Checking Differential and Common Mode Responses of an ADC Input RC Filter

An RC network on a differential ADC input does not have one universal cutoff frequency. The response depends on whether the two inputs move together or in opposite directions. Separating those cases helps explain why a filter rejects differential noise yet allows a common mode disturbance to become an error.

Define the two excitations

For differential excitation, the relevant signal is the voltage difference between inputs. For common mode excitation, both inputs move together relative to the circuit reference. Draw the current paths for each case before calculating a pole.

The TI report on noise filter design for precision conversion distinguishes these responses and explains that mismatched common mode time constants can unbalance the input. A capacitor connected directly between the inputs serves a different role from separate capacitors to ground.

Calculate using the actual network

Include source impedance, any resistor divider, and both input branches. Use the equations for that topology rather than applying a single resistor and capacitor formula to whichever pair is most visible on the schematic.

As a simple limiting case, a capacitor across two inputs sees no changing voltage from a perfectly equal common mode movement. It therefore cannot provide the same filtering action for that excitation as it does for a differential signal. Ground referenced capacitances introduce additional paths.

Evaluate mismatch as an error source

Vary the two branch time constants independently within their realistic ranges. Equal nominal components do not guarantee equal response. Include asymmetric source impedance or protection capacitance when it materially affects the branches.

Apply a common mode stimulus and examine the resulting differential voltage. Compare that converted error with the measurement budget across frequency. A disturbance outside the desired band may still matter if its conversion occurs where the differential response provides limited attenuation.

Verify conversion and settling together

Measure the complete input network with the intended ADC operating mode. Check signal settling and any interaction with its sampling input in addition to noise rejection. Increasing resistance or capacitance can improve one response while slowing acquisition or changing gain.

Save separate differential and common mode response results, component assumptions, source impedance, and measured conversion error. Keep the applied common mode voltage within the permitted range during testing. A useful filter specification states what is rejected in each mode and how much imbalance is acceptable. Describing the network only by a nominal cutoff leaves the most important coupling mechanism between those modes hidden from the review.