8-bit analog input card??

I am using a 8bit analog input card on a siemens 315f-2 cpu.
This is used with a LVDT and its amplifier...I read back a value of between 10 000- 28 000 with about a 128 jump in 1mm of movement.Is it posible to mease more acurate?if so how do i calibrate it to do so as the jumps is always 128 unit.Or is it not posible with a 8bit card?the LVDT rated 1.5V/V

Also for some reasone my cpu does not want to subtract real values,it just gives me 0 as a anwser for any combination of subtration FC105 also returns 0 as a answer for everything......HELP!!!

In S7 analog cards data are left justified. This means that after conversion data looks as follows:
sign - 1 bit
converted input value - 8 bits
trailing zeros - 7 bits
That's way smalest change of converted value is 128.
Resolution is still range/2^8. If it's not enough then you need card with higher resolution.