Proper method to filter inputs of a (true) differential ADC

Question

In datasheets of different true differential ADCs I noticed two kinds of a low pass filter at the inputs. First one with 2 capacitors to the ground

The other with a capacitor between differential inputs:

So what is the proper way to place a filter at the inputs? When each (or perhaps both) of these filters should be used? in general at first ?

And now for my case where I have DC signals as input, so a filter would be used not for antialising, but rather for bandlimiting the input noise? I guess it is not a good idea to try to bypass the inputs of the ADC?

Answer 1

It depends on desired SNR and level of CM noise how to best reduce CM levels.

• Balanced paths & impedance
• high CMMR Differential Amp with wide bandwidth
• Shielded pairs
• Active Guarding on shield using buffered CM signal
• Baluns, raise CM series impedance followed by shunt caps
  • no effect on differential mode impedance

General best differential buffers input signals with common mode caps.

Using unity gain buffers gives the highest BW for signal and CMRR

• It is assumed Transmission Line terminal impedance matching for broadband signals on long paths, is a separate issue depending on situation.

Answer 2

The proper way to provide an input low-pass filter is to do both. The first case provides common mode filtering, and the second case provides differential mode filtering. To calculate the filtering needed, determine your Fc for the common mode case, where Fc = 1/(2 * Pi * R * Ccom).

For the differential case, you simply add C4 (Cdiff) between the two inputs. This filter is needed to filter any signal differential caused by component mismatch in the common mode filters. Therefore, you need to make Cdiff about ten times Ccom.

As far as determining Fc for the DC case, you need to filter both input noise and aliases due to it. Therefore, choose your Fc based on the lowest expected noise frequencies. While anti-aliasing filtering is still needed, it will be handled by this filter as well.

A good reference paper can be found here - https://e2e.ti.com/blogs_/archives/b/precisionhub/archive/2015/11/06/three-guidelines-for-designing-anti-aliasing-filters.