What about this?
The LPF has a corner frequency of 10Hz. The 10K resistor just after the LPF acts as a load for the op-amp.
The 1.5k resitor has two roles:
1) protects the op-amp from capacitive loading.
2) If the test point TP? is used to inject a testing signal, the 10k/1.5k divider causes the ADC to see a mix of about 0.9 times the op-amp output and 0.1 times the injected signal. In that case, the 1.8n/1MEG RC could work as a LPF (~10kHz) for the injected test signal.
I think this could be an explanation, because the 1MEG resistor doesn't seem to be RC filtering the op-amp, the 10K+1.5K take care of that.
Edit: No. Of course, a signal injected from the test point will see an RC filter with 1.8n and 1.5k, so the 1MEG resistor does nothing in that respect. What is that 1MEG resistor for?