Ferrite bead for long cables

Thread Starter

Rikh

Joined Jan 19, 2020
3
Hi,
with reference to the attached schematic, I need to minimise noise picked up by the 3 meters long cable. Noise can be picked up by the cable and fed back into the driving (instrumentation) amplifier which in turns is amplified in its output. According to Fig 19 Page 8 of the attached application note, when driving long cables, you can use a resistor in series with a ferrite bead on the instrumentation amplifier output to reduce that issue.

Three questions:
1) is resistor required? why and how do you choose the value?
2) I am thinking of putting one also in the input (i.e. receiving side op-amp input). Is that correct? How do I choose the value for this one? Based on the op-amp input impedance maybe? How?
3) if the cable is both shielded and twisted pair, is the ferrite still required? In other words, is it worth putting it in or not for a 3 meters long cable?

Many thanks all feedback :)
 

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Janis59

Joined Aug 21, 2017
1,537
One of most effective method of fringefield securing is double braiding where one is normal braiding but other with antiphase (just apply the -1 factor operational). Teoreticians speak it may give a 100-110 dB attenuation to trouble-signal.

If contrary, RF induction is the main cause, then as more lossy the ferrite, as better result.
 

Lo_volt

Joined Apr 3, 2014
280
Take notice of where the return current to your signal is traveling. Is your 3 meter cable two conductors twisted and one of the conductors signal ground? Are either of the amplifiers grounded to building earth ground or chassis ground? Are the power supplies for the two amplifiers connected together?

When you connect signals through such a long cable, best practice is to use twisted pair cable with the signal on one conductor and the return for the signal on the other. The return conductor may be ground. All current flowing through the signal conductor will flow in the opposite direction on the return conductor. This will ensure that minimal differential noise can be induced on the signal at the input to the receiving amplifier as that same noise will appear on both signal and return. Any induced noise will be canceled out.

If the grounds for the two amplifiers are connected, this may act as an alternate path for your signal return current. This separate path for the return current will cause problems as current induced in it by external noise will not be the same as the current induced by that noise in the signal conductor. That difference will show up at the input to the receiving amplifier and be amplified accordingly. You may need to isolate the grounds on the two separate amplifiers.

Along the same thought path, if you are supplying power via a separate cable, this cable may be subject to induced EMI. This may present itself at the output of the transmitting amplifier. You must ensure that you properly decouple the power supplies with capacitors. As well, you may want to use an isolated DC-DC convertor at the transmitting amplifier to separate the signal ground from the power supply ground. This will avoid the situation where signal return current flows through your power supply ground (the fabled ground loop).

To sum it up:
  1. Make your signal return the second conductor of a twisted pair.
  2. Isolate the ground of the transmitting amplifier so that no signal return current flows through that path.
  3. Decouple your amplifiers properly.
  4. You may want to use shielded twisted pair, but if you use it connect the shield at one end only so that no signal return current travels through the shield.
 

Janis59

Joined Aug 21, 2017
1,537
RE: Lo-Volt
Absolutely sure when we speak about RF signals, but hardly untrue when about sound frequencies. So, jumpig off the coaxials toward twisted pair created tenfolding and later hundredfolding of frequency response of internet. The same time would be brainless to switch all microphone cabling off the coaxials towaard twisted pairs, the result would be rather catastrofical. For each task is the best suited instrument, however here the topic starter havent revealed the puspose of cable, thus any even the best advice is wrong per se.
 
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