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2

There is a path through R28 and R32 to give crosscoupling between Diff1 and Diff2 that is giving the small signal in OA4. The two opamps share the same offset adjustment. Any signal coming in the D input goes through R31, the R32 to the offset adjustment. Because that acts like a 2.5k resistor to ground there is about a 4:1 attenuation to this point. (expect ...


5

You are violating the the "common-mode input voltage range", which defines the maximum and minimum acceptable voltages that may appear at the opamp's inputs, given its power supply potentials. From the TL071 datasheet, on page 19, we find \$ V_{ICR} \$. It tells us that this range could be as constrained as ±11V, but is more typically +15V and -11V,...


5

That op-amp has a minimum recommended supply voltage of 6V and an input range that typically gets to the negative rail +3V. You are outside both of those constraints, so whatever it does is not clearly defined. Probably you're preventing the bias circuitry from functioning and causing the output to shut down. Give it supplies such as +/-10V (or even +8/-4) ...


2

You need an op-amp that can function with the input at the lower supply rail, or you need a negative rail. Once you go outside the input limits (which you are doing with an input so close to the negative rail) anything can happen. Since you have 12V available you don't need a Rail-to-rail op-amp, a single-supply type will likely do, or one with input common ...


4

Does anybody have an idea on what's going on and how to deal with it? Thanks It's called signal inversion (also phase reversal) and happens when the input goes beyond the upper or lower limit specified in the data sheet. Basically, the output polarity flips and things become fairly meaningless: - 2nd picture from here. My advice is to add a small negative ...


2

According to the datasheet this is not a rail-to-rail device: The datasheet does not provide detailed specs for the voltage rails you are using, but you can see that the output can't reach the positive rail, and this is worse the larger the output current. You should also not expect correct behavior with 0V inputs.


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