I bought my SDM3065X from Telonic a fortnight ago. It was delivered the very next day (for once I'd
not placed my order on a bank holiday Friday evening as is my usual purchasing habit of high value items
). I've been playing with it ever since, mainly to monitor voltages in a temperature stabilised Rb reference oscillator (Efratom LPR101) project I've been working on over the past 13 months since acquiring the LPR101.
I've tried running a few "sanity checks" with a well rested AA alkaline cell and a CR2032 with a 10M and 610K potential divider to test the 200mV range down into uV territory. Although neither of these "references" are temperature stable, they did at least permit me to check the behaviour of the least significant digit's 'dance routine'.
The AA cell test with its very slow variation of voltage allowed me to see a well behaved transition from one 10uV readout to the next, typical of any DMM that's faced with slow analogue variations and the task of assigning a digital representation of a messy analogue quantity. So far, so normal.
I didn't bother adding a 10Mohm in series to create a 2:1 divider with the 10Mohm impedance of the meter to get into the 1.5v range to examine the meter's behaviour on the 2v scale, electing instead to 'go for broke' and look at the 200mV scale with the CR2032 and voltage divider described above. Here, the result wasn't quite as smooth, hinting that the meter was right at its stability limits, very likely aggravated by microvolt scale thermo-electric effects (which appear to be the weakness of trying to measure sub 1 ohm resistors using only a 1mA test current regardless of whether it's a 2 or 4 wire measurement setup).
I was so intrigued by the difficulty in getting stable uV readings in the 200mV range, I decided to check how accurately and stable a reading I could get out of a 'Zero Volt Reference" (something we can all make up for a fraction of a penny's worth of solid copper wire). The result was a little disturbing compared to my previous experience with pretty well every hand held DMM I've ever owned, including the two 9999 counts Mestek DM91As I've been using of late, where such a zero volt reference would halt the random digital dance at a reading of exactly 0.00V or 000.0mV.
I was seeing a readout hovering close to either +2uV or - 2uV and anywhere in between and was beginning to think I had a faulty unit until I took a closer look at its specifications after remembering that I was looking at a resolution three orders of magnitude better than those Mesteks (0.1mV versus the 0.1uV of the SDM3065X). Interpreting the full scale accuracy figure given for the 200mV range, it would appear that the meter is actually (but only just!) within specification.
Despite this 'reassurance' by the specifications, I still find this digital dance with the final
two digits a little disturbing, even if using auto zero can get this down to a single count either side of a zero voltage reading, so I am impelled to enquire whether anyone else has used a zero volt reference to quantify these meters' accuracy at the zero volt end of their 200mV scale. IOW. am I the only one to be plagued by this less than perfect representation of a zero volt reference? I suspect not but I'd appreciate some feedback from other SDM3065X owners as a sanity check of my own experience with my one and only 6.5 digit bench DMM.