Absolute capacitance for AD7147

Hi, Farad,

Have a look at figure 18 in the datasheet or CDC output to input capacitance conversion.

Hi, Farad,

Have a look at figure 18 in the datasheet or CDC output to input capacitance conversion.

Thank you Dcanny,

I looked into the fig but s not easy to get the values, if there is a formula for the conversion it would make it very easy to get the capacitance.

Hi Farad,

I looked into this a little deeper. I don’t know what the setup conditions were for the Fig 18 graph – e.g. I don’t know what the Offset DACs were set to and I don’t know what was used to measure the Input Capacitance for the graph. So it's not the best graph to go  by.

Something to note however, this part was designed to measure a ‘change in capacitance’ and not absolute capacitance.

The resolution is 16-bits so there are ~ 64k CDC codes. This is to cover a typ CIN range of +/- 8pF, resulting 0.24pF/bit. However, from talking to my manager, this is only the typical value and it’s accuracy may only be 20 – 25%.

Ideally there is 0pF on a CIN with nothing in proximity to the sensor, and at startup this will give a reading of CDC = 32k and then for every 0.244pF increase the CDC will increase by 1 (when the sensor input is connected to the +Input; the CDC count will decrease by 1 if the sensor input is connected to the -Input). Of course there is usually some stray capacitance on the Input so the reading at startup won’t be exactly 32k … and that’s when we recommend you use the DAC values to bring it back to 32k.

But to answer your main question – this part is not designed to measure absolute values of pF and sub-pF capacitances.