AD7147 real capacitance reading and resolution

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

Parents
  • Hi Maithil,

    thanks for you reply.

    i don't have it clear.

    one of the problems is that when i change the offset for example from 9 to 8, see below, the reading is changing from 32223 to 33242, this is around 1000 counts. so is i offset step is 0,32pF and it is 1000counts, then 1 count is 0,32fF, and not 0,244fF. correct?

    all this is being tested on the eval board EVAL-AD7147EBZ.

    the other question about this example, if as in the first picture below the offset is set to 9 and the reading is 32223,

    can you please tell me what is the real capacitance readed and how do you calculate it?

    thanks

Reply
  • Hi Maithil,

    thanks for you reply.

    i don't have it clear.

    one of the problems is that when i change the offset for example from 9 to 8, see below, the reading is changing from 32223 to 33242, this is around 1000 counts. so is i offset step is 0,32pF and it is 1000counts, then 1 count is 0,32fF, and not 0,244fF. correct?

    all this is being tested on the eval board EVAL-AD7147EBZ.

    the other question about this example, if as in the first picture below the offset is set to 9 and the reading is 32223,

    can you please tell me what is the real capacitance readed and how do you calculate it?

    thanks

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