Hello. I am requesting some assistance on a circuit I am wanting to design. This design is a gas analyzer with a local lcd display. I have a Figaro KE-25 oxygen sensor, that has a 0% to 100% oxygen level range. This oxygen level is represented by a mV signal comming out of the sensor. The problem i have with this sensor is that when oxygen concentrations are low, there is a voltage offset due to the sensors characteristics (http://www.figarosensor.com/products/offset.pdf). When I put span gas (25% oxygen) to the sensor my existing circuit displays the value correctly. When i put zero gas (0% oxygen) to the sensor my existing circuit displays 2%~3% oxygen. First, i need to find an instrument amplifier ic that has very small offset voltage and input bias current (<10nA). Second, I need this amplifier to accomodate adjustable external zero and span components, which i am assuming is gain and offset. Once the amplifier's gain is set to whatever i choose, i need to be able to adjust the output at low levels of oxygen (0%) for correct display of oxygen percentage. If anyone can help me with the amplifier selection and the zero and span portion of the circuit, i would be entirely grateful.
Ok. I have decided to get away from the loop powered DAC (AD421) so i can utilize dual power supplies. Attached is my proposed circuit. Can you all critique (especially the change to the low pass filter as well as a suggestion for the values of the 3 resistors on the output of the inverting amplifier)? Any thoughts on using a quad op amp (AD8659) to save space on board?