At one time in the distant past, Fluke commissioned a dual-slope integrating DVM chip (40-pin DIP).
A very similar 40-pin version was released by Intersil: the ICL7106.
The ICL7106 was designed to drive LCD 7-segment display of type "1.999" pin-for-pin direct.
The ICL7107 was designed to drive LED 7-segment display. Since a lot more
current was required, it ran hotter, so that the built-in voltage reference was less reliable
than the internal reference of the ICL7106
The ICL7106 is ubiquitously used as the heart of very many "3+1/2 digit" DVMs.
I had the opportunity to investigate closely its operation. It is a work-of-art!
Its linearity and offset performance is very impressive.
Many DVMs that use the ICL7106 have poor accuracy that can be traced to the
front-end resistor divider which scale down input voltage to be compatible with the
chip's input voltage range. The tolerances of these front-end resistors limit the
meter's accuracy. The chip itself is very linear.
So you can calculate a "scaling factor" for each DVM DC voltmeter scale if you have
a known good voltage reference. Once a scale factor is found, you can used it
to correct any reading on that scale.
Its not a good idea to try this on AC scales, since AC-to-DC conversion circuits
external to the ICL7106 chip are notoriously non-linear.