The general head's up on LTC24xx and other LT switched cap parts - If you use these for measuring resistors (even for relative value shifts) you have to really pay attention to how you drive those inputs. When you take the time to actually test these 24 / 32 bit ADC's on accurate & calibrated sources, you find out in a hurry where the datasheet is a bit optimistic. With further conversations with LT and more product testing we've found that what the datasheet doesn't tell you when you're chasing PPM - in a nutshell:
A) Keep the source impedance <<< lower than the first input switch Ron (which is usually around 5k). Which means your source impedance wants to really be <<500 ohms for lower gain error. Keep source impedance <<50 ohms for closer total INL error datasheet spec. This also reduces that input protection diode leakage effect (which applies a TC effect error to your data); measured leakage values can tend to run around 20~30% higher than datasheet values across full temp span. Otherwise your data will be shifted in gain / offset or INL.
In other words, these are made to use an input buffer
in almost all cases, unless your signal source is very low impedance to begin with.
B) Pay really really close attention to slapping capacitors or RC filters on these ADC inputs or Vrefs pins. It seems counter-intuitive, but those can degrade INL and cause offset in a hurry if you're not careful.
C) Remember that the input pins and Vref inputs will have an AC current component driven onto your signals, be aware. If you're after PPM level accuracy, this can have a observable effect, so watch out for that.
However you're testing resistors, it's always a good idea to test & verify your method or device data against a known good resistance measuring system (which will always be a resistance bridge, either manual or automatic...NOT a DMM) AND your resistors under test should be at a typical BIAS load and TYPICAL thermal flow + mechanical mounting condition for whatever application you're testing.
If you don't have a resistance bridge available: that's why I suggested running LTZ ratio resistors ON an LTZ - That's the most accurate test for that application.
By the way -
If you really want to test resistors correctly, here's a nice kit for a fraction of the "new" price. If you haven't seen how a precision resistor oil bath works, here's an example:
https://www.ebay.com/itm/Measurements-International-Automatic-DC-Resistance-Bridge-Complete-System/282828252605?You get some nice goodies in there like the Hart oil bath to maintain your references, an SR-104 (which is 10k, not 10M as the description reads), a 3458a, a few Low EMF switch scanners, automatic resistor bridge ($$$$$), etc.
You even get Windows 98!! WooHoo!!
I'd offer seller ~~$70k to get the conversation rolling, and you might score yourself a VERY sweet deal.