Electronics > Beginners
Low Cost DIY Differential Probe
JS:
Current limiting is easy enough, just two transistors could do it. You could also change R38 with a current source, also not overly complicated or space consuming, a jfet and a resistor could do.
For the CMRR trim, I don't think you need that much cap trimmers for the compensation, just two should do, not tapping the cap networks to the trimmer ends, just one network by side. You are dealing with pretty low frequency to worry too much about compensation.
Then, for the resistors choice, I'd go with 1%MF as I have and here parts are also hard to get and expensive. If the CMRR trim is reachable from the outside you can tweak it every use, and just rely the trim caps are reasonable fine. Note that an INA wouldn't improve CMRR, as the input divider would be the limiting factor. And even then, you can't get it perfect as source impedance unbalance will make it change in every measurement.
Let's say you have your CMRR perfectly calibrated, now you connect the probe to a real world application, where the output impedance is different in both sides, like if you are probing a 10k resistor to measure a low current floating around, now the CMRR is just 40dB, no matter how nicely are the resistor and caps matched on your circuit. Things get worse in HF as the impedance gets lower by the caps in the divider. That's why a higher input impedance is good, other than loading the circuit and generating some error. In this case the error due to the loading will be 1%, which could be totally acceptable but if you are trying to measure 1µA variations in that 10k resistor in a signal that has 10V CM now the CM noise is 10 times greater than your signal variations...
Note that for measuring small NM signals on top of big CM noise isolation probes are better, as their CM input impedance is really high, and ridiculously high at DC, being just the leakage across the different components on the isolation barrier from the floating side to ground. The capacitance tends to be a bit high so HF CM input impedance is not as great tough, still much better than a differential probe.
JS
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