1.how did u choose that the opamp can solve this issue? i simply blind before ask this doubt to u
You wanted to convert a voltage range of (2.0V - 0.5V) = 1.5V into a voltage range of (2.0V - 0.4V) = 1.6V. Converting a 1.5V voltage variation to a 1.6V voltage variation requires gain, so you can't do it with just voltage dividers. Op-amps are the usual way to provide accurately controlled gain. The circuit characteristics can be defined very accurately by just the external components (resistors). The "op" in "op-amp" stands for "operational" which comes from the fact that op-amps were originally used to perform arithmetic "operations" - multiplication, addition, subtraction, division etc. So multiplying by (1.6/1.5) with a reference voltage of 2V is easily done with an op-amp.
I chose that specific op-amp because of the requirements for low input offset voltage (needed for accuracy), and low-voltage operation with the inputs and output able to swing close to the supply rails. I originally planned to use a 3V supply, not 4V, so I eliminated the Maxim MAX4236 (
http://www.digikey.com/product-detail/en/MAX4236BEUA+/MAX4236BEUA+-ND/1702410) which is a bit cheaper than the OPA376 I specified. Then I realised I had to use a supply voltage of at least 3.15V because of tolerances in the XTR111, so the MAX4236 is also suitable.
and in data sheet we don t have any other ways to solve this issue right?
Yes, your persistence in asking the same question for a third time has paid off - I've come up with a simpler solution.
This design applies the adjustment using the op-amp inside the XTR111 so no external op-amp is used. Adjust the multi-turn trimpot for exactly 4V on pin 5 and use good quality components for the two 30kΩ resistors and the 1kΩ resistor.
Very neat trick Kris, although I think your formula works out that it has gain, how does this work?
Thanks Adam
I don't have time to explain now. Both of these designs just use gain and offset adjustment methods that can be understood in terms of Thevenin's and Kirchhoff's laws.