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tried one of the 4.5 digit Digital Panel Meters?


Anyone played with one of these or something similar?

Which I think is really this:

So when these come up on Ebay, the specs are kinda interesting.

# Accuracy: +/- 0.1% <<
# Type: LED (Blue)
# Power: 5V
# Range: 0-19.999V DC
# Resolution: 0.01V

0.1% accuracy?  Really?  Do I believe that?  And how can the resolution be 0.01v --
what is the last digit of the display used for then?  There's an unmarked 28 pin chip
running the show, so it's clearly not the ICL7101 which is used in most DPMs.  Hmm,
maybe a 28 pin PIC?  Haven't been able to find a schematic or really any documentation
at all.

I'd think that to hit any decent specs with this much resolution you would need a
real A/D chip with a good voltage reference chip, but I see really nothing on here
but that mystery chip.


There appears to be a 2.7V zener, maybe this is used as a reference? There are some very stable zener diodes. An internal band gap reference in the IC might also have enough accuracy, especially if it's calibrated after assembly.

0.1% isn't that great for 4.5 dgits, at 19.999V, the uncertainty would be +/- .020V, so the last digit is garbage, and I wouldn't trust the second to last digit to be very accurate either (if it's 5, it might be between 3 and 7). You would need something like a 16-bit ADC for that kind of resolution, but that assumes that all those digits are actually real. Given the accuracy, they might as well use a 12 bit ADC or even 10 bit DAC (just use some random number or averaging for the extra digit). Without a datasheet, I wouldn't assume anything.

My guess is that the mystery chip is either a modern equivalent of the ICL710x, or some sort of 8-bit micro (I'm sure there are micros with 16-bit ADC).

One issue is if it needs a power supply that's isolated from the unknown voltage, I really hate this with many digital panel meters, and tend to prefer analog (if I don't need accuracy) or rolling my own with a MCU for this reason.

It sounds impressive, but don't forget 20,000 counts is only just over 14 bits. They might be using a microcontroller with (say) an 12 bit onboard ADC, and over sampling 8 times. Inherent analogue noise dithers the signal and you get an effective higher resolution. Don't confuse precision with accuracy. Getting 0.1% precision is easy, you just add ADC bits. But accuracy is a whole different kettle of rice, just because you have lots of ADC counts doesn't mean they are linearly spaced. The "precison voltage source" will be built into the chip.

The 7107 had (has?) a known problem where heating of chip caused by driving
the LED displays causes the internal reference to drift, changing the display :-(

So I tend to be kind of suspicious of things that look like a one chip sampling
and driving solution.  Yes, I suppose you could try to compensate for the
heating, but a better solution would be to use an external reference.


Gentlemen  :) 

This version its an overpriced toy ...

Its best to add few bucks , and get an complete used Fluke 8050A or an hand-held version. 

I was looking for the above display , for some time .
My conclusion was , that its not worths this price tag ..
So I acted wisely .
And got an true multimeter instead of that. 


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