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Coaxial cable assembly for current sense measurement?

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T

treez

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Hello,
We are getting too much noise scoping across a sense resistor in series with our 100W offline SMPS input. We are connecting a diff probe across the sense resistor, and then into the picoscope 2205A.
The loose diff probe leads are giving too much pickup.
Therefore instead, we wish to just use a piece of 50 ohm coaxial cable, but do you know if its possible to buy say a 50cm to 110cm piece of coaxial cable with SMA connectors on its ends? (we would then connect a BNC to one end, and put it into the scope,.. the other end we would cut the SMA connector off, then solder it to the sense resistor pads.
Can you buy such a cable assembly?
We are supplying the SMPS from an isolated AC power source PSU
 


You can buy coaxial cables of any length and with any connector termination, e.g. one end SMA other end BNC. Popular variants even from catalog distributors.
 

treez said:
Therefore instead, we wish to just use a piece of 50 ohm coaxial cable, but do you know if its possible to buy say a 50cm to 110cm piece of coaxial cable with SMA connectors on its ends? (we would then connect a BNC to one end, and put it into the scope,.. the other end we would cut the SMA connector off, then solder it to the sense resistor pads.
Click to expand...
You should still expect some interference if you don't use a good diff probe between the sense resistor and oscilloscope. I would recommend using CTs for high frequency measurements.
 

Thanks, its basically the 50Hz mains input current so indeed i am now going to see about a current transformer for it.
Ill have a look for a core and have to wind it myself i can imagine.

- - - Updated - - -

The trouble with CTs as you know, at the AC side of the mains rectifier, is that the magnetising currrent corrupts the measurement, unless you can make LS very very high.
 

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