ADC Input Stage Linearity
Per the MCHP datasheet, Figure 4.4, extrapolating out past the 0.01uF upper value, I'd replace the 10_Ohm R21 output dampening value with 100 Ohms. The figure, extrapolated, suggests at least 50 Ohms for 0.1uF Cload.

The datasheet gives no hint on output impedance.

Unless we examine how the phase margin changes from 90_degrees at 1MHz with NO Cload, to 45_degrees at 1MHz with 500pF Cload; Xc[500pF, 1MHz] is -j318 Ohms; thus the Rout is also 318 Ohms, to produce the 45 degree phase shift. If we dare proclaim this is inductive reactance, knowing 1uH at 1MHz is +j6.3 Ohms, we compute 318/6.3 = 30uH inductive equivalent reactance; 0.1uF and 30uH will resonate at 50KHz.

Thus you may have an oscillator, caused by the heavy C_load.