I have a pressure sensor, MPX2100 and I need to read its output with an Arduino. To do so I want to use a differential amplifier with a gain of 40 times (3k and 120k resistors). The impedance of the sensor's output is 3k. So is this input impedance too high that I will get non-linearities in the output?
The amp op that I am going to use is LM258.
My schematic:
Even though a In-amp is better I assembled the circuit above just to test my sensor while I wait my AD623 to arrive, however the gain is about 3 times! When my sensor's output is 50mV the output of the amplifier is about 170mV!
Another odd thing that is happening is that when I connect the sensor to the op-amp the voltage at the sensor's output changes to -4mV.
I am not sure whether my sensor is not working properly or the amplifier circuit is messing everything up.
Best Answer
I'll assume you're using the common op-amp differential amplifier like this:
with R1 = 3 kOhm and R2 = 120 kOhm.
The input impedance of this circuit is approximately 2xR1, or 6 kOhm in your case. This won't necessarily cause nonlinearities but it will cause you to lose about 33% of your signal to the the voltage divider formed by the sensor's output impedance and the amplifier's input impedance.
If you are not tightly cost-constrained, consider using an instrumentation amplifier instead of an op-amp. In-amps are generally optimized for differential amplification with high accuracy at dc and high input impedance. TI has in-amps in their catalog with budgetary pricing in the $0.50 to $0.60 range, which probably means $1 - $2 in onesy-twosy quantities.