I found that in CN0200 and CN0191, AD8676B and AD8675 are used for both force and sense pins and the output buffer of AD5781/AD5791. The specification of AD8676 is superior, though, the slew rate is limited. We are considering replacing the amplifier to ADA4004 and LT6018, is it worth doing so?
What is the selection criteria for the voltage reference (force and sense) amplifiers and the output amplifiers for 18/20 bit precision DAC?
For the reference buffers, the primary consideration is its accuracy. You have to check the amplifier output noise, offset and drift. You don't need a high slew rate amplifier here. If I am going to suggest for the best amplifier here, it would be the ADA4077. It has the best offset & drift performance and its noise is still way below 1 LSB of the AD5781/AD5791.
For the output buffer, the AD8675 should be fine for AD5781 & AD5791. Criteria is also the drift and offset voltage and it also has lower output noise.
And considering that OPA227 has a SR of 2.3V/us, which is similar to AD8675's 2.5V/us, will AD5781 has the same settling issues as in the following figure? Can these phenomenons be simulated using LTSpice?
Since you are constantly varying the DAC code, yes you should opt to use the AD8676 for the reference buffer.
You should also use the LT6018 as the output buffer if you need the fast settling time.