differential input ADC for single ended DC measurements?

[SArepairman]

What is the deal with using differential input ADCs for single ended DC measurements?

There are many delta sigma ADC which demand differential inputs.. but the data sheets seem to like showing AC signals when making these demands. What is the case with DC signals?

Can these techniques be used with all differential ADCs or are there any special cases that we should worry about?
http://www.ti.com/lit/an/sbaa133/sbaa133.pdf


Are there better methods?

[calexanian]

My take, which is probably wrong is it is all driven by noise rejection. a differential input can be used as a single ended input with proper handling on the unused side. As a matter of fact one thing I forget regularly is that AN ins on many pics default to differential or comparator modes on some types! 

[SArepairman]

does anyone have a schematic for a multimeter that uses a ADC chip properly? I mean a ADC ASIC, not a traditional capacitor based one like a bench multimeter.

[jmaja]

I know top end Flukes use LTC2435-1 (e.g. U3 in http://www.eevblog.com/images/Fluke28pcbbottom2.jpg), which is a 20 bit version of the 24 bit LTC2415-1, which I have used a lot. It is a differential one and it can be used to measure DC. Actually it can only measure DC, since it has 13-15 Hz sampling rate unless you use an external clock signal for sampling.

You can measure single ended with differential, but you need to be careful about maximum range and absolute voltages. E.g for LTC2435/LTC2415 the range is +-2.5V at 5V reference and both inputs must be 0-5 V. Also you need to have no capacitance in the input unless you have very low impedance source. The newer generation ones are a bit more tolerant about input capacitance and you can also choose via SPI if you want to measure ground referenced or differential. E.g LTC2492 is from this newer generation. With these you can get ~1 uV noise, thus ~22 bit resolution at 5V without any averaging. I have built a current monitor with ~1 mA noise, ~1 mA offset and at least 1000 A maximum current using LTC2492 using differential measurement. Actually a single LTC2492 is measuring the voltages and currents of two batteries. Usage is very simple, just a voltage divider for measuring higher voltages and a protection resistor for the current measurement. I also needed a small capacitor at the current measurement input for getting low noise in this case due to noisy environment, but very often that is unnecessary and almost always leads to offset etc. errors with this kind of sigma delta ADC.

I haven't used other brand sigma delta converters. thus I don't know much about them.