Hello,
Thank you for your question,
The threshold settings work in conjunction with the time varying gain(TVG) settings of the PGA460 device to accurately qualify the return echo over distance.
Below is a figure from the PGA460 datasheet displaying the echo signal in red and the threshold setting in blue. Here, you can see the threshold setting changing with respect to time. This allows the initial burst energy to be ignored, and the expected return echo to be qualified .
There are many methods to set threshold, but most require trial and error for best results. The PGA460-BOOSTXL EVM has a GUI that allows the user to easily see the receiver output plotted with TVG and threshold maps, see below:
Generally, most threshold maps look similar to this, A high initial threshold(white) is used to avoid triggering on the burst, then the threshold is lowered to somewhat follow the ring down of the transducer, and finally stabilizes around a stable value for accurate threshold detection. Note, I use the TVG(blue) to ensure the receive echo amplitude is high enough for detection yet also avoiding saturating the ADC. I ramp my TVG down around 4ms as I do not want to detect any TOF measurements longer than 4ms in this particular implementation. A similar result can be be obtained by raising the threshold settings after the maximum desired TOF time is reached.
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This document includes additional information on device settings, and recommended setup for threshold maps: PGA460 FAQ
Please let me know if you have any questions,
Regards,
Jacob
Hello,
The analog gain range exists to give adequate resolution to the gain settings without requiring excessive number of software bits for programming thresholds.
Generally, it would be unlikely to require both the maximum (90dB) and the minimum (32dB) setting in the same read, so TI separates the gain ranges into 4 groups for better resolution.
We recommend users select the gain range that suits their highest value gain setting. Below are the four selectable gain ranges for the AFE.
TVG is used to normalize the echo response across long distance and select where the echo will exist. Since the echo will possess less amplitude the further out the target moves, TVG can be used to make a 2 meter TOF the same amplitude as an 8 meter TOF.
As I showed in my example, the TVG amplifier can also be used to selectively amplify time ranges of interest.
Therefore, the higher the TVG setting, the more gain will be applied to the receive path. It is necessary to balance the TVG setting with the receive echo amplitude for accurate TOF measurements.
Here is an example of a well tuned PGA460 used for range measurements from 1m to 5m
Please let me know if you have any questions,
Thank you,
Jacob
