I am concerned about the noise immunity of AM1805AQ and AM0805AQ. VCC drop causing POR is described on page 44 of AM18x5 Datasheet. High frequency noise (about 10 ns pulse) exists in the power supply or GND of our product, and it may cause VCC to momentarily be lower than the threshold for VCCRST. In "Table 13: Power on AC Electrical Parameters" of the datasheet, TLOW:VCC (Low period of VCC to ensure a valid POR) is spec'd as 0.1 sec (typ) at 25 °C, but is there a maximum low period duration for VCC below which data will be retained and POR will not occur?
No, the spec that you refer to is only meant to define the typical VCC duration that would trigger a POR. Even though the RTC consumes such little current that it can retain internal register data even through brief VCC voltage drops below VCCRST, any VCC drop below VCCRST could cause a reset and a loss of data.