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I have this long cable which is carrying two differential voltage signals (around 300kHz maximum frequency) through a very "noisy" environment. I need to acquire this signal through a differential op amp (PGA204) and I was thinking about putting a filter at the input of the instrumentation OP AMP.

I found this nice filter:

filter for in-amps from Analog Devices tutorial MT-070

(Image from: Analog Devices Tutorial MT-070 - In-Amp Input RFI Protection)

But I cannot deeply understand the purpose of the two different filters (DM and CM). From what I know, a common-mode noise is a noise present in both lines with the same "polarity", while a differential-mode noise is a noise which is complementary in respect to the ground in the two lines.

What I see in the schematic are three LP filters, two of them (R1 C1 and R2 C2) filter the signal in the two lines independently, with respect to the ground, while the third one (R1 R2 C3) filters the whole differential signal. But aren't we just adding poles to filter the input signals? I mean, if I set the cutoff frequency of the two CM filters to a certain value, these two filters will just filter the signal within the range set. In the same way, the differential filter will just filter the input signal within the range set.

  • How exactly do these two filters work together?
  • How can they act on different kinds of noise if they are just LP filters?

Thank you

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From what I know, a common-mode noise is a noise present in both lines with the same "polarity", while a differential-mode noise is a noise which is complementary in respect to the ground in the two lines.

Yes, that is correct but I would have said that a differential-mode noise is seen as an unwanted signal between the two wires of the differential pair.

if I set the cutoff frequency of the two CM filters to a certain value, these two filters will just filter the signal within the range set. In the same way, the differential filter will just filter the input signal within the range set.

Absolutely and this can be a problem; if your unwanted signals are in the same part of the spectrum as your wanted signals then trying to filter-out the "unwanted" inevitably filters out the "wanted".

How exactly this two filters works together?

They make an attempt to filter out noise that is significantly higher in the spectrum than your desired signal bandwidth.

How can they act on different kind of noises if they are just LP filters?

They are just simple filters with simple mechanisms - they can't do magic and in many cases they fail to deliver but, on the other hand, in many situations they do a good job. Generally, filtering is not a great solution if the interference spectrum overlaps your signal spectrum.

On a side note, you might struggel to get the bandwidth you want from the chosen InAmp: -

enter image description here

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The value of the circuit shown in Figure 1 is realized when the frequency of the common mode noise is much greater that of the differential signal. Notice the constraint tDIFF > 10x tCM. In your case, your differential signal shares spectrum that is near many sources of RF noise: SMPS, AM broadcast, arcing switch and relay contacts, etc. I suspect you will have to reduce the CM noise in other ways. To start, you should be using a shielded, twisted pair cable, with the shield attached only at the IN-AMP analog ground. Is the 300 kHz sinusoidal or rectangular? If the latter, consider a termination resistor.

A systematic approach would be to know what the offending noise components are. Use a scope or spectrum analyzer to get a handle on it. Once you know what you're dealing with, you will have a better idea how to address it.

Also, if you're using a gain much greater than 1 on your PGA204, 300kHz will be near or beyond the the IA's 3dB point.

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