Understanding the Op Amp output

I’m using a LM318N in the below configuration with a Force Sensing Resistor

My output voltages with various RMs are as follows,

5K 1.3 to 3.1
10K 1.3 to 3.5
47K 1.3 to 3.6
100K 1.3 to 3.4

I do not understand why I get a minimum of 1.3V to start with all RMs
Secondly the range and top pressure readings are all about the same not varying as per the FSR data sheet
Is this a function of the op amp and will I get a different result with say an LM358 or a LM324 ?


First, can you use a meter to test the resistance of the device?? The curves seem to show that the resistance should be very high with no force.

I suspect the resistance does go very high with now force, and the Opamp is just showing you that.

Check by connecting the OPamp input to Gnd. The output should then be ground... 0 volts..

LM318 is not "rail-to-rail" OPA, and: operates from supplies of ± 5V to ± 20V. This is why 1.3 - 3.5V all you could get at the output. Search for better chip, that can works from single power line and provides full range voltage swing at the output. Second, the way you connect FSR, you shouldn't expect linear results.

Magician, good catch! I Shudda Thought about that.

Idealists always assume Ideal Opamps :roll_eyes:

Thanks -Havn't used them in years . Coming back to me now Sine wave in -sine wave out centered on half of the available voltage approx