SLASEC8B February 2017 – January 2019 PGA460-Q1
A digital gain feature after the low-pass filtering is implemented to improve the SNR of the received echo without lowering the threshold values. Because this gain is applied after the band-pass and low-pass filtering, the digital gain does not amplify the out of band noise. This gain feature can help in suppressing false detection such as ground reflection and detecting farther objects with more accuracy.
Two sets of digital gain ranges are available: short range (SR) and long range (LR). The SR and LR gain levels are set using the Px_DIG_GAIN_SR and Px_DIG_GAIN_LR parameters, respectively, in the Px_GAIN_CTRL register independently for Preset1 and Preset2. The LR gain is applied starting from the threshold level point set by Px_DIG_GAIN_LR_ST parameter to the end of the record period. The SR gain is applied from time zero to the start of the selected LR-threshold level point.
To prevent false detection of an echo at the point in time where the digital gain is applied, the defined thresholds are also changed as shown in the example plot in Figure 9. Here, the LR gain is applied starting from the threshold level point 9. If the LR gain is different than the SR gain at threshold level point 8, the threshold level 8 is multiplied by the ratio between the LR gain and SR gain (DIG_GAIN_LR/DIG_GAIN_SR) 1 µs after the end of the SR threshold level 9 point. Although this creates a discontinuity in the threshold level, the object detection is not affected (a false threshold crossing is prevented) because the echo signal is also scaled by the same gain ratio. After this point, the threshold level is changed to the next set threshold level (point 9 in example below) using a linear interpolation scheme. The threshold levels should be adjusted by taking the digital gain and the ratio between the LR and SR gains into account.