixed? I hope Siglent can show us that their goal is to match and maybe even go beyond other companies in this industry.




« Last Edit: January 01, 2022, 03:27:10 pm by Amaruk »
 

Offline eloso

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Re: Will this 2V signal damage my bench multimeter over time?
« Reply #32 on: January 01, 2022, 05:12:30 pm »

Sensible - The 3045X's other little foibles I don't mind quite as much as this one so if they are able to fix it then fine.

What is the best way to make a bug report that hopefully gets looked at?  My experience with 99% of companies out there is the real guys that know about things are protected from customers behind a wall of  people who don't.  Unpalatable but for understandable reasons of course.


Incidentally, do we know if other machines in the same range exhibit this kind of behaviour ?   Awaiting the outcome of tautech's sporting activity with the 3055 firmware with bated breath !

 
Eloso
 

Online Kleinstein

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Re: Will this 2V signal damage my bench multimeter over time?
« Reply #33 on: January 01, 2022, 05:23:58 pm »
The SDM3055 has a 2 V range as the limit for high Z mode and may thus switch ranges at 2 V in auto mode.  I don't have very much hope the 3055 firmwire would fix the problem - chances are more that accidently some of 3055 firmware made is to the 3045 in the first place and triggers the switching.
They seem to use the same base for the FW, but have still different hardware.

The question is more if this problem was allredy there with much older FW.
 

Offline tautech

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Re: Will this 2V signal damage my bench multimeter over time?
« Reply #34 on: January 01, 2022, 07:17:36 pm »

Sensible - The 3045X's other little foibles I don't mind quite as much as this one so if they are able to fix it then fine.

What is the best way to make a bug report that hopefully gets looked at?  My experience with 99% of companies out there is the real guys that know about things are protected from customers behind a wall of  people who don't.  Unpalatable but for understandable reasons of course.


Incidentally, do we know if other machines in the same range exhibit this kind of behaviour ?   Awaiting the outcome of tautech's sporting activity with the 3055 firmware with bated breath !

 
Eloso
I'll report it when I have further info and write something up. I too strongly suspect a bug.

Sporting activity status:
Still doing due diligence on internal difference between models.
Need to further investigate the China only SDM3055X-E datasheet for ranges and general specs as to how well they match SDM3045X.
Have just 1x SDM3045X in stock that may have to become the workshop DMM......
Hoping Notepad will suffice for FW code edits.  :-//

Unknowns
Western SDM3055 use a Caddock divider whereas SDM3045X use passives but China SDM3055X-E not known.....highly suspect same as SDM3045X.
Avid Rabid Hobbyist.
Some stuff seen @ Siglent HQ cannot be shared.
 
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Offline AmarukTopic starter

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Re: Will this 2V signal damage my bench multimeter over time?
« Reply #35 on: January 10, 2022, 02:16:29 pm »
I have no updates to report yet unfortunately. But I just realized that with instruments that are this good it is easy to forget that it is almost impossible to sell instruments for these consumer friendly prices and offer the same level of customer support as you get from brands that offer products that cost much more (2-3X). Somethings going to give. It is that simple. Having said that, I hope we get a fix for this but I think that is not very likely... I hope I am wrong. :)
 

Offline audiotubes

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Re: Will this 2V signal damage my bench multimeter over time?
« Reply #36 on: December 18, 2023, 08:54:47 pm »
specs as to how well they match SDM3045X.
Have just 1x SDM3045X in stock that may have to become the workshop DMM......

Sorry, didn't see an appropriate place for this, too many Siglent threads on the forum :D

Anyway, can we please get some firmware that stops the horrible green flashing on the power button? I have the SDM3045X, the SDS2104X Plus scope, and the SDG 1032X signal generator, and only the DMM has this "helpful" annoyance.

I have taken apart more gear than many people. But I have put less gear back together than most people. So there is still room for improvement.
 


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