Hi Musach,
Nowhere in the ANs for the AD5933 IC does it say that extra low-pass filtering is required when using a lower clock frequency. With 16MHz clock, the sampler operates at 1 MSPS and to adhere to the Nyquist sampling condition, the cut-off frequency of the low pass filter should be below 500kHz. I believe the receive stage of the AD5933 provides a 2 stage low-pass filter with characteristic frequencies in the range 200-300kHz, so for the 16MHz clock, no additional filtering is needed.
However, do the filters internal to the AD5933 scale with the clock frequency?
If not, then it seems additional low pass filtering external to the receive stage of the AD5933 seems to be required to avoid aliasing. I.e. using a clock of 50 kHz means we are sampling at ~3 kHz and we need to insert a low pass filter with cut-off below 1.5 kHz.
Thanks in advance for your answer!