I am working on a temperature measuring circuit using a pt100. The input of my instrumentation amplifier (AD8226) is between 80-200mV. The Gain is set to 20.84 so I get an output range of 1.7-4.2V. I want to shift down those voltages to 0-2.5V for my ADC.
I only have a precise 2.5V Voltage reference. Whats the best way to get -1.7V for the reference pin or is there a different way to shift the levels down so I can use the entire ADC voltage range?
You can use an op-amp to generate -1.7V from the 2.50V reference. Most instrumentation amplifiers require a low impedance input for the reference input.
simulate this circuit – Schematic created using CircuitLab
If the op-amp has very low bias current you may not need R1. Of course the op-amp requires a negative supply for this to work.
Usually you can modify the input circuitry to the instrumentation amplifier to offset the output voltage.
If it's a three-wire RTD then there is usually a second current source to work with. The details depend on your arrangement- in the simplest case (two-wire RTD) you'd use a Wheatstone bridge arrangement with the resistor opposite the sensor determining the zero balance. There's little other reason to use an instrumentation amplifier anyway- a regular op-amp would do as well for a single-ended input.
The simplest way to do what you want is to use a couple of op amps. Taking the output of the first amp as V1,
simulate this circuit – Schematic created using CircuitLab
will do the job. A few notes:
Use 1% resistors. They're cheap these days, and you don't have to worry as much about tolerances stacking up the wrong way.
OA1 is needed because as the input changes, so will the voltages at OA2s inputs, and this will affect the loading on the pot output if you don't have a buffer.
OA2, in particular, must be a rail-to-rail op amp that will operate at 3.3 volts. And even then, be aware that it won't go exactly to either 0 volts or 3.3 volts, but will be within 10 or 20 mV, depending on load.
