Led Display Digital voltmeter problem with 200mv scale

[karter1]

I've the digital voltmeter with 1cl7107.
I've used the schematic and components recommended on the datasheet for 200mV scale, so the max number on the display is 199.9
My problem is that the last digit seems to be very unstable.. it changes between +-4 numbers.
It's only a noise problem or there is other reason?

 

[audioguru2]

Hi Karter,
Welcome to our forum.
Did you use Maxim-ic's improved IC or Intersil's old one?
Did you use metal-film resistors that are very stable, or ordinary carbon-composition or carbon-film ones that are unstable?

 

[Kevin Weddle]

It sounds pretty stable to me. When you consider you are measuring fractions of a millivolt, I would suspect noise and missmeasurement. I'm just guessing, but the thing is probably accurate in the few millivolts range simply because the best meters only display the millivolt range. A millivolt is such a tiny amount. In situations where the voltage is very small, they try and linearly amplify the best they can. It is surprising that they can demodulate the way the do. It goes to show how accurate amplfiers can be in the low voltag range. But I would not count on that voltmeter simply because it has to average and while the process may be accurate, the actual voltage you are measuring is not. It is kind of a two fold situation in which things can be done with low voltage but you would be hard pressed to witness every aspect of it.

 

[audioguru2]

Maxim-ic's improved (at no extra cost) IC guarantees a 000.0V reading with 0V input and its accuracy is guaranteed to be plus or minus 1 count, which is 0.1mV. It is actually better than that but can't display it. Its noise and temperature drift are guaranteed to be 15uV or less, but most IC's are much better. If you don't use good stable resistors you are throwing away that stability and accuracy. Their datasheet is here:
http://pdfserv.maxim-ic.com/en/ds/ICL7106-ICL7107.pdf

 

[karter1]

Hi audioguru.. I've the Intersil one.
Reading the Intersil datasheet the accuracy is about +-2 counts and it would be fine for me.
But the last digit it's much more unstable.
So, i guess that the only way is to improve the passive components quality to reduce noise (I use carbon film resistors) or just use the Maxim ic!
Maybe Kevin is right.. you can't pretend too much from a meter.
Thank you for the answers (sorry for my english! ;D).

 

[chai98a]

Hi karter and all
Who.. do you have PBC 1 side of led display dogotal vol pls send to me or upload for share.
thank you