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LT1990 50Hz pickup

Question asked by NormanVST on Jul 26, 2018
Latest reply on Jul 28, 2018 by krisf

I am using the LT1990 as a bi-directional current sense amplifier. The power rails are +/-3v3.

Circuit attached.

The I_Sense+/- connections go to two 4mm banana plugs. The these then have two wires (about 30cm in length) which connect to the device where the current needs to be measured (i.e. DUT).

The problem I am having is a significant amount of 50Hz pickup. I get the pickup with and without to 30cm wires connected. 

The DUT is a completely standalone device. It does not share any common power or ground connections with the LT1990.

In my previous circuit I used the AD626 as the current sense amplifier. This did not suffer from 50Hz pickup, even with the 30cm wires connected.

I am thinking that the pickup is due to the increased input impedance of the LT1990  over the AD626 (1M vs 200k).

I need to be able to measure currents and changes in current from DC to a few KHz so I can't have a 50Hz notch filter as this is valid within my measurement range.

Are there any techniques that I can do to stop the 50Hz pickup in the first place?

Is the pickup due to the high input impedance or have I misunderstood this?

For reference my PCB is a 4 layer board with internal power and GND planes.

Thank you.

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