I want to observe the influence of the mos switching on phase current. It happens in hundreds of ns. So the amplifier should have at least a 10ns resolution. Does it indicate the bandwidth of the amplifier should be around 100M?
The rated phase current is 3.68A and phase resistance is 200mΩ. The voltage across the 5mΩ shunt resistor is around 15 mV, at most 50mV. But the DC voltage is 24V. So the CMRR of the amplifier should be above 10000:1.
At first I want to use a current probe to observe the current directly. I scanned the current probes in tek.com and found that my requirements are satisfied except a new emerged problem. There is input impedance reflected into the phase when using the current probe. And the least value is 36mΩ at 1MHz. I’m not sure if it’s a disturbance comparing the 200mΩ phase resistance since it is not constant, it varies along the frequency and the value can’t be neglected comparing 200mΩ. Is it a disturbance? By the way, the phase to phase inductance is 0.065mH.
Then I turned to differential probe. But the CMRR is 1000:1 at just 1MHz. Obviously, it isn’t enough.
Now I plan to amplify the differential signal first and observe the output of the amplifier. Is there an amplifier meeting all the requirements above? I’ve noticed INA240, but the bandwidth is just 400KHz.
Any other amplifiers or any other solutions?
Best regards.