amplifierinstrumentationlevel-shiftingvoltage
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?
The output of the AD8222 simulation makes sense. 250Ω for R1 gives you a gain of about 200. Input signal is 1V. The output would have to be 200V, and it gets clipped near the supply rails.
Lower the input amplitude, and/or the gain, and you should see a sine wave at the output.
The output of the LT1920 simulation doesn't readily make sense to me. The gain and the input are roughly the same, so I would expect it to swing and clip like the AD8222 simulation. Hopefully someone can spot the error there.
edit: The negative input pin (In-) in the LT1920 schematic looks odd. Maybe it not connected to ground.
Because "for best performance, the source impedance to the REF terminal should be kept below 2 [ohms]".
If you made a resistive divider to meet that requirement, it would be 4 ohms in each arm, or 8 ohms between power and ground, which would likely consume a lot more power than you'd want to use for this function.
Why does the impedance need to be below 2 ohms?
It's explained in the text you posted:
Best Answer
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.