New ANENG multimeter

[Mr. Scram]

When you have to stop and thing under what circumstances is your meter not going to work, what's the point?

Any tool has limitations. It is up to the person using it to know, respect or purposefully exceed them.

[Mark Hennessy]

If you watch the video you will notice that there is a DC and AC component to the waveform.  I have stated many times that the output from this generator is a 220VACrms 60HZ that was full rectified and unfiltered.  This normally provides me with a quick check of the AC, DC and AC+DC functions.   In this case, the meter can not seem to handle the rectified signal reliably.  The 8002 had a similar problem.

Ah - I hadn't seen reference to a DC offset before. That changes things...

I used my arb to get us away from the AC mains so others could perhaps repeat the test in a safe manor.   Again I full wave rectified the output of the Arb.  Yes, a basic function generator is all you would need.   At 400Hz, sine, 10Vrms, full rectified  the 8002 will read 10.01VAC.  My HP will read 10.002.    Switching the 8002 to DC, it reads 20.19 and the HP reads 20.198.  Switching back to AC and the 8002 read 10.01.  So no problems.   

Now lets increase the amplitude to 15VACrms with no other changes.   The 8002 reads 15.01AC and the HP reads 15.004.  Switch to DC and the 8002 reads 30.62 where the HP reads 30.63.  No problem.  Now switch the 8002 back to AC and it reads 3.093AC!  Of course, the HP still reads 15.004.  With the 8002 still in the AC mode, disconnect the leads and reconnect and the meter still reads 3.040.  I disconnect again and reconnect, 15.01.  Hit and miss. 

I'd done similar tests, but none of my function generators create enough voltage to replicate this problem. So I (carefully!) set up a test with a variac+isolation transformer and a bridge rectifier, and can confirm the behaviour - but read until the end

I found that it was absolutely fine with an AC input voltage (to the bridge) of 11V, which gives an AC component after the bridge of 3.8V and a DC component of 9.7V. No problem at all...

But once you go beyond that, the AC reading from the AN8002 and AN860B+ start to fall. Adjusting to give 15V prior to the bridge gave an AC component of 5.1V as measured by my 87V, and 4.3V according to the AN8002. The AN860B+ does a very similar thing. At 30V AC in, the post-bridge AC component is 10.7V, but the AN8002 reports it as just 0.7V!

I might have imagined this, but the AN860B+ did give correct readings initially, but once I switched it to DC and then back to AC, the bug came and hasn't gone away since. It's almost like something was changed by that initial test, as unlikely as that seems.

On the upside, I can report that the AN8008 doesn't have this problem

I wondered if other meters with the same IC could be affected. The BSIDE ADM08A seems fine, as does my UNI-T UT210E - although it has to be said that the DC offset did cause the auto-ranging to take quite a while to sort itself out.

And quite by chance, I discovered that the BM235 also does strange things with this test. With the same 30V AC into the bridge, it gives the expected 10V AC after the bridge. But cycle through DC -> VFD -> AC and it displays 0.6V AC. If you unplug the signal and put it back in again, it does sort itself out, so in practice this is less likely to catch you out if you're probing a circuit. Interestingly, if you turn the meter on and off again with the signal still plugged in, it continues to give incorrect readings.


BUT...

Having said all the above, I wasn't happy about some of the numbers I was getting, and especially as I started to get some inconsistencies after those tests. I wasted a bit of time checking my connectors, etc, and then decided that my large bridge rectifier might be causing problems with the junction capacitance or similar. So I added a 10k resistor to the output of the diode bridge. This improved things a lot.

Now, with 30V AC going in to the bridge, I was getting 26V DC and 13V AC after the bridge (verified with a Fluke 187 and an 87V - I brought out the 187 for the AC+DC mode). And, the AN8002 and AN860B+ behave perfectly. But the BM235 didn't change...

Time to reach for the 'scope. As expected, with no 10k resistor, the waveform is nuts. The screen-grab shows it with 3 multimeters connected to the output, but if you disconnect those, it get even more funky! With the 10k resistor, all is well.

I've no idea what causes the AN8002 and 860B+ to give such strange results without the 10k resistor - it's possible that some unfortunate combination of the waveshape combined with the particular properties of the AC conversion is to blame - but it looks like in a realistic power supply circuit, this particular problem shouldn't rear its head. But certainly, it's worth bearing in mind. And the BM235 behaviour is worthy of further investigation.

Does your setup have any sort of load across the output of the bridge, beyond the meter itself? And does adding a resistor help at your end?

All the best,

Mark

[evava]

Guys, thank you all!
Very useful information!

[ModemHead]

Yes, you need to encourage a minimum of current through the diodes or they will be decidedly non-linear.  10 Meg won't do it.  Junction capacitance and all that.

... And the BM235 behaviour is worthy of further investigation. ...

I don't have an AN8002 to test, but I do have a BM235.  To see the interesting effect easily, set the input voltage so that the AC component after the bridge is about 4Vrms.  Increase through 5 and 6Vrms.  If your BM235 behaves like mine, the reading will peak at roughly 5Vrms, then start to decrease.  It does not up-range unless you disconnect and re-connect it with the voltage at or above about 5.3Vrms.

I assume the DC component is saturating some buffer amp in the chip, causing the AC component to be clipped, and hence the erroneous readings.  To be fair, AC+DC arbitary waveforms are going to be iffy with any multimeter at some point, better to use a scope to be sure.

[joeqsmith]

Does your setup have any sort of load across the output of the bridge, beyond the meter itself? And does adding a resistor help at your end?

Yes, I do have a 10K resistor on across the output of both the transient generator as well as the last test I did with the arb. 

I repeated the 15VACrms test with 8002 in parallel with a second meter and scope. The only difference between the fullrect2&3  is toggling the 8002 to DC and back to AC.   
The last two are with the transient generator.     

A few of us have shown basically every meter can show errors in the AC, DC  or both under the proper biased AC signal.  What's different about these meters is they can show the data correctly at times.
 

[Mark Hennessy]

Curious, isn't it 

I spent a bit more time this morning playing around. As you were using 15V AC, I upped mine to 20V AC post-rectifier, just to see if I could provoke it. I did go quite a lot higher, but rather than fry my 10k resistor or dig out one that was more highly rated, I kept those tests short.

Anyway, I could not get either the AN8002 or AN860B+ to mis-read in these tests. I must have worn out the yellow button and range switch in those meters, but no luck - they just consistently read correctly. Noting comments in the video, I tried both polarities, but no difference. Likewise, with no 10k resistor, polarity didn't change the incorrect readings. At the same time, I noted that the issue with the BM235 is polarity-sensitive (it behaves OK with -DC going in).

Picking up on another comment in the video (34 minutes in), I also noted that the AN860B+ does indeed behave oddly when switched off. Partway through the tests, I added a small incandescent light bulb in series with the bridge - just to act as a safety limiter should I do something daft with the wiring. Easier than a fuse, but low enough resistance to not change the results... This bulb lights dimly when the AN860B+ is turned off. But if you measure the resistance between the terminals, you get an open. So there's obviously a voltage-dependant factor here, and sure enough, if you turn to a function like Ohms, the bulb comes on at the same brightness. Of course, doing that with any meter produces similar results in those modes - no-doubt the clamp+PTC+series resistance for those modes are in circuit. So it seems that, as you suggest, the 860B+ is not isolating the jacks in the usual way when switched off. How strange...

Anyway, as I said earlier, it is peculiar that the AN8002 and AN860B+ behave the way they do for the waveform I initially put in. Until I mentioned it, no-one else suggested trying the tests with some damping resistance after the bridge, and that's curious given how explicit the instructions were. For anyone else wishing to reproduce these tests, that resistance (even 100k will do) is essential, as without it, the waveform produced after the bridge is essentially completely arbitrary, depending as it does on several variables. Even with a x10 'scope probe, I'm not convinced I'm even seeing the same as the meters were seeing - just the capacitance of a metre of RG58 made about 100V of difference on the 'scope with no other loads present.

Ultimately, more data is required before concluding that these two meters definitely have an issue that will cause a real-world problem. At this stage, I'm more concerned with what I've found with the BM235 - especially as that's a meter that is often recommended as being well behaved and solid.

[joeqsmith]

I spent a bit more time this morning playing around. As you were using 15V AC, I upped mine to 20V AC post-rectifier, just to see if I could provoke it. I did go quite a lot higher, but rather than fry my 10k resistor or dig out one that was more highly rated, I kept those tests short.

Anyway, I could not get either the AN8002 or AN860B+ to mis-read in these tests. I must have worn out the yellow button and range switch in those meters, but no luck - they just consistently read correctly.

[Mark Hennessy]

I'm not doubting you, Joe, and I'm sure no-one else is. What we need now is a larger sample size 

I'll have one more go tomorrow morning before I put away the variac and other stuff.

Just a thought: would you be willing to look inside your new AN8002? The version number and date is on the silkscreen - mine is V0.4 and 2016/09/13. I'm just wondering if there could be distinct versions out there with this bug...

[kalel]

I don't think I'll be able to test this once my an860b+ rebranded meter comes (I don't think I have the right gear). But until then, you will probably have figured out all of the details anyway.

[joeqsmith]

I'm not doubting you, Joe, and I'm sure no-one else is. What we need now is a larger sample size 

I'll have one more go tomorrow morning before I put away the variac and other stuff.

Just a thought: would you be willing to look inside your new AN8002? The version number and date is on the silkscreen - mine is V0.4 and 2016/09/13. I'm just wondering if there could be distinct versions out there with this bug...

After your post of running above 15, I thought I should at least repeat it. 

The one I am showing is marked:
ZT101/102 V0.4
2016/09/13
SN#170209...

[Mark Hennessy]

OK, so same version as mine. Oh well... My s/n is 170206748, so a little older than yours.

So this time, I dug out a beefier 10k resistor and tried some higher voltages. Finally I got my AN8002 to play up, but I needed 30V AC/63V DC (about 70V prior to the rectifier). I thought I'd gone at least that high yesterday, but only very briefly because of my wimpy resistor, so I must have missed it.

I found that if you leave the meter on AC and disconnect and reconnect the source, it does come good without having to press buttons or re-power. Not sure if that's been mentioned yet...

I also discovered something else. With the meter behaving "properly" on AC, it gave good readings as I increased the input until it got to about 50V AC on the meter (about 110V AC prior to the rectifier). At this point, the AN8002 started under-reading, and then the reading started to decrease as the input rose. The polarity applied only made a small difference to this. Presumably this is the same root problem, only manifesting itself at these higher voltages because the meter is in a different range.

Regarding the 860B+, it also gave inaccurate AC readings at about 50V AC (actually a little less - perhaps 45V AC). But in terms of the gross errors when switching between AC, DC and AC again, I had to go to 60V AC/120V DC (as shown on the Fluke 187, because the 860B+ was reading 46V at this stage - though if you reverse the polarity it does give correct readings, and the problem does go away).

(The BM235 also starts to under-read the AC when you get to about 50V, though it's completely OK if you reverse the polarity, and sometimes rights itself if you remove and replug the signal. The VFD readings seem OK)

So there we go - I finally saw it! It's interesting that there is such a large spread of voltages at which the fault reveals itself. It's also interesting that none of my other meters showed any problems during the same tests, including several with the same chipset. Indeed, just for fun I went all the way to 240V AC (prior to the bridge). All meters - including the AN8008 - behaved perfectly.

The AN8008 result is interesting, because apart from the slight change in chipset, I'd imagine that everything else would be pretty similar between this and the AN8002. It would be good to reverse-engineer or otherwise obtain the schematics for both of these...

Hope this is of interest,

Mark

[joeqsmith]

... but I needed 30V AC/63V DC (about 70V prior to the rectifier).  ...

So there we go - I finally saw it! It's interesting that there is such a large spread of voltages at which the fault reveals itself. It's also interesting that none of my other meters showed any problems during the same tests, including several with the same chipset. Indeed, just for fun I went all the way to 240V AC (prior to the bridge). All meters - including the AN8008 - behaved perfectly.

The devils in the details.    If you are running your autotransformer off AC mains I can believe there is this much of a difference as the problem is frequency dependent.   You never specifically stated what frequency you were testing at.  If you were running at 50Hz full rectified,  I am surprised the 8002 would even settle at the lower voltages.   Mine just hunts non-stop.  The meter is too cheap to include a manual range, so you have no idea what the signal is without grabbing a better meter.   

I set my arb for 50Hz and with 24.74VACrms (50.2VDC), the 8002 can be toggled between AC and DC without any error.  I then slowing increased the amplitude while toggling the AC/DC selection button until the meter faulted.  This happens at 26.0VACrms.   And, yes I can remove the connection when leaving the meter in AC and it will read the correct value. 

As I continue to increase the AC voltage, while I continue to toggle between AC/DC the meter will begin to work correctly but it will go through areas where it is really messed up.

At 37.89 the 8002 reads 2.297V
At 62V the 8002 reads 49V
At 64.98 the 8002 reads 64.9
At 74.5 the 8002 reads 74.5

Other meters may have this same problem.  I suspect that the meter can read correctly, it is some sort of math error.  Again, no manual range but it's possible it picks the wrong range and could read it correctly.  This would make sense with the unplug to reset as it would reset the autorange as well. 

You say the BM235 has the same problem.  That meter has a manual range so if you are able to replicate it, just manual range it and see if it throws up the correct numbers.

[kalel]

... but I needed 30V AC/63V DC (about 70V prior to the rectifier).  ...

So there we go - I finally saw it! It's interesting that there is such a large spread of voltages at which the fault reveals itself. It's also interesting that none of my other meters showed any problems during the same tests, including several with the same chipset. Indeed, just for fun I went all the way to 240V AC (prior to the bridge). All meters - including the AN8008 - behaved perfectly.

The devils in the details.    If you are running your autotransformer off AC mains I can believe there is this much of a difference as the problem is frequency dependent.   You never specifically stated what frequency you were testing at.  If you were running at 50Hz full rectified,  I am surprised the 8002 would even settle at the lower voltages.   Mine just hunts non-stop.  The meter is too cheap to include a manual range, so you have no idea what the signal is without grabbing a better meter.   

I set my arb for 50Hz and with 24.74VACrms (50.2VDC), the 8002 can be toggled between AC and DC without any error.  I then slowing increased the amplitude while toggling the AC/DC selection button until the meter faulted.  This happens at 26.0VACrms.   And, yes I can remove the connection when leaving the meter in AC and it will read the correct value. 

As I continue to increase the AC voltage, while I continue to toggle between AC/DC the meter will begin to work correctly but it will go through areas where it is really messed up.

At 37.89 the 8002 reads 2.297V
At 62V the 8002 reads 49V
At 64.98 the 8002 reads 64.9
At 74.5 the 8002 reads 74.5

Other meters may have this same problem.  I suspect that the meter can read correctly, it is some sort of math error.  Again, no manual range but it's possible it picks the wrong range and could read it correctly.  This would make sense with the unplug to reset as it would reset the autorange as well. 

You say the BM235 has the same problem.  That meter has a manual range so if you are able to replicate it, just manual range it and see if it throws up the correct numbers.

Speaking of manual range, the an860b has the "range" button, while having I assume the same chip as an8002. 

[joeqsmith]

You say the BM235 has the same problem.  That meter has a manual range so if you are able to replicate it, just manual range it and see if it throws up the correct numbers.

Speaking of manual range, the an860b has the "range" button, while having I assume the same chip as an8002.

I don't have one to check but it would be interesting to know if its the autorange causing the problem on the AN860. 

[kalel]

You say the BM235 has the same problem.  That meter has a manual range so if you are able to replicate it, just manual range it and see if it throws up the correct numbers.

Speaking of manual range, the an860b has the "range" button, while having I assume the same chip as an8002.

I don't have one to check but it would be interesting to know if its the autorange causing the problem on the AN860.

I will have a rebranded AN860B+ eventually (when/if it comes) but unfortunately I'm not able to do these tests.

[Mark Hennessy]

The devils in the details.    If you are running your autotransformer off AC mains I can believe there is this much of a difference as the problem is frequency dependent.   You never specifically stated what frequency you were testing at.  If you were running at 50Hz full rectified,  I am surprised the 8002 would even settle at the lower voltages.   Mine just hunts non-stop.  The meter is too cheap to include a manual range, so you have no idea what the signal is without grabbing a better meter.   

I set my arb for 50Hz and with 24.74VACrms (50.2VDC), the 8002 can be toggled between AC and DC without any error.  I then slowing increased the amplitude while toggling the AC/DC selection button until the meter faulted.  This happens at 26.0VACrms.   And, yes I can remove the connection when leaving the meter in AC and it will read the correct value. 

As I continue to increase the AC voltage, while I continue to toggle between AC/DC the meter will begin to work correctly but it will go through areas where it is really messed up.

At 37.89 the 8002 reads 2.297V
At 62V the 8002 reads 49V
At 64.98 the 8002 reads 64.9
At 74.5 the 8002 reads 74.5

Other meters may have this same problem.  I suspect that the meter can read correctly, it is some sort of math error.  Again, no manual range but it's possible it picks the wrong range and could read it correctly.  This would make sense with the unplug to reset as it would reset the autorange as well. 

You say the BM235 has the same problem.  That meter has a manual range so if you are able to replicate it, just manual range it and see if it throws up the correct numbers.

Yes, 50Hz AC. All those details were given in post #104.

I don't see the strange up/down behaviour. Once you go past 45V, it just gradually and smoothly falls, down to about 35V at 105V.

There's no doubt it's an auto-ranging thing. The AN8002 has that internal range switch if you want to try it for yourself (ought I point out the hazards of that for anyone else who might be thinking of trying that?).

It seems that something is clipping internally ahead of the ADC, resulting in a signal that has an average value that doesn't exceed the 6200 that's needed to make it range up. The fact it often comes good when the signal is briefly removed suggests it might be a capacitor storing charge from being in DC mode (or some near equivalent in the digital domain given that the RMS detection is done with DSP). With a more detailed datasheet, and access to the pins (which luckily you have with the 860B+), more diagnosis might be possible. In the meantime, the key thing is that we've been able to characterise it to some extent, so I can eventually update my reviews of these meters.

Of course, the under-reading at higher voltages is a similar thing caused by clipping. Changing up to the next range resolves that. But of course, you have to know that

Whether this is something that most users will run into is a whole other question, but it's definitely something that needs reporting.

If I can find a small, low-current variac for a good price, I'll build up this test setup into a small box for future testing with a bit more safety and convenience in mind...

Thanks for your input

Cheers,

Mark

[MacMeter]

Whether this is something that most users will run into is a whole other question, but it's definitely something that needs reporting.

Been following these threads on ZT102/109 8002/8008, as I bought both. Excuse my newbie ignorance, but perhaps one of you experts can post ALL the gothas and "issues", and readings "shortcomings", INACCURACIES of these two models in a single post. As of now I am totally lost as to what bugs, and what modes are NOT trustworthy with these two meter models. I also own the EEVBLOG BM235, not sure of reported bugs on that one either. THANKS!

[floobydust]

Shame on Dreamtech for not displaying the f/w version on power-up 
Without that, this is all a mess to understand what DMM0660 models have this the firmware bug.
It would be a product recall in North America due to the safety aspect.

I looked at the DMM IC to see what is involved in reading ACV.
Autoranging - it's not the LCD value; in software, if the A/D is near fullscale, you switch the input voltage divider down a decade. But the A/D gain can also be stepped down.
DMM IC firmware is switching to keep your A/D resolution up there but not saturate the input amplifier.
On ACV you have two registers: A/D gives a DC result and an (HPF) AC result.

It sounds like the firmware is grabbing the wrong register while doing it's internal autoranging, at a particular input gain and A/D count.

[Mark Hennessy]

Does anyone have a link to the original datasheet? Kerry Wong's translation is great, but I'd like a better look at the diagrams which are rather low-res in the PDF.

Presumably you're looking at something else to arrive at those conclusions? The details in Kerry's version imply that pre-ADC gain is used for the lowest millivolt ranges only with this meter.

[Mark Hennessy]

Whether this is something that most users will run into is a whole other question, but it's definitely something that needs reporting.

Been following these threads on ZT102/109 8002/8008, as I bought both. Excuse my newbie ignorance, but perhaps one of you experts can post ALL the gothas and "issues", and readings "shortcomings", INACCURACIES of these two models in a single post. As of now I am totally lost as to what bugs, and what modes are NOT trustworthy with these two meter models. I also own the EEVBLOG BM235, not sure of reported bugs on that one either. THANKS!

The bug being discussed occurs when you have a mixture of AC and DC present at once.

It must be emphasised that many meters can be confused by such conditions. The tests we've been doing perhaps represent an extreme condition, where large amounts of both AC and DC are present - and to my mind it's debatable as to how realistic this is, based on what I get up to with my multimeters.

For that reason, I've tried a different test this morning. I've taken 1V from my function generator and passed it through a coupling capacitor. This is connected to a 1k resistor from a DC bench power supply set to 30V, and the junction goes to the DMM. This - to me, at least - represents a much more likely scenario for electronics, where small AC signals riding on large DC offsets often occur. With this setup, I've been able to confuse a whole load more meters than just the Aneng models discussed here. Though no Flukes, I'm happy to say.

I would also emphasise that for this sort of work, an oscilloscope is the proper tool in my opinion. I would always look at a signal on a screen if that's possible and safe. Perhaps I'd then use a multimeter to get a more accurate reading if needed, but having seen it on the 'scope screen first, I'd know roughly what to expect, so wouldn't fall into the trap of automatically believing the numbers convincing presented on the LCD (that's a really important point to take away if you are a "newbie" ).

It's vital to know your tools and understand their limitations as nothing is perfect. That's why Joe and I have been explicit about our test methodologies so that others can repeat the tests themselves and characterise their own meters. No special equipment needed - you might not have the same stuff as Joe and I, but if you understand what you're trying to achieve, then you should be able to think of something that replicates the conditions closely enough.

That's the AC+DC "bug". I think that any other issues - such that they are - are covered on my reviews:

http://www.markhennessy.co.uk/budget_multimeters/aneng_an8002.htm
http://www.markhennessy.co.uk/budget_multimeters/aneng_an860bplus.htm

I will add a summary of the AC+DC bug to these at some point soon.

Hope that helps,

Mark

[glarsson]

The bug being discussed occurs when you have a mixture of AC and DC present at once.

One of my two AN8002 shows incorrect AC without any (significant) DC.  Select V and AC. Put probes into 230V AC outlet. Display shows 234VAC (or similar). Toggle to DC; display shows 0VDC. Toggle to AC; display shows 2VAC.

Looks more like a bug when switching between AC and DC, with or without an DC voltage present.

[Mark Hennessy]

The bug being discussed occurs when you have a mixture of AC and DC present at once.

One of my two AN8002 shows incorrect AC without any (significant) DC.  Select V and AC. Put probes into 230V AC outlet. Display shows 234VAC (or similar). Toggle to DC; display shows 0VDC. Toggle to AC; display shows 2VAC.

Looks more like a bug when switching between AC and DC, with or without an DC voltage present.

As I said back in post '96, that doesn't happen with my samples. I have just checked again...

When yours is reading incorrectly on AC, what happens if you remove a probe and re-connect it?

[crazyguy]

Does anyone have a link to the original datasheet? Kerry Wong's translation is great, but I'd like a better look at the diagrams which are rather low-res in the PDF.

Presumably you're looking at something else to arrive at those conclusions? The details in Kerry's version imply that pre-ADC gain is used for the lowest millivolt ranges only with this meter.

original copy

[Mark Hennessy]

Does anyone have a link to the original datasheet? Kerry Wong's translation is great, but I'd like a better look at the diagrams which are rather low-res in the PDF.

Presumably you're looking at something else to arrive at those conclusions? The details in Kerry's version imply that pre-ADC gain is used for the lowest millivolt ranges only with this meter.

original copy

That's great - thank you 

I'll take a proper look later, but certainly looks like there's lots of good stuff in there!

[glarsson]

When yours is reading incorrectly on AC, what happens if you remove a probe and re-connect it?

I can't reproduce it now (don't know why) but as I remember it I had to move the rotating switch away and back.

One thing is clear. Now when I switch back from DC to AC it auto ranges 0.000, 00.00, 000.0, 234.00. When it showed 2VAC it did not auto range and just displayed 2.something directly.