Problem of amplitude with LTC6261 amplifier (inexplicable attenuation of input signal)

Hello,

I am using the LTC6261 amplifier to design a second order low pass filter. My low pass filter has a 5 Hz cutoff frequency. Here is the scheme.

So as you can see I have two resistors of 220 k and two capacitors : one 100 nF and another of 220 nF. The output of the amplifier and so the filter goes to the ADC input of a MCU. My problem is that I have an attenuation of the input signal of the filter at the output. I mean when I put for example a 1 Hz sine wave at the filter input with a 2 Vpp voltage, and at the output I have only 1.8 Vpp, which is incorrect since I have a 5 Hz cutoff frequency. Also, to be sure that the problem doesn't come from the filter, I have retired the capacitors and just kept the resistors, which finally give us a simple buffer. Despite that I wasn't able to have the same signal at the output, I still had 200 mV attenuation (1.8 Vpp instead of 2Vpp). Also I was wondering if it's not the impedance of the ADC that is too important that is the cause of the problem. So I have disconnected the amplifier output and the ADC and still the same attenuation. I have tried everything I also soldered a new amplifier and still nothing. I really don't know from where this attenuation comes. I also checked the datasheet but I don't know if I missed something that could explain this. Any help would be appreciated.

Thank you very much,

Best regards,

Mike