Wow! All the way down to 10 Hz!
Is 500 kHz the upper limit for the AD5933?
With internal 16.776MHz clock it will do down to about 10kHz, below that it will no longer capture a full wave. But with a variable external clock signal it's no problem getting down to 10Hz .
It does take a slightly more elaborate algorithm to scan the lower frequencies, something like: set the clock to desired frequency (I use mcu PWM as clock for the lower frequencies) to capture at least 1 full wave for the 1024 point DFT, then scan 1 octave and repeat.
Downside is that it's pretty slow at low frequencies, as it needs to lock on a few waves first. But speed is not really high priority, beyond faster is preferred
The official, linear until, upper frequency is 100kHz as far as I remember, but it will do up to 500kHz with a roll off that starts around 200kHz. I can compensate for the roll off above 200kHz, with some loss of resolution.
I have considered trying e.g. a 20MHz external clock just to see if it will still run reliably