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06-29-2017 10:09 PM
I assume you are referring to the datasheet "Capacitance per Code" specification: 1 fF (typical) under Default cap sense configuration. The datasheet specifies "Maximum External Capacitive Load" for the default CSO Gain Adjustment (CS0CG[2:0] = 111b = "8x") as 45 pF (typical), and 500 pF (typical) for CS0CG = 000b = "1x". Hence, as you stated, for capacitive loads <= 45 pF, in the default configuration the capacitance per code is 1 fF. Exceeding 45 pF in this scenario would violate the max external capacitive load spec, and thus be unspecified (though I would expect the cap sense output to saturate at the max perceived value).
If you decrease the CSO Gain Adjustment to add margin to the max external cap load spec for your anticipated load > 45 pF, without changing any other settings from the default I would imagine you can expect the capacitance per code to scale accordingly with this gain adjustment. It's not something we specify, but I'll look into this and get back to you. Let me know if this isn't what you are asking.
06-30-2017 10:18 AM
Thanks Phillip. That is indeed what I'm looking for. I am using a variable capacitor that will reach up to 70 pF, and I need to know what the resolution would be once it crosses 45 pF. With 8x gain and 45 pF, the product is 360. Assuming the maximum product can be 360, then with a 70 pF load the maximum gain should be 5x. In that case, perhaps the resolution would be (1fF)*(8/5). This makes sense to me but I wanted to confirm.
06-30-2017 05:30 PM
I spoke with our cap sense experts and have some insight to share on this matter. The resolution of the cap sense code steps does scale with the CSO Gain Adjustment setting, but there are some nuances that inhibit following a simple path from the datasheet specifications to the answer you seek:
- While the CSO Gain Adjustment does manipulate the effective gain of the capacitance-to-digital conversion (such that increasing CS0CG increases the gain, which lowers the capacitance-per-code - i.e. enhances the resolution - and reduces the maximum capacitive load), the relationship is not linear. Thus, the description of each setting ("1x", "8x", etc.) indicates its index in the sequence of available gain settings, but does not precisely describe the magnitude of the gain. So a simple ratio-based approach built from the gain descriptions isn't going to yield expected results.
- The two datapoints in the DS (referenced in my prior response for CS0CG = 111b/000b), which provide the maximum capacitive load at either end of this gain adjustment range, are more appropriate to use here. Though the max loads available at the intermediary settings are not specified in the DS, the trend is such that max capacitive load is lightly sensitive to CS0CG at higher settings, and more strongly influenced by CS0CG at the lower end of the gain range. Hence, to realize a configuration with max load to support your 70 pF design, you might need to select a lower CSO Gain Adjustment setting than would be suggested by a simple "nx vs 8x" mapping from the 45pF default configuration.
- Note also that the DS specifies the typical values. The actual value can be influenced heavily by environmental factors (humidity, etc.) and part-to-part variation. A robust design implementing this capacitive sense module is one based upon the relative sensed capacitance of one load vs. another (i.e. the state of a "button" when pressed/touched vs. when it is not). Attempts to rely on absolute capacitance values instead would require careful calibration of each device, including dynamic tracking of environmental factors that can influence the conversion results.
Can you share more about your particular design? Feel free to open a support case with us directly if you'd like to more carefully discuss any proprietary aspects of your system.