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Use AD7746 with an input range of 0-100 uF

Question asked by jeremy@twisthink on Aug 10, 2015
Latest reply on Dec 9, 2015 by wasim_22382

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)

 

My questions:

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,

JW

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