ADG821 can operate from 1.8V to 5.5V.
Please tell me the VINH/VINLvoltage of the digital input at Vdd=1.8V.
I checked the reports about the device however, we do not have data available for the Vdd range above. The ADG821 can work at 1.8V but it hasn’t been fully characterized.
You might want to consider using ADG811, Quad x SPST switches, as an alternative. The ADG811 also has <1 ohm on resistance and is fully characterized at Vdd=1.8V with VINH = 0.65 x Vdd and VINL = 0.35 x Vdd.
Let me know if this helps.