Well, that´s kind of noble understatement! Substituting relays with a
bunch of N-ch MOSFETs, maintaining a THD < -120 dBc even for signal
voltages around 10 Vrms, is certainly not a trivial task, especially
because you might run into Vgs max (usually +/-20 V) issues at these
high levels!
As a matter of fact, nearly all conventional analog FET-switches "of
the shelf" are composed of parallel p- and n-FETs with static gate
voltages, yielding the typical "bathtub" Rds-on curves, causing quite
a lot of distortion when they have to conduct more than just a couple of
µA. The only exception I know of is the MAX14756 with an almost
perfectly flat Rds-on vs. signal voltage characteristics, yielding
very low THD also for high signal amplitudes.
But this IC is of course too expensive to realize 10 different gains,
so your solution to do this with your own discrete solid state solution
is admirable!
And I agree, relays are outdated in an ever shrinking electronic
world. And not necessarily better performing than a well made solution
with MOSFETs. Good luck with this project with the ES9842!
By the way, did you notice? The "2 Vrms input" (differential) in the
Functional Block diagram are just another blunder in the data sheet.
In reality, the modulators surprisingly accept quite higher voltage
before hitting 0 dBFS.