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Input Capacitance range extension  for AD7745/46

Question asked by engineer_adc on Sep 9, 2014
Latest reply on Sep 12, 2014 by seanbrown

hello all,

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