Lets just say we need 6.5 digits for dynamic range AND resolution at the same time.
A contrived example without going into offset drift, picoamp bias, chopper amps, regulator standby currents and all the cool stuff that needs a 6.5 digit meter (or differential voltmeter but bugger that thread fork):
Consider a 20mV signal on a DC bias of 20V. What resolution can you measure that to on a 4.5 digit meter (assume 22 based count on both)?
4.5 digits: 20.020V -> 20.??mV
6.5 digits: 20.02005V -> 20.05mV
Adding count uncertainty or say +/- 2 counts:
4.5 digits: 18 - 22mV
6.5 digits: 20.03-20.07mV
There was a time when everyone knew what the voltage, current or resistance was and there was never any question about it, it was what it was precisely. When was that, it was when everybody could afford to relinquish their analogue meters and go digital and we all had 3.5 digits and that new meter also gave us other things to worry about. That was because most of the service manuals/charts had their voltage references taken and stated as being measured with a meter @ 20,000Oopv and voltages we were reading were much higher because the new meters had an input impedance of around 10,000Mopv had a significantly lower loading on the circuits under test.
Then someone came along with a 4.5 digit meter, which is not too bad as it happens, then came a 5.5 digit, a 6.5, 7.5 and 8.5 digits and suddenly no one really knows what the voltage is anymore. A case in point, I'm sitting here looking at my 3.5digit DMM's connected to a voltage source, I've connected 10 up and they are agree with each other, 1.948V fine. Then I connect the same voltage source (untouched) to my 4.5 digit DMM's and now that extra digit is constantly changing, its now 1.9485, no wait its 1.9482 no wait its 1.9488 and so one.
Next I swap out the 4.5 digit meters for my 5.5 digit ones and WTF, I now have the last 2 digits constantly changing so what is the actual voltage that I'm reading?? The true constant here is the 1.943v which remain steadfast anything after the 3 just really cannot be relied upon as it only takes a gnat to fart and the reading changing again and again and again.
Its much the same story when switched to measuring resistance but this time the only digit that changes is the last on the 5.5 meters.
So the results that I'm getting is that the more digits the meter has, the more unsure of the actual value you become but tend to to go with either the first 3 or 4 digits after the decimal point so it would seem that the extra digits become just points in a game of brag or top trumps rather than serve a useful function. How many times have I either read it or heard people say that that they would have to have their meter professionally calibrated, especially so if it were to be 6.5 or higher digits because it cause them to worry about those last digits really were? As if they really matter in the grand scale of things
Now then the real big question is if there was to be a 6.5 digit DMM that came within my price budget would I go for it? You'd better believe I would so I can join in the game of top trumps and besides there is something hypnotic about seeing all of those digits light up, even if they aren't giving me or the equipment I'm working on any real advantage
Nice unit, why not add a splash of colour to the lab?
It's the variability of the source that drives you nuts.
I'm in the garage tomorrow looking some angle brackets so I can fix my function generator and LCR meter to underside of my shelf. I'd like to mount the TF930 up there too but I'm worried about spoiling the resale value of it later if I upgrade it.
Looks like I've started a trend. You must post pix
It's the variability of the source that drives you nuts.You are spot on, if the source is varying, even slightly, with the higher resolution meter, the more the least significant digits are going to keep altering in an effort to reflect the changing input. Whereas with fewer digits, that change has got to be much bigger in order to alter the least significant digit. 3.5 meter, no change in display, 4.5 meter sometimes the last digit will go up or down by 1, 5.5 meter, last 2 digits were changing all the time on the power supply. Its like a proverb, man with 3.5 and 4.5 digits, knows the voltage, man with 5.5 or more digits is unsure of the voltage due to last digits changing.
I'm in the garage tomorrow looking some angle brackets so I can fix my function generator and LCR meter to underside of my shelf. I'd like to mount the TF930 up there too but I'm worried about spoiling the resale value of it later if I upgrade it.
Looks like I've started a trend. You must post pixAs promised here is my revised (for now) bench, what you can't see is dotted around the room in any suitable space I can are 4 CRO's, signal generator, numerous, microscopes, handheld DMM's, analogue meters, portable LCR devices, soldering kit, jigs, isolating transformer, variac, power meter, parts racks, calibrators and tools etc. I'd love to have a room big enough to take a proper 2 or 3m bench and shelves then I could have all the kit out and ready to be used, maybe this week its my turn to win the lottery....
It's the variability of the source that drives you nuts.You are spot on, if the source is varying, even slightly, with the higher resolution meter, the more the least significant digits are going to keep altering in an effort to reflect the changing input. Whereas with fewer digits, that change has got to be much bigger in order to alter the least significant digit. 3.5 meter, no change in display, 4.5 meter sometimes the last digit will go up or down by 1, 5.5 meter, last 2 digits were changing all the time on the power supply. Its like a proverb, man with 3.5 and 4.5 digits, knows the voltage, man with 5.5 or more digits is unsure of the voltage due to last digits changing.
Quite frankly the 5.5 digits is overkill for me. I could get along fine with 4.5 or even 3.5 digits. But it's nice to have....just in case.
6.5 digits? Only if drops from heaven for free.
I'm in the garage tomorrow looking some angle brackets so I can fix my function generator and LCR meter to underside of my shelf. I'd like to mount the TF930 up there too but I'm worried about spoiling the resale value of it later if I upgrade it.
Looks like I've started a trend. You must post pixAs promised here is my revised (for now) bench, what you can't see is dotted around the room in any suitable space I can are 4 CRO's, signal generator, numerous, microscopes, handheld DMM's, analogue meters, portable LCR devices, soldering kit, jigs, isolating transformer, variac, power meter, parts racks, calibrators and tools etc. I'd love to have a room big enough to take a proper 2 or 3m bench and shelves then I could have all the kit out and ready to be used, maybe this week its my turn to win the lottery....
It's the variability of the source that drives you nuts.You are spot on, if the source is varying, even slightly, with the higher resolution meter, the more the least significant digits are going to keep altering in an effort to reflect the changing input. Whereas with fewer digits, that change has got to be much bigger in order to alter the least significant digit. 3.5 meter, no change in display, 4.5 meter sometimes the last digit will go up or down by 1, 5.5 meter, last 2 digits were changing all the time on the power supply. Its like a proverb, man with 3.5 and 4.5 digits, knows the voltage, man with 5.5 or more digits is unsure of the voltage due to last digits changing.
It's the variability of the source that drives you nuts.You are spot on, if the source is varying, even slightly, with the higher resolution meter, the more the least significant digits are going to keep altering in an effort to reflect the changing input. Whereas with fewer digits, that change has got to be much bigger in order to alter the least significant digit. 3.5 meter, no change in display, 4.5 meter sometimes the last digit will go up or down by 1, 5.5 meter, last 2 digits were changing all the time on the power supply. Its like a proverb, man with 3.5 and 4.5 digits, knows the voltage, man with 5.5 or more digits is unsure of the voltage due to last digits changing.
Quite frankly the 5.5 digits is overkill for me. I could get along fine with 4.5 or even 3.5 digits. But it's nice to have....just in case.
6.5 digits? Only if drops from heaven for free.
It's the variability of the source that drives you nuts.You are spot on, if the source is varying, even slightly, with the higher resolution meter, the more the least significant digits are going to keep altering in an effort to reflect the changing input. Whereas with fewer digits, that change has got to be much bigger in order to alter the least significant digit. 3.5 meter, no change in display, 4.5 meter sometimes the last digit will go up or down by 1, 5.5 meter, last 2 digits were changing all the time on the power supply. Its like a proverb, man with 3.5 and 4.5 digits, knows the voltage, man with 5.5 or more digits is unsure of the voltage due to last digits changing.
TEA Proverb #382: Man with more digits knows when he needs to buy a more stable power source (or replace the filter caps).
It's evolving.
I really wish people wouldn't post geekporn pictures.