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

  • Hi Josebly

    the mid count shown is for adjusting any stray capacitance's as its corresponds to zero and any value of offset can be removed, sensor is good across its range.

    Now offset register 1 count is 0.32 pF, so total 64 counts correspond to 20pF

    and data register 1 count is 0.244fF, so total counts correspond to 16pF(+8pF)

    if you change offset count by 1, the difference in data count would be 0.32pF/0.244fF which would more than1000 counts which is correct behavior as you have observed., the 0.32pF can be considered across all the ranges.

    also if you read CAPACITANCE SENSOR OFFSET CONTROL section in the datasheet there are 2 DAC's mentioned there, which one are you using for setting the count of 45? can you repeat the steps as mentioned in that Section and let me know What you see there?


    thanks

  • 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

  • hi

    please let me know the readings at 0 offset and the offset value at which you get midcount values.

    also let me know the input connected to it and i hope its connected to channel 0

    thanks

    wasim

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