Hello GustavoKF,
Sorry I didn't understand your application -- I thought you were looking at audio tones, not the low-frequency modulation of your flanger. With the latter, there's no trouble with the frequency, but as Brett noted, the -1940 truncates the lowest 4 bits of the readback -- thus a resolution problem.
With hardware handy I tried the multiplication and initially got the same strange result you described. After some experimenting, I concluded that the Real-Time Display doesn't work with settings of 24.4 or smaller -- likely because the -1940 doesn't provide the full number of bits down there. Apparently the sign bit is also lost, causing the polarity flip we both see. To work around this I multiplied the display's input by 32, effectively shifting 5 bits to the left -- thus converting 28.0 to 23.5 format, which the -1940 can faithfully read back:
The Real-Time Display works fine at the 23.5 setting, as shown below:
.
Hope that helps,
Bob