Multimeter measurement causes output deviation of op amp

1. Background

When doing boost, the following circuit is used to sample the inductor current

Use a current sensing resistor and an op amp differential circuit.

As the current increases, the CT2 voltage increases, and the op amp output increases.

2. Measured data

The first measurement was with a feedback resistor of 100k, the second test with a feedback resistor of 51k, and the third test with the same circuit parameters as the second.

Question 1: Why is there such a big deviation between the reverse end and the non-inverting end?                                     

Question 2: The output leg of the op amp should be low resistance, so why does adding a multimeter (the DC voltage range of the multimeter is used with another multimeter to measure the resistance to be 10M) have such a big impact?   

 After finding the problem and changing the measurement method, the third volume measured ideal data.

Problem 1: The multimeter probe wire is too long, resulting in inaccurate measurements.                                                                    

Problem 2 is that the multimeter probe wires are parallel to the load wires (just like the windings of a transformer, the AC signal is coupled to the probe wires).

The load power line is coupled to the multimeter's probe line, affecting the closed loop and causing the op amp's output to be unstable. In actual measurements, the op amp's output voltage is different when the probe line position is changed, as shown in the following figure.

The blue-green line is the output load line

3. Reflection and summary

1. After testing, it is found that in actual measurement, the negative probe of the multimeter needs to be shorter, and the positive probe line should be folded several times by hand, and the two probes should be measured in parallel, which is better. (Measurement with the probes in parallel.)    

As shown below        

2. As with oscilloscope measurements, when you need to measure weak signals, you need to reduce the loop.

3. At the same time, in PCB wiring, power signals and weak current signals should not be routed in parallel