Hi tompazntua
The ADE7758 features and measurements have been designed and tested for line frequencies between 45 and 65Hz. I have done some testing on the FREQ register measurements before, and have observed that the register measurements do not stay good below 40Hz- hence not recommended to use them. You can however read the period measurement for such lower frequency cases and calculate frequency from that.
The RMS and energy measurements have not been tested at such lower frequencies for ADE7758. The reactive energy works by phase shifting the fundamental current signal by 90 degrees and that would not work well for frequencies outside the 45Hz-65Hz range.
If you can let me know some details about what features you need to use for this 25Hz line frequency case and the performance expectations, if any, then I can help you figure out if ADE7758 can be used in that application or not. Based on your inputs, I can also suggest any of the newer ADE ICs ( like ADE7878A or ADE7880) to you, which have different VRMS and reactive energy implementations and better performance in general, if you are interested.
Regards
hmani