PLC Millivolt Input Standards

This is likely an easy one for anyone who works with more modern PLC's and their related sensors.

I have a Scadapack 350 series PLC system that has four counter inputs. #0 is a standard 5 volt 5 ma open collector type but #1 and #2 are millivolt types setup for low voltage inputs.

Here is the problem. Presently someone else designed the system and is running standard magnetic reluctance type speed sensors directly into the #1 and #2 millivolt inputs which causes them to have very inaccurate readings due to what I feel is because they are being grossly overdriven on input voltage at the higher input signal frequencies. As tested our magnetic pickup units will put out 15 - 25 volts AC open circuit at the higher frequencies they need to work at.

Now that said I have tested our PLC inputs using a digital signal generator and they seem to have a very specific voltage bandwidth of ~.5 - ~1.2 volts. Any input signal above or below that seems to get ignored.

The problem as I am seeing it is when the millivolt inputs get over driven that far the high impedance inputs are saturating and not able to read the higher frequency rates with any accuracy.

So with that what is the present standard for millivolt type inputs now a days?

Back when I had my last PLC classes these were typically reserved for input signal of less than a volt at the most and the literature for our PLC systems does not give a specific range for these two inputs other than to say they are to be used for direct connection to standard millivolt output type sensor systems.