I'm using the AD7767 for a high precision measuring device. There is an input stage that reduces my input voltage from +/-12V to +/-2,4V and a Diff Amp to drive the ADC. My ADC input filter consists of two 15 Ohm resistors and one 10nF capacitor. Input stage works very well but I recognize an ADC linearity error six times higher than the promised error of 7,6ppm.
It seems to me, that the input resistance of the AD7767 is not constant and causes the error together with the resistor of the input filter. If I reduce the filter resistors to 5 ohms the linearity error decreases from +/-44ppm to +/-14ppm.
All your application notes show an input filter with 15 ohms resistor and even the eval board uses this values.
Is it possible to get more information about the AD7767 input resistance and its behaviour?
Please can you tell me how I can get the typical 3ppm performance without eliminating the input filter?