MSP430FR2633: CapTIvate Electrical Characteristics: Counts to Capacitance?

Hi Sam,

The CapTIvate peripheral is a relative sensing engine; that is to say that it is very well suited to detecting changes in capacitance (% change in an electrode over time) but it is not well suited to precisely measuring absolute capacitance. There are several non-linearities in the operation of the front-end that make it non-ideal for an absolute measurement. You bring up one source of variation (the integration capacitor on-chip with collects the charge transferred from the external electrode) which varies somewhat with process (not so much with temperature). There are other characteristics which do vary with temperature (the delta-V on the charge transfer moves with the threshold voltages of the transistors used for the charge transfer, as an example).

I have some formulas which I could provide to you that would allow you to convert changes in the count value into equivalent % changes in the external capacitor, but it doesn't sound like this is what you are looking for.

One method of approximation to get closer to an absolute capacitance is to measure your external electrode, and then measure the external electrode with an additional on-chip reference capacitor connected in parallel with it, and look at the difference. Based on the known value of the reference capacitor you could try to back out and estimate the capacitance of the external electrode by itself, but there is still some tolerance to the on-chip reference capacitor due to process variations that would make this non-exact.

Another thing to consider with CapTIvate is that because the architecture is based on transferring charge from the external cap onto the internal integration cap until the internal cap reaches a certain voltage, the IP is more sensitive to reductions in capacitance than it is to increases in capacitance (due to the 1/x nature of the architecture). In other words, if you start with 20pF, adding 5pF creates a smaller absolute change in counts than removing 5pF would. This can be backed out mathematically if needed.

If your application requires knowing precise capacitance values, CapTIvate may not be the best IC we have to offer. I'm happy to see if we can make CapTIvate fit for your use-case, but I would also point out that for lower channel count / higher precision applications we have the FDC family of devices (www.ti.com/.../FDC2214) which is better suited for more absolute and higher resolution measurements. Let me know how I can best help here.

Regards,
Walter