FDC2114EVM: range

Hi Janine,

Your reference frequency should be much faster than your sensor frequency. Fref (after the Fref divider) should be at least 4 times larger than Fin (after the Fin divider). You can find this information in Table 1 of the datasheet.

For your 3.3 MHz Fin, your Fref should be at least 13.2 MHz after the divider. However, the higher the reference frequency is the better your resolution and accuracy will be. You should use the 40MHz reference frequency with a divider of 1. Currently you have Fref set to the same frequency as your sensor frequency, which will give you very inaccurate readings and could explain the odd range behavior you observed.

Best Regards,

Hi Janine,

You may set the Fin divider to either 1 or 2 for your sensor frequency. This is not set incorrectly.

Fsensor and Fref should absolutely not be the same. The equation below is taken from the FDC2114's datasheet:

The FDC2214 has 28 bits to output any possible sensor frequency between 1kHz and 10MHz. If Fsensor = Fref, then Fsensor/Fref = 1, and the minimum sensor frequency that DATAx can represent is 2^12. Fref needs to be at least 4x larger than Fsensor for the DATAx equation to work and to maintain reasonable resolution.

Also note that whenever the capacitance of the sensor changes, Fsensor will change in response. Even if you set Fsensor equal to Fref initially (which you should not do), they would not stay equal to one another.

With L=56uH and C=33pF, your sensor frequency would be ~3.7MHz. Using equation (1) from above and assuming your Fin and Fref dividers are set to 1 and Fref = 40MHz, the FDC2114 would output 378 (in binary). Note that this is assuming that the total capacitance in the LC tank is 33pF. If 33pF only included the surface mount capacitor in the LC tank, the additional sensor capacitance would change the sensor frequency and result in a different output code.

With L=100uH and C=33pF, your sensor frequency would be ~2.771MHz. Using equation (1) from above and assuming your Fin and Fref dividers are set to 1 and Fref = 40MHz, the FDC2114 would output 283 (in binary). Again, this is assuming that the total capacitance in the LC tank is 33pF. If 33pF only included the surface mount capacitor in the LC tank, the additional sensor capacitance would change the sensor frequency and result in a different output code.

Fin = Fsensor/(Fin divider code)

The Fref divider code, Fin divider code, settlecount, and Rcount values are all user controlled in the FDC2114's registers. You can use page 17 of the datasheet to determine your settlecount and Rcount values. Use 1 for the Fref divider and either 1 or 2 for the Fin dvider.

Hi Janine,

The purpose of the Fref divider is so you can use an external clock that is faster than the 40MHz maximum Fref. Fref should always be as high as possible without exceeding 40MHz after the Fref divider.

Best Regards,