Hello

i have question regarding capacitance sensor connect to ad7746 evaluation board.the range of capacitance sensor changes in different medias.

like in air its arounf 10-15pF

in water its between 50-65pF

the project i am working on is to differentiate between the medias(water/air) based on the output of the AD7756.this is what i did so far

connecting the capacitance sensor to Cin+ and EXC A as in CN-0129

the maximum value my sensor can get is 60 pF taking this into account

my calculations for extended input are:

R1=100.5K

R2=180.6

my range Factor F is 3.50374

CDACEFF=0.5100 (Cap gain calibration =5CC0)

Cdynamic=14.3513904

Range=60+/-14.3513904

CAPDAC set to 76 Hex

please correct if i am making mistake

1. when the capacitor is in water i could of the range 50-65pF

but for air as the capacitance range is 10-15pF do i have to change the CAPDAC value as this range does not fall under the dynamic range of 60+/-14.3513904

2. As the range 10-15pF fall under the 21pF input range for AD7746 i think i dont have to use CAPDAC REGISTERS?

3. Is it a good choice to use AD7746 if yes how am gonna deal with it because if it is required to change the CAPDAC value every time(provided the range doesn't fall in the dynamic range) i change medium from air to water or vice versa ?

P.S thanks to all the moderators and senior members for all the help i got through this forum.

Hello Adnan,

You have followed the example in the Circuit note too closely. In the circuit note, the Fixed Extension factor is larger than the Dynamic, which is why it was chosen for the rest of the calculations.

In your case, the Dynamic extension factor is (65pF - 10pF) = 55pF/8.192pF = 6.714. This is larger than your Fixed Factor.

Keep in mind that you want the CAPDAC to offset any non-changing capacitance and allow the range of the CDC to encompass your entire range.

I have not calculated the required resistors, but the later calculations show that you will have a dynamic range of ~±27.5pF. If you place your CAPDAC in the middle of your range: 37.5pF (0x25 or 0x26) then you should be able to measure the two mediums. Air will give you a low reading in the 0x000000 range and water should give you a high reading, in the 0xFFFFFF range.

Regards,

Sean