Hi MARS
1. VN shouldn't be tied to ground (or neutral) directly. It should be tied to ground via a RC filter, like I have shown in my post. The resistor and capacitor values must be the same as what you have used in other voltage legs VAP, VBP and VCP and in current channels (IAP-IAN, IBP-IBN, ICP-ICN). Based on your figure, the resistor should be 1kohm.
2. The frequency calculation is done via voltage channel zero-crossings. The zero-crossings are not very well recorded when the voltage signal is small. So, that is why you are observing a bad result with low voltage values. In the application, the line voltage is usually around the nominal value (110Vrms/220Vrms). With a 1000:1 attenuation on voltage channel, the voltage signal entering the ADE IC will be 110mVrms or 220mVrms. This is very close to the fullscale of the ADCs and hence you should not have a trouble in those cases. Do you have any specific reason why you might need to monitor frequency values at low voltages? Is that an expected application scenario?
3. VRMS results are accurate till a dynamic range of 20:1, which is 17.67mVrms at the ADE IC inputs. I am assuming you are able to get good rms results above this voltage level. If not, offset calibration can be done to get accurate results within this dynamic range. Do you need to accurately report lower voltages as well? If so, what is the lowest and highest voltage value that you need to accurately report? May be you can change your attenuation network to accomodate this range.
Doing offset calibration to get accurate results below this 20:1 dynamic range, may not be very useful. However, the method to do offset calibration is explained in page 17 of ADE7754 datasheet and page 9 of AN624 app note.
Let me know if you have further questions.
Regards
hmani