i am working on input capacitance range extension for Ad7445/46.i have few confusions regarding the circuit note available on analog website.
i am following the circuit note and made my circuit on eval board for ad7746
my calculations and settings are :
i used capacitance 146pf a fixed value capacitance connected to cin 1 and exc A
based on that
my calculation for R1=100k R2=126k
the gain factor F=8.78
c dynamic range=36.03 (does it mean that the input can be anywhere in range of 146(+_ 36.03)
based on gain calibration factor value of 5CC0 Cdaceff turned out to be 1.26
Dacset value for 146pF is 115.8(0x74)
after setting up the registers using EXC setup=0x1B,cap setup=0x80 and capdac A=0x74
capcitance data register read as 0x784982 and capacitance is shown as -0.246812
i am not sure if the settings and procedure i am following is correct if its correct then how can i calculate the real capacitance value using eval software for eval board.
secondly if i connect a value less than 146 let say 120pF will the value change accordingly to the change in input capacitance?do i need to change the capdac A value every time i change input capacitance value?
and again how can i calculate the real capacitance value.
P.S i have attached pictures of my settings as well