# 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

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• Hello Mike,

What is the input and output waveforms like? What's the DC offset of the input waveform? What's supplying the voltage at the input? Function Generator?

One issue I see here is that the bias current of the amplifier is rated at maximum of about 100nA. With 440kohms (220k+200k) at the input, that would shift the DC level and potentially cause the output to clip. If you can provide the waveforms, that would be fantastic.

You can also reach me at Kaung.Win@analog.com

Kaung

Reply
• Hello Mike,

What is the input and output waveforms like? What's the DC offset of the input waveform? What's supplying the voltage at the input? Function Generator?

One issue I see here is that the bias current of the amplifier is rated at maximum of about 100nA. With 440kohms (220k+200k) at the input, that would shift the DC level and potentially cause the output to clip. If you can provide the waveforms, that would be fantastic.

You can also reach me at Kaung.Win@analog.com

Kaung

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