Multiplexer autoranging - idea for accurate multimeter? - Page 1
Forum section: electronics / projects

author: TriodeTiger
author_rank: Regular Contributor
timestamp: March 26, 2014, 09:44:28 am
content: Accurate as in, more accurate and general purpose than a cheap chinese metre. Any catches?So, I figured, lets say I have a high voltage or low voltage: I just use a multiplexer to change the feedback ratio from 0.01x to 100x until it is within the desired range of my ADC.Same with current shunts I suppose, selecting the appropriate shunts, and then amplifying the sense voltage(dual auto-ranging? sweet!) - I assume if I calibrate it with the actual shunt values and such, would it be pretty darn accurate on the low end?Resistance divider to find resistance (can't be darned to make some sort of CCS..), select right one until voltage is roughly half of VCC, or enough to amplify maybe? Found. It seems if I have like a 10mOhm on the range, I can measure pretty darn low compared to any DMM!Frequency maybe too? Schmitt trigger on signal once middle point is found or however -> then divide the largest scale possible, if frequency is too low, keep going up until it is within the desire reading range.Assume I choose one of those ultra low noise opamps, and I assume the gains are not too big (100) or too low (0.01x) for these things.Would that work? Is this what autoranging multimeters kinda do? I kinda like I can just add interfaces to it, such as the frequency divider, or amplifiers, or whatever it may be and I know the ratios already as they were selected by my microprocessor.Alexander.

author: han
author_rank: Frequent Contributor
timestamp: March 26, 2014, 09:54:23 am
content: In most of auto-range DMM i use i can tell the scheme:1. If the reading higher then +/- xxxx (few reading below max ADC reading) attenuate the signal 10X2. If the reading lower then +/- XXX (few reading from 1/10 max ADC reading) reduce attenuation by 10Xthere is a issue with the technique:1. Slow2. have a problem with near max value (like 3.999V in 4000 count ADC) , can oscillate the attenuation rangeif there is any way to clamp the voltage to ADC without any negative consequence, then multiplexing the ADC from Range 1X to 1000X is very good idea, you get a fast response without jumping from one range to other range

author: TriodeTiger
author_rank: Regular Contributor
timestamp: March 26, 2014, 10:45:34 am
content: Ah, interesting, I'm ranging just based on voltage actually. My software simply derives the real value, (voltage * range ratio, i.e. 1.25V on shunt @ 100x = 0.0125A) and formats it in to 4 digits with its proper metric suffix (12.50mA)1.000k can still oscillate with 999.0, but no crazy re-attenuation loop and possibly loading the source differently, etc, as it is software only! (edit: I guess it still will do this, if it's say, 0.1V and I declare that as too low for my ADC and bump it up 10X more to 1.0V)I figure now I'm just looking at any catches at lower ranges, such as if the multiplexer or opamps add noticeable leakage or something I cannot begin to understand, when amplifying small voltages and such and doing math off of it. Maybe it will just work. Soon I will built, and suppose calibrate eventually and measure its accuracy.Just seems like an awesome way to accurately read 1mOhm to 10GOhm, 1nA-1kA 1Hz-1GHz, just by ranging with large ratios to keep sane values at the ADC. Limited by some factors of course (quality of connections for high frequencies, how I read nano-amps with noise and all, etc.) but it fundamentally is sound.

author: dannyf
author_rank: Super Contributor
timestamp: March 26, 2014, 11:22:45 am
content: QuoteYou can always try. But in the end, you will find that those digital meters, DT830 or the likes (cheapest of the cheap), are actually very accurate.

author: TriodeTiger
author_rank: Regular Contributor
timestamp: March 26, 2014, 12:03:27 pm
content: Quote from: dannyf on March 26, 2014, 11:22:45 amI suppose that is right. I guess I am just extending on what they do, by adding low resistance measurements and very low voltage measurements? Maybe it is just too niche or pricey or large for them to add the extra ranges/low noise opamp to do that, so it is not done by cheap players, and all that is left is the expensive stuff. I just didn't realise it was so relatively easy to get the expensive stuff (minus the grand quality) with DIY. It'd be fun measuring wire resistances and stuff.99% of my questions turn in to things I already know in the back of my head, hmm... Well, any other catches would helpI suppose I'll build it and ask more.

author: dannyf
author_rank: Super Contributor
timestamp: March 26, 2014, 12:16:35 pm
content: QuoteBoth are fairly easy to do, like through adapters to your existing meter (so the output could be current or voltage).A millivolt meter is easy to build; Once you have that, you can build a milliohm meter, etc.QuoteIt is fairly easy to do but quite difficult to calibrate.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 26, 2014, 12:29:20 pm
content: So if you have 1000V at the input and your 1:10 divider is putting 100V into the mux that's supplied from 9V, what happens?Better to use a divider chain with switched resistors, so you only ever have to switch a few volts at a mA or so. Also, nulling the switch resistance is a must, particularly on the lowest ranges. Nice part is, once you see how it works, it's much simpler and more elegant than a regular chain, let alone a set of dividers in parallel.Tim