Electrical – Common mode input impedance

You've asked quite a few questions. Let me try to answer them in the order you asked them.

1) "What I read about common mode noise is that it is a noise generated on all signal lines and is common to all lines. How can that be?" You should re-read that again as your first link specifically states that it is in a common direction on all the "signal lines." Basically what that means is that the lines where you care about noise have the same noise present on them, and the noise is in the same phase with the noise on the other lines.

2)"Now, my doubt being, how is the common mode noise generated ? What components generate common mode noise ?" CM noise is generated by anything with a varying signal, which includes noise, that is close enough to couple that signal into the line in phase with each other. That sounds complicated but lets break it down a bit more so that it's simpler to understand. Everything has some variation to it, be it noise or signal or anything else. These variations are called AC waves. These can couple to other lines either through capacitive coupling or through inductive coupling. You have two line that you care about. If the AC on the line near by the two you care about couples in it's outside AC onto your signal in phase (going the same direction) on both lines then that is a source of common mode noise.

3)"Now, in the link1 document it says power sources generate common mode noise. Is this applicable to both ac and dc power ?" Yes. Even DC power has some AC components to it, and all AC can be considered a noise source. Any noise source can be coupled commonly into any other signal.

4)"This means We connect one end to Vcc and gnd and then the other side to a dc rail and GND ? Is that it ? So that choke will have a current rating sufficing enough for the load ?" This is where you start getting a little fuzzy. The picture you included has a great example of how to hook up a common mode choke. The Vcc should pass through one inductor of the choke, and the GND should pass through the other inductor of the choke (Don't short them together). This also means that YES, the choke must have a current rating high enough for the power source's supplied current. Your understanding is correct.

The ADS1675 is a high speed ADC which runs of a \$5\mathrm{V}\$ Single-Ended power supply. If you note the following tables from the datasheet:

The differential input must be referenced to mid point of the supply rails which in this case requires a common mode voltage of \$2.5\mathrm{V}\$.

You then have an input full scale range of \$\pm3\mathrm{V}\$. I can only presume there is some attenuation internally which means that this full scale range is brought to within the specs of the device (need to clear that up with some more reading).

The circuit you show is for a THS4503 which is a high speed fully differential amplifier. The purpose of this is to either buffer or amplify the incoming signal to the full scale range of the ADC, and critically to drive the sample/hold capacitance of the ADC input.

Because of the high speed, you need a pretty strong driver to ensure that the capacitance of the sample capacitor in the ADC is driven quickly to the level of the incoming signal. This requires a fairly high current to drive the capacitor at high frequencies. So if you have a weak signal source it is important to amplify it before driving the ADC.