Strange AC RMS measuring behaviour - OWON XDM1241

[Phil1977]

I´ve noticed a strange behaviour of my OWON XDM1241:

If I measure current AC and the measured current falls below 1% of the display range, the display shows just zero. If it´s in manual 500mA-mode and the current goes below 5mA, it´s not displayed anymore. The display resolution of this meter is 55000 digits, so principally there are around one thousand digits around zero that are artificially nulled!

Of course if you use autorange then it shows the correct value. But especially in current mode I e.g. want to use manual range to keep the shunt resistor low, and also in the 500uA-mode there are no measurements possible below 5uA.

Has anyone observed a similar behaviour with this or similar meters? The measurement magic is done in a HY3131 chip from Hycon which usually is known to be a good piece of silicon. I´m afraid this fault comes from OWONs software which may want to fake a better than real zero stability.

PS: I´ve just noticed it´s doing the same in AC voltage mode. There it is better to get over it because autoranging usually doesn't hurt there. But even for a cheap benchtop multimeter it´s hardly acceptable to throw so many digits away.

[Kleinstein]

Some dead range at the lower end of the AC ranges is very common with analog RMS, though 1% is rather high.
The HY3131 supports digital RMS and should not have such an issue.
Looks like this is a software issue, either to get a more stable zero or just a wrong initialization / calibration of the AC ranges.
It may be worth for someone else to check if they have a similar problem. It could be a one off problem with calibration (too much noise in the zero reading during calibration).

[Phil1977]

It´s happening at absolutely exactly 500 digits. In the 5V-range 0.0500V can be displayed, as soon as I set the function generator to 0.1mV less I get 0.0000V.

So it´s quite clearly digital, and it would have been quite a coincidence if the noise during calibration would have been exactly 500 digits in all ranges - thanks anyhow for the possible explanation.


I´ve written to the OWON-support. I don't have too high hopes there, but let´s give them a chance.

[coromonadalix]

maybe with the eevblog 121 meter  some compare would be possible   it use the same ic ?  no ?

[David Hess]

...

Has anyone observed a similar behaviour with this or similar meters? The measurement magic is done in a HY3131 chip from Hycon which usually is known to be a good piece of silicon. I´m afraid this fault comes from OWONs software which may want to fake a better than real zero stability.

PS: I´ve just noticed it´s doing the same in AC voltage mode. There it is better to get over it because autoranging usually doesn't hurt there. But even for a cheap benchtop multimeter it´s hardly acceptable to throw so many digits away.

I have only noticed that behavior in test instruments when the manufacturer wants to hide the noise or drift.  In some applications it is appropriate, but not in a multimeter.

Lots of Chinese electronic scales do the same sort of thing at the values used for calibration, so if the sample weight is slowly increased, there are bands where it sticks to the calibration value instead of showing the actual weight, making the meter seem more accurate.

[Kleinstein]

maybe with the eevblog 121 meter  some compare would be possible   it use the same ic ?  no ?

AFAIK the EEVblog 121 meter uses the HY3131, but in combination with an analog RMS chip. And thus a different AC solution.
The XDM1241 seems to use the internal digital RMS solution that is lower bandwidth but ideally with less issues at a small fraction of the range.
There is a chance that the exactly 500 counts could be some kind of default setting and a cal step could have been skipped.

I see no reason to cripple the AC readings that much. Normally the standard circuit with the correct "offset" for the meters own noise should work well. The weak point is the limited bandwidth (the specs are only for 1 kHz). The limited BW also makes noise less of an issue than with the analog RMS. So even without cheating one should get a rather stable zero and OK readings even below 0.1 % of FS. The "cheating" is more from the low bandwidth, that can make it look better than a higher BW solution.  So I see absolutely no need for cheating with a fixed zero - it's more like a bug or missed/failed calibration step.
A solid zero is more a solution that analog RMS solutions may use / need, as these by design work poorly at low amplitude. These often requite more than 1% or 5% of the FS to work. So there 500 count dead zone may be possible, though already a pretty bad case.

[electr_peter]

As discussed on this forum, some Hioki TRMS multimeters artificially go to zero in AC range while others (like Keysight U1272A) show residual AC TRMS value without trickery. I much prefer the later, as it show things as is without any artificial tricks. Besides trickery, there are implications w.r.t. sensitivity and accuracy.

I see no reason to cripple the AC readings that much.

The reason (from DMM manufacturers point of view) is that RMS DMM never shows "0.00V" and they get constant complaints from regular users why it is so and why their DMM is "broken". To not bother with this artificial zero function is added.

[Phil1977]

This is incredibly frustrating but probably you really have to spend >1k€ for a keysight if you want to be taken as serious by the instrument manufacturer.

[Phil1977]

In fact this behaviour is accordant to the specification sheet - though the stupid zeroing is not mentioned:

[4] Specifications are for amplitude of sine wave input > 5% of range. 0.1% errors will be added when the
range of input sine wave is 1% to 5%.

In other words: The error below 1% of range is 100% of value