The sampling window is the actual duration per scan cycle for active data sampling of the sensor frequency. It is programmed with the exponential parameter, LCDIV, in Register LC_DIVIDER (Address 0x17), and the individual linear sensor cycle counter SENCYCn (n = 0, 1, 2, or 3) in Registers SENSORn_CONFIG (n = 0, 1, 2, or 3, Addresses 0x20, 0x22, 0x24, 0x26). For most touch button applications, the button sampling window should be set to between 1 ms and 8 ms. For sampling rate of 160 SPS, the window has to be less than 6.25 ms. For continuous sampling, the data becomes available at the configured sampling window period rate. The recommended minimum sensor conversion time is 1 ms. Longer conversion time can be used to achieve better signal-to-noise ratio, if needed. The active channels in Figure 8-6 will sample sequentially if multiple channels are enabled.

Figure 8-6 Configurable Scan Rate and Sampling Window

The LDC3114 is designed to work with LC resonator sensors with oscillation frequencies ranging from 1 MHz to 30 MHz. Equation 7 calculates the exact definition of the sampling window.

Equation 7.

where:

• t_SAMPLE is the sampling window in μs
• SENCYCn and LCDIV are the linear and exponential scalers that set the number of sensor oscillation cycles
• f_SENSORn is the sensor frequency in MHz

In Equation 7, LCDIV (0 to 7, default 3) is the exponential LC divider that sets the approximate ranges for all channels, and SENCYCn (0 to 31, default 4) is the linear sensor cycle scaler that fine-tunes each individual channel. Together they set the number of sensor oscillation cycles used to determine the sampling window.

For example, if the LC sensor frequency is 9.2 MHz, and it is desirable to get 1-ms sampling window, then this can be achieved by setting SENCYCn = 17 and LCDIV = 2.

Alternatively, from the sampling window and sensor frequency, the LCDIV can be read off from Figure 8-7Figure 8-7. For example, 1-ms sampling window and 9.2-MHz sensor frequency intersect in the region closest to where LCDIV = 2. Then SENCYCn can be calculated accordingly.

Figure 8-7 LCDIV as a Function of Sensor Frequency and Button Sampling Window