Integrated ADCs are fast approaching the performance previously only achieved by integrating ADCs. The AD4630 is a fine example, with a 24-bit (slightly more than 7.5 digits) resolution at a 2Msps sample rate. With the right analog front end and some signal processing, performance comparable to 6.5 digit meters is possible. The AD4030, a close relative of the 4630, is reported to have true 8.5 digit linearity. I have acquired samples the AD4630 and plan to investigate it further in the future. 

High-resolution integrated ADCs have not yet found use in bench multimeters, where advanced forms of the multislope ADC still operate. One of the more interesting implementations is found in the HP/Agilent/Keysight 34410A. The Art of Electronics has a short side-note about their multislope implementation that piqued my interest. 

"Looking at Multislope IV waveforms with a scope, you see a much different beast. A1b has been replaced with a faster AD829 op-amp (120 MHz, 230 V/μs), with the integrating capacitor C1 reduced by 5x. A hardware engine forces the error integrator to produce consistent 10 Vpp ramps with a 2μs period, using coarse data from an 80MHz AD9283 converter digitizing the 10V ramp at 14 ns intervals, and fine data from an AD9200 10-bit converter with a limited 2V range, clocked at 75 ns intervals near zero volts. Slope changes and the counter record are made at 75 ns intervals, and the AD9200 makes starting and ending readouts with 0.02% resolution. As a result, a Multislope IV converter can measure 4.5 digits in 20μs (0.001 PLC), or 20× faster than Multislope III."

(The Art of Electronics, page 921, footnote 61)

The brief explanation leaves a lot to be desired, and acquiring a 34410A to reverse engineer is beyond my means as a student with no source of income. However, this description forms the basis of my plans for the next revision of the multislope ADC, called Multislope 4(IV). 

Aside from excellent op-amps, TI recently released the TMUX113x series of analog switches. These feature incredibly low charge injection (1pC) and single ohm on-state resistances. The TMUX1133 matches the pinout of the 4053, while TMUX1134 has four switches. The latter opens up some interesting possibilities like using the fourth switch to short the integrator between measurement cycles, which would do wonders for dielectric absorption and save some PIO instructions.