The following schematic:
formatting link
is a "capacity to voltage" converter based on two 555s (non CMOS version): the first one is the astable, the second one is a monostable which pulsewidth depends on the capacity, so I have a V(DC) proportional to this capacity.
The period of trigger pulse is (measured) about 21 us.
V(DC) - after RC filter - should be about Vmax*D.C.
where Vmax is about 4,4V and D.C. (duty cicle) is about 0,69
When I measure V(DC) I got 2,6V instead of about 3V as expected.
Where I am mistaking? RC filter bandwitdh is about 30Hz so is much lower than 47 kHz (pulse frequency).
Any hint/suggestion is welcomed.
P.S. I noticed that the min.level in output to the second 555 is greater than 0.