What you get on analog input resolution is based on the type of A/D converter used and is always a compromise between speed, noise immunity, resolution and cost. PLCs tend to go for the middle ground, partly because PLC system design is generally optimized for fast sequential logic solving, so adding something slower but with higher resolution is going to potentially bog down the primary mission of the device. DCS systems on the other hand use much more powerful processors so they can deal with the higher resolution inputs (i.e. up to 24 bit). But that capability comes at substantially higher cost as a control system, hence their primary mission being plant wide process controls, not machine controls. PLCs have been used for plant wide control systems and SCADA systems, but not typically when high analog resolution is a necessity. Jut like in horse racing, you have to know your horse before you put your money on it.
As my old mentor first told me when I asked about the difference between a DCS and a PLC, he said "PLCs make things, quickly; DCS systems make stuff, accurately."
All that said, Siemens uses their S7-400 PLC I/O hardware for their PCS-7 DCS system, and because of that, they have Analog cards with 24 bit resolution A/D converters (although you only can use 20 bits + sign I think). Of course, that is their most expensive PLC system as well. I think the S7-300 has higher than 16 bit A/d converters but you only can use up to 16 bit on some types of signals.
Have you considered using a separate stand-alone PID controller with higher resolution for direct control of that process and then extracting only pertinent data from is for use in the PLC program?
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