I would like to design an RC input filter for the AD7799 for a load cell application. I have read this article: Front-End Amplifier and RC Filter Design for a Precision SAR Analog-to-Digital Converter | Analog Devices , but I am having trouble determining the proper specs (e.g. conversion time and input capacitance) in order to apply the same equations to the AD7799.I suspect the equations for an SAR ADC are different from what they need to be for a sigma-delta ADC.

I am taking measurements at the 19.6Hz rate. What should the Fc be for my RC filter for this chip at that rate? What are the equations I should be using?

Thanks,

Scott

After some searching and testing, I found this article - Delta-sigma antialiasing filter with a mode-rejection circuit | EDN, which along with its referenced articles, it appears that Fc should be something greater or equal to my sampling frequency. In my tests, I designed an RC filter with an Fc=45Hz, which is more than double my sampling frequency.

However, despite my resistance (35k) being well below the maximum level specified by the reference article, the filter appears to increase the noise level. I have also tried simply placing a capacitor between the pos & neg signal lines at the input of my board. A 1uF capacitor seems to have helped considerably. However, it still does not solve my fundamental problem - sensitivity to noise.

With both the analog portion of my circuit shielded with a grounded copper cover, along with a shielded cable also grounded, I am able to obtain stable enough readings. However, if I touch the unshielded portion of the cable, my readings jump from the noise. The common mode capacitor helps as well, but I am still not getting as low of noise sensitivity as I would like.

I have utilized every technique I can think of - optimum frequency for AC noise rejection, proper shielding, and the cap between the two signal lines. An RC input filter does not appear to help, but make it worse. I know that better noise immunity can be obtained. What am I missing or doing wrong? Any ideas or suggestions are appreciated.