I'm trying to put together the suggested circuit for the following sensor

http://proto-pic.co.uk/flexiforce-pressure-sensor-25lbs-1-area/

the circuit is part of the datasheet.

-1V does not really seem to be required. One option is to replace the -1V with +5V, and the grounded input by a 3.3V input. This inverts the behavior, but that shouldn't be a problem, and the advantage is that those are probably voltages you already have, and the output is still between 0V and 3.3V, so can be directly put on the ADC of your mbed.

The sensor modulates its resistance. What the circuit does is put a constant voltage over the sensor. Depending on the resistance a current will flow, which is again converted to a voltage by the resistor. But it shouldn't matter if those voltages is -1V and 0V, or 5V and 3.3V. Only the current flows the other direction.

Thanks for the detailed help :)

Even a simple resistive divider will do: Vout = Vref * R1 / (R1+R2); So the voltage is not directly proportional (or inversely proportional) to the resistance but maybe that's not required.

Taking your original suggestion could I do the following:

1) Connect the +ve of the op-amp to 0V. 2) Connect the -ve of the op-amp to 3.3V. 3) Use a simple resistor voltage divider to achieve a voltage of approximately 1v on the sensor.

Reason I suggest 3.3V is that I don't have a +5v source on my board.

I need to maintain a linear relationship between the pressure applied, so I don't think a resistive voltage divider would work in this case.

You can always calculate it digitally what the corresponding pressure would be. Unless you are very busy with other calculations that shouldn't be much of an issue, especially on an 1768.

The output voltage you get is at no pressure equal to the voltage at the positive input of the opamp. So that needs to be between 0V and 3.3V. If the other terminal of your sensor has a higher voltage, the output will decrease at increased pressures, if it has a lower voltage, it will increase. So if you have 0V on the + input, you need a lower one on the pressure sensor, which is negative and a problem.

You can use a resistive divider to put 0.5-1V on the positive input, and ground the other side of your sensor. The downside is you have an offset, the output starts at 0.5V-1V, so you cannot use your complete range of the ADC. But it means you don't need other voltages.

Don't you have any other stable voltage above 3.3V? Doesn't need to be necesarily 5V.

Thanks for taking the time to explain I now understand, at the moment there isn't a +5v supply but I can add one so no problems. You see this is why I only do software and some digital electronic design.