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,
Differential input resistance is the resistance between inputs; this resistance of the amplifier is multiplied by the loop gain coefficient. Common-mode input resistance is the resistance between any input and ground; this resistance is not affected by negative feedback. High-speed amplifiers with bipolar input have low input resistances.
Apparently, you look at the amplifier as a black box and therefore it seems to you that these resistances exist literally. But this is wrong, as it is wrong to forget about their existence. They are not connected to anything inside the amplifier. This is only a calculation model.
Another issue that interests me is the noticeable phase shift between signals. This is 43 degrees at such a low frequency - 3 Hz, and you said that you excluded all capacitors from the circuit. This is strange...
Thank you very much for your answer. It's clearer now! And as you suggested, the problem comes from the common mode resistance, because when I did calcultion considering this common resistance I was able to find the attenuation I had.
KirV. said:Another issue that interests me is the noticeable phase shift between signals. This is 43 degrees at such a low frequency - 3 Hz, and you said that you excluded all capacitors from the circuit. This is strange...
No for the figures I have posted, this was with the capacitors included. I explained in the post that I have tried to remove the capacitors and despite that I had the same attenuation. But the signals you see in that post it's with the capacitors included.