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AD7147 real capacitance reading and resolution

Question asked by JOSEBLY on Mar 14, 2016
Latest reply on Mar 30, 2016 by Wa$iM

Hi everybody,

 

i am using the AD7147.

 

i have some questions about offsets and fF per count.

 

i understand that the best way to use the sensor is around the mid count 32750, using offset compensation to be in that range.

 

i had read that offset is .32pF per step, with 63 steps, and the reading is 0.244fF per count.

 

the real capacitance will be calculated as the (offset steps)x0,32 +/-(delta from 32750)?

 

for example,

if with an offset set at 45, and a reading of 33600,

will the capacitance readed will be 45x0,32 + 850x0,244e-3= 14,4 + 0,2074 =14.6074pF?

 

then i have another doubt,

 

if i change the offset using the eval kit, i observed that every offset counts changes the CDC reading about 1000 counts when i am centered around 32750. If each offset step is 0,32pF, then shouldn't be 0,32fF per count instead of the 0,244fF per count as i read on another post?

i know that the CDC output code is not completely linear in all the range, but looks like 0,32fF will be more adequate at least around the center scale 32750.

 

how accurate is the offset 0,32pF compensation per count? can i really consider 0,32pF per offset step in all the range?

 

 

thanks

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