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

Parents Reply
  • Hi Kirill,

    Thank you for your reply. 

    the common-mode input resistance of this amplifier is 10MΩ typical, which forms a voltage divider together with two resistors at the input.

    The common mode input resistance is the resistance between the + and - input pins. How then it could form a voltage divider with two resistors ? I mean we have the voltage source then the two resistors which forms an equivalent resistor and then the common mode input resistance, but the last one is between the two input pins so how it could be a voltage divider (the common mode resistance is not connected to gnd) ?

    Thank you,

    Regards,

    Mike

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