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| EEVBlog 121GW Multimeter Firmware Details |
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| chronos42:
hi joeqsmith and 1anX, i noticed this issue first with firmware 1.57 and blamed this firmware, but i was wrong in this point, sorry for the missleading. I also can get very accurate values in the low pF range, but only one time after the meter was laying around unused for some time. After switching on the first time it is within its specs. So far so good. But from now on the behavior is not predictable. When I switch off and on it sometimes is spot on again, sometimes it reads about 80pF too less. When I use the mode button and step through the different modes until I am back to capacity mode it also reads 80pF too less. Switching off and on in this state does not reset the meter to a correct reading. I have to wait some time, then it comes back. The reading counts up with about 1pF all 15 seconds until it has reached the correct value again. That makes it so hard to find out a predictable behavior. For me that looks like a port that is not correctly set and floates in tristate mode. In this case there is no stable operation possible and everything can happen and also every meter can behave different. |
| CDaniel:
Sorry about the pictures being tilted :-\ So , meter on the bench , no probes , no powerfull electric field nearby ( everything was shut down ) At first was 0.000 , and then slowly is drifting up ( in maybe 30min ) . Doesn't matter if is on or off . With leads connected is much faster and bigger . This offset is adding when I measure a real capacitor . This is not something heating up inside , it is like some capacitor in the measurement path is charged by external electromagnetic field . If I switch the rottary selector to off and then on again to capacitance nothing is changing . To zero out this offset I have to cycle through cap manual ranges or to switch to resistance and back ... I see that you are curious about the shim , my multimeter was bought from Welectron with it already installed ( I looked inside ) . And is working just fine ... for now :P |
| CDaniel:
Mister joeqsmith , if you read my post carefully I said that with leads the drift is faster and bigger . I tested without probes on purpose so you can't say that I moved them or I did something ... or my test capacitors are not good enough ;) If you don't observe anything wrong doesn't mean that everybody else is stupid ... |
| CDaniel:
Some issues are hard to replicate if you don't have the exact meter , it was pre-production or modified in any way . And the firmware was released just recently , before this , with low resolution , was useless to test small capacitors . So be patient , if there are contradictions is because we are testing right now , ideas and observations can change . Probably many others will test and share the results . |
| CDaniel:
OK , I have read the HY3131 Configuration , but we can't know yet how the firmware is implemented . How the configuration bits are . For the noise in resistance mode I discovered that they didn't used the input low pass capacitor C21 , now they fixed that . That's like using an ADC for DC without any capacitor at input , any electomagnetic field will make the reading to fluctuate . Maybe this is something simple , or not ... if it is something hardware after all , that CMOS 4053 switch leakeage current is the first suspect ;) In my shoping list are some MAX4053 low leakeage switch |
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