It's all about settling time

Fellows,

My idea is having forward settling time less than ADC conversion time.

AD7982 conversion time is 710ns;

To get 18 bit accuracy, ADG1204 transition time plus settling time < 400ns, AD8475 specified the 16 bit settling time 50ns, I guess the 18bit settling time should be less than 100ns. RC filter settling time should be less than 200ns if i chose the value right.

So the forward settling time is sqrt(400^2+100^2+200^2)=458 ns.

Here comes the problem, AD8475 requires very low impedance input source to achieve gain accuracy and high CMRR. So I need find a fast settling buffer to drive AD8475, and the supply voltage for this buffer should be no less than 24V to cater to the input signal.

I went through and found AD8021 might be the only one suitable.  But it only specified 1V step, 0.01% settling time, which is 23ns. How can I know the 18 bit settling time with large input, like +8V step with ±12V supply ?

Please correct tme if i'm wrong and it's appreciated if you had suitable OpAmp to recommend.

Cheers,

Jason