Measuring settling to 18-bits (4ppm) is a very difficult measurement to make. This graph shows that the LTC2757 settles a 10V step to within ±1 LSB in 1.8µs. Application Note 120 “1ppm Settling Time Measurement for a Monolithic 18-Bit DAC” by Jim Williams, offers a thorough discussion on measuring 18-bit settling time.