Hello DoubleDang,
Hi everyone!
I need to calibrate a particular device to 20V within +/- 2mv,
so as I understand it I'll need a DMM with a combined accuracy of 0.01%, is that right? Or do I need to find something that's an order of magnitude better than that i.e. 0.001%?
No. With an external reference the accuracy of the DMM does not play a role, only short-term stability of the DMM
- as long as it's only about this particular measuring point.
![Smiley :)](https://www.eevblog.com/forum/Smileys/default/smiley.gif)
Example:
I have a cheap handheld UT61E-Asian DMM - one of those that Dave Jones ("pure opinion") dislikes so much.
The 20V DC?! figure you need to calibrate fits perfectly into its 22V measurement range right at the upper end as it should be.
It has a resolution of 1mV in the 22V range which seems to be sufficient to achieve that 20V +-2mV goal.
IF you provide 20V (say 20V+-0,2mV) by means of an external reference to the UT61E you just have to keep in mind that value
and trim your device to exactly to that value shown before.
Even when your reference is between 19V and 21V the linearity is not an issue with my UT61E - measured against a 6,5 digit DMM.
Given the exact value of the reference you would simply calculate the error of the UT61E and take it into account when calibrating the device.
The accuracy that you were asking for is only needed when calibrating the reference.
Regards
try