# How does a CM choke reject common-mode noise while allowing the signal of interest? [duplicate]

As far as I know a CM choke is nothing but a transformer(?).

How does it block common-mode noise and pass the signal of interest? Should we think of it as an LR filter? And how to size it if we know the signal of interest? A pictorial explanation with a circuit diagram ect helps to understand.

Edit:

Below are two scenarios where an isolated(floating/battery powered) sensor S signal is measured by an ADC device and the system is single ended. The coaxial cable’s two wires somehow are victim of CM noise. As you see the CM noise is the spike superimposed on the half sine wave desired signal:

So in the first scenario(upper diagram) I can see how the CM blocks this spike now. But in the bottom diagram there is an attempt to remove the same spike with a LPF where the LPF’s ground is the signal ground as usual. Will this attempt also remove the spike or not?

• it is somewhat like a transformer, but a 1:1 transformer and it's connected differently. For CM signals, it tries to induce the same signal as already present and they cancel out, differential mode is allowed through Commented Sep 13, 2017 at 9:28
• If I use a CM choke I cannot use it for conveying DC signals ? Because it is like a transformer? Commented Sep 13, 2017 at 9:29
• No, it is connected differently, so it has a DC path between input and output, unlike a transformer.
– user16324
Commented Sep 13, 2017 at 9:48
• @BrianDrummond One of my difficulty is that when to use a CM choke. If I observe noise superimposed on my DC channels(BNC coaxial cable carrying DC-like analog signals to a dataq device); in this case upto what common mode noise freq. I can use a CM choke? Im confused some say CM noise can only filter the noises at RF frequencies. Commented Sep 13, 2017 at 9:53

How does it block common-mode noise and pass the signal of interest?

Let's say you have an unspecified 1:1 transformer and wire the primary to a 1 V 1 kHz AC source. The output from the transformer will be a 1 V, 1 kHz voltage as expected. Now if you were to apply a separate 1 V, 1 kHz to the secondary no current will flow in the secondary because it's like putting two identical batteries in parallel.

Let's also say that the transformer is ideal in that each winding has infinite inductance. In the scenario painted above there will be zero current taken from the primary and secondary sources. This, in effect is the device blocking common-mode voltages i.e. two identical AC sources are applied to both input and output and no current flows.

If you applied different AC sources there could of course be significant current flow. So this tells us how re-wiring a 1:1 transformer as a common mode choke works.

But, the primary and secondary inductances are nowhere near infinity so there will be current flow but, that current flow reduces as frequency increases so, a common mode choke performs better as frequency rises but, up to a point and that point is determined by the winding capacitances and the core losses.

Should we think of it as an LR filter?

Thinking of it like a transformer is just fine but remember the limits mentioned above as frequency gets too high.

If I use a CM choke I cannot use it for conveying DC signals ?

Transformers don't work at DC so, DC passes unhindered.

• Please see my edit I could express the core of my confusion: So in the first scenario(upper diagram) I can see how the CM blocks this spike now. But in the bottom diagram there is an attempt to remove the same spike with a LPF where the LPF’s ground is the signal ground as usual. Will this attempt also remove the spike or not? Commented Sep 13, 2017 at 11:14
• I don't understand the relevance of the low pass filter comparison in your question. As far as I am concerned the basic question is answered irrespective of your edit but please do elucidate if you feel I've misinterpreted what your edit is all about. Commented Sep 13, 2017 at 11:38
• I was wondering why they use choke for CM noise instead of a LPF. This lead me to ask this question. I would be glad if you have a comment on my edit. Commented Sep 13, 2017 at 11:40
• A low pass filter only filters differential line signals. It doesn't filter signals that appear on both lines. And, conversely, a CM choke doesn't filter differential signals i.e. a difference voltage at the input will appear at the output largely unaltered. Different beasts doing different jobs. Commented Sep 13, 2017 at 11:58
• So in my figure II the spike will pass and the LPF will be useless right? I tried to remove a 50Hz noise once with a similar setup like in figure II(with proper cutoff), maybe that was the reason it didnt work. So CM noise cannot be filtered by a LPF like a usual line signal noise. I hope I got it, let me know if what i wrote is not correct. Thanks. Commented Sep 13, 2017 at 12:02

You can think of the common mode choke as a 1:1 transformer turned sideways.

To common mode signals it acts like an inductor. So at DC it has a very low impedance and the impedance rises with frequency.

For differential signals the magnetic fields cancel. So a low impedance can be maintained for high frequencies (exactly how high depends on how the choke is wound).

Most common mode chokes are designed to block RF interference. You could design one to block low frequency AC but it would be bulky.

• Please see my edit I added diagrams: So in the first scenario(upper diagram) I can see how the CM blocks this spike now. But in the bottom diagram there is an attempt to remove the same spike with a LPF where the LPF’s ground is the signal ground as usual. Will this attempt also remove the spike or not? Commented Sep 13, 2017 at 11:15
• Another question is Im dealing with DC like signals. And the sampling of ADC is 4000Hz max. In my case using choke for RF interference is meaningless? Or can RF interference have still impact on a DC signal which is sampled at 4000Hz? Thanks Commented Sep 13, 2017 at 11:19