Metrix MTX3250 strange behaviour reading Ohm

[munnik]

Hi,


I've just acquired a Metrix MTX3250 multimeter, and I'm very happy with it. There is only one strange issue that I can not understand or resolve. Hopefully you can help me with this.

When I put the multimeter in Ohms mode and the test leads are disconnected, the multimeter shows 000.00 Ω. I would have expected to show .OL MΩ. When I connect the test leads from the MTX3250 to another multimeter and put that other multimeter in VDC mode, the MTX still reads 000.00 Ω. The other multimeters shows 5.6077 VDC. When I change the MTX from auto range to manual range, I can find the expected value of 10.926 MΩ. The other multimeter then shows a value of 0.6142 VDC. When I step down one range, the MTX3250 shows .OL MΩ and the other multimeter shows 1.1034 VDC. All other ranges show 000.00 kΩ, 00.000 kΩ, 0.0000 kΩ and 000.00 Ω. For all these ranges, the other multimeter reads between 5.1113VDC and 5.608VDC. When I have the correct reading in manual mode and switch back to auto ranging, the reading remains correct. Continuity seems to be working fine on the MTX3250.

Is this maybe a software issue? The current version of the software is 1.05.

[coromonadalix]

Unless you have some antenna effect with probes, or high impedance who charge up the input stage   ....

You should ask Chauvin Arnoux   .....

could be FW issues ...  check if there are updates

[shakalnokturn]

That's a bit of an unusual problem.

Are capacitor and current measurements working as expected ?

I would tend to think that it's software or at least bus related.

I don't think the schematic for this model is around but if I'm not mistaken the ASIC is the "DMM5" (new version).

There are schematics for a handheld model using DMM5 posted by Vince in this topic:
https://www.eevblog.com/forum/testgear/metrix-mx56c-bk-precision-5390-multimeter-teardown

That should give you a general feel of how things work. Note that the DMM5 (ITT) used in this schematic although functionally almost identical isn't the same as the DMM5 (Metrix) used in the MX24B and I suppose in your MTX3250.
They made minor changes to the silicon between the two, I've tried swapping them in the past: Everything works but all the measurements are wrong by a set factor so likely some modification was made to ADC reference voltage. In case you come to the conclusion the autorange in the DMM5 is whacked look for nothing older than the MX24B for a parts donor.

First try to figure out how the CPU and DMM5 are communicating during range switching see if it looks FW related, could be worth checking on calibration EEPROM accessibility too.