AnsweredAssumed Answered

capacitance calculations for ad7745/46 input extension circuit

Question asked by engineer_adc on Sep 11, 2014
Latest reply on Sep 12, 2014 by seanbrown


i have made an input ectension circuit on the ad7746 evaluation board.

i have calculated the extension circuit values for an input capacitance of 146pF

based on that my resistor values are R1=100.04k R2=125.7k


Dac set=146/1.24

dac set(146pF)=0x76

F gain_factor=8.79

C_dynamic range=+/- 36.03

i have few confusion

Does the dynamic range means i can connect any capacitance within the range of (146+/-36.03)pF

Now i have connected 155pF to the input with

setting capdac A as 0x7D

enabling capacitive channel

exc setup as 1B

the data registers  in continuous conversion mode read as=7A4572

capacitance is shown as(-0.183319)

how can i calculate the real capacitance based on these values?