Quality multimeters using sigma-delta?

[nfmax]

Agilent/Keysight U3606A & B 5 1/2 digit DVM's use a sigma-delta ADC, it says in the specification. This type of converter gives a much more frequently updated display than does a multi-slope type converter, although the actual measurement averaging period may be similar. With multi-slope you don't get any result at all until the conversion finishes, with sigma-delta you can get updates as often as you want. This has both benefits and drawbacks, you can see if a voltage is stable or not, but if it isn't stable, the display is difficult to read.

[Kleinstein]

The faster SD converter Chips like LTC2440, MCP3911 and others usually use a kind of multi-bit system internally. This likely also applies to many audio converters (even if they call them single bit) to get the required resolution at high speed. The multi-bit loop is essentially needed to get higher than 2 nd order or to get the required speed with a 2 nd order SD. The trick is using a thermometer style DAC and internal dithering/swapping the sources - this way one can get good INL, not so much limited by the DAC, as the error is largely averaged out.
Some of the Multi-slope ADCs (especially those without a dedicated run down phase) could also be interpreted as a kind of first order muti-bit SD converter with PWM used as an DAC.

With a SD converter one does not have to use a rectangular integration window and thus sin(x)/x response, but many converters use it, because it is simple. The advantage over classical dual slope is that an SD can use continuous integration and thus don't miss large parts of the signal like the old dual slope does. Modern integrating converters are integrating the input most of the time, so not missing much.

Besides classical integrating multi slope and SD there are also a few high resolution successive approximation ADCs that could in principle work very well in a high resolution DMM. The Fluke/Phillips recirculating reminder could be seen as such an approximating converter. The LTC2376 ADC chip and similar have rather impressive specs (e.g. better INL than SD ADC chips - maybe except the prema ones) - though I don't know a DMM that uses them.

[TurboTom]

The Rigol DM3068 (6.5 digits) utilizes an ADS1256 (http://www.ti.com/lit/ds/symlink/ads1256.pdf) which is specified to be a sigma-delta converter. I would consider the DM3068 a "quality multimeter" though opinions may differ... . At least I'm quite satisfied with its performance and accuracy.

Cheers,
Thomas