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PGA460-Q1: Temperature Sensing

Frank Aloe
Prodigy 20 points
Part Number: PGA460-Q1
Other Parts Discussed in Thread: PGA460

I am using the burst and listen function of the PGA460 and capturing a profile of upto 5meters. 

The question I have is does the PGA460 automatically compensate for speed of sound over temperature.  The data sheet seems to elude to this but I am uncertain. 

I am doing a outdoor sensor that will see temperatures range from -20C to +45C and need to continuously measure distance.

  • 0
    TI__Mastermind 27950 points

    Hi Frank,

    The PGA460 reports the time-of-flight for a measurement as a 16-bit microsecond value, not as distance. Compensating for the temperature changes must be performed at the master controller when converting time-of-flight to distance as shown in the datasheet's table 3 foot note 2:

    (2) To convert the object's time-of-flight in microseconds to distance in meters: distance (m) = [vsound × (MSB<<8 + LSB) ÷ 2 × 1μs] . For improved burst-and-listen accuracy, add the additional burst offset to the originally calculated distance: distanceburst_offset (m) = [vsound × (Pulses / Frequency) ÷ 2 ]. The speed of sound is typically assumed to be 343m/s at room temperature. Adjust the speed of sound as a function of ambient temperature: vsound= 331m/s + (0.6m/s/°C × Temperature(°C)).

    The PGA460 includes a built-in temperature sensor, which the master controller can also read the value of. Apporximating the ambient temperature is explained in equation 8 of the datasheet:

    Without calibrating TEMP_GAIN and TEMP_OFF, the ambient temperature can be approximated from the die temperature reading using Equation 8
    TAmbient (°C) = TDie - [RθJA × (VPWR × IVPWR_RX_ONLY)]
    where:
    • RθJA(°C/W) is the Junction-to-ambient thermal resistance of 96.1°C/W.
    • VPWR (V) is the input voltage.
    • IVPWR_RX_ONLY(mA) is the supply current from VPWR pin during listen only mode of 12mA.

  • 0
    Prodigy 20 points
    Just to be sure I understand correctly.
    When I do a echo data dump I would apply the equations listed below in table 3 noet 2 to array values and this would compensate for the temperature?

    Frank


  • 0 in reply to Frank Aloe
    TI__Mastermind 27950 points

    Frank,

    Table 3 Note 2's equation refers to the ultrasonic measurement result command response, but the general conversion from round trip time of flight to one way distance is applicable for the echo data dump and any other ultrasonic system.

    The echo data dump is a 128 byte representation of the record length, which can be set between 4 - 65ms (set in the preset 1 or 2 record length register). Since the echo data dump is always 128 bytes, the delta time between each step increases as the record length increases. So a 4ms record length will offer the highest resolution, while a 65ms will offer the lowest resolution. When converting the echo data dump's record length to distance, consider the case of 4ms for instance. 4ms divided by 128 yields 31.25us step resolution. Assuming room temp, 4ms is 68.6cm. Similarly, 68.6cm divided by 128 yields 0.53cm step resolution on the echo data dump plot. If temperature increaes, such as 45C, then 4ms is 71.6cm, meaning your max range increases, but the step resolution decreases. This is how temperature factors into and affects the echo data dump result.