I am trying to find a capacitance to Digital converter for a range of 0 - 100 uF +/ 10 uF. I need a resolution of about 1 uF.
I took a look at the CN-0129 (http://www.analog.com/media/en/reference-design-documentation/reference-designs/CN0129.pdf) which shows how to extend the range of the AD7746 (originally 17 pF).
I worked through the example in CN-0129 and found that my FFix = 64,700,588, CDYN = 100,000 pF or 100 uF, FDYN = 7,324,219.
Using FDYN for my F (since it is the larger F value), and an R1 = 100 k, my R2 = 99.99997 k. This leaves me with a calculated 1.06 uF / LSB. Also note that I believe CN-0129 has an algebraic error where it suggests that R2 = R1 * (F + 1) / (F - 1), but actually R2 = R1 * (F - 1) / (F + 1) -> the + and - signs are flipped.
(I attached my calculations)
Is extending the AD7746 to a range of 100 uF possible? in other words does this IC have a maximum extension and have I exceeded it with my desired range?
If I have exceeded the range of the ADD7746 is there an IC that can work in my larger range?
Thanks for any help that can be offered,