Hi Guys,
Thanks for such prompt replies.
The gain varies from about 46, at 10mV, to 51, at about 60mV. At first I thought that it was insignificant, but when I re-did my maths it threw the PIC's reading out to a value that was unacceptable.
I have measured the sensor output voltage with and without the op-amp plugged in. As well as with the voltmeter connected and disconnected. Unfortunately it made no difference.
It is not the ADC, I have been doing all of my calculations with a voltmeter.
The 10mV offset is prefferred. The unit is therefore, easily calibrated at around 13mV and has an origin at 0,0.
I assume that the offset voltage of 7mV is the minimum voltage that can be amplified, which the sensors signal will always be greater.
The final output does not have to go to the full 5v, the 3.5V will be adequate.
Is a rail-to-rail chip going to make much difference to the output's linearity? Id rather not have to order in new components, I'm really excited about this project and would like to continue with it as soon as possible.
Thanks so much for your help,
Tom