Electronics > Beginners
Common mode choke for Instrumentation amplifier input
ricko_uk:
Hi all,
with reference to the attached schematic with the common mode choke at the instrumentation amplifier input.... The instrumentation amplifier input impedance is extremely high (so the currents are extremely low) and the voltages at the CM choke inputs are also extremely low making the currents even more microscopic (pico amps or even femto amps??). So does the CM choke work at all? Would it not need some "decent" currents to create a magnetic field so the noise cancel itself?
Thank you
unitedatoms:
Impedance is frequency dependent. There is input capacitance in pF range. So the current is not femtoamperes in AC, but rather nanoamperes or more (million fold for MHz interference).
ricko_uk:
Thank you Unitedatoms :)
would it still work at removing noise between 50KHz and 200KHz?
How do I choose it correctly? What parameters do I need to check in the datasheet to choose the most appropriate CM choke?
Thank you again
T3sl4co1l:
Right, to make a filter, some impedance is required. A filter for infinite impedance is meaningless.
Tim
unitedatoms:
Perhaps the chokes have some kind of specs for common mode loss, blocking. I saw example of choke in HP LCR meters, where coaxes from BNC ports go through large rings, two or three turns. I'd look at there part numbers and make conclusions.
The correctness of choice is possibly debatable. May be the most common chokes are most correct. The most common ones are this lossy ferrite rings for power supplies and VGA cables, can be found as pair of semi rings with plastic clips to mount on cable.
That kind of chokes is designed for DC DC converters frequencies in high kHz range.
Edit: @Teslacoil. Well, I can not follow that logic that countermeasures is meaningless. For receiver with infinite differential and common mode impedances and non zero sensitivity to common mode signal, the required filter simply has to have common mode impedance greater than infinity. The practicality of such filter is debatable. :)
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