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TI Application Report SNAA267 mentions the transducer LMT70.

Other Parts Discussed in Thread: LMT70, LM94023, LM61, ADS1148, ADS1146, ADS1147, LMT90, LMT87

Kuglestadt writes, "The sensing element of the LMT70 consists of stacked BJT base emitter junctions that are biased by a current source. The output of the sensing element is buffered by a precision amplifier whose class AB push-pull output stage can easily source and sink currents of up to 3 mA.
The amplifier output connects to an output switch that is turned on and off by the digital control input T_ON."
What is NOT clear to me is whether that ADC circuit can resolve a temperature change of 0.01 deg C in the course of a SLOW change in temperature T(t). In my application, a pulse of heat warms soil. Then T(t) arrives a few seconds later at the thermometer and it decays away for 0.5 hr or so. Thus I need sampling of T(t) of about 1 per s AND I want to ship the voltage readings to a microcontroller, which might be a yard away or more.
My initial questions to Bogdan Nicola were numbered 1-2238579220.
Charles KenKnight

<chaskenk@yahoo.com>

  • Hi Charles,

    Getting to 0.01C will be a challenge as the LMT70 temperature sensor is rated to +/-0.05C typical accuracy at the temperature ranges 20C to 42C. The LMT70 does not have a built in ADC in the sensor itself, rather you will need to interface the output with an external standalone ADC or integrated ADC inside a microcontroller. Controlling the microcontroller to tell the ADC to sample every second should not be a big challenge.

    What is the application of heating soil?

    -Michael Wong
  • 28 July 2016
    Hello,
    Michael Wong stated, "Getting to 0.01C will be a challenge as the LMT70 temperature sensor is rated to +/-0.05 C typical accuracy at the temperature ranges 20C to 42C." He asked, "What is the application of heating soil?"
    Clarification of my design issues are linked by the above paragraph.
    In a new application, not yet in prototype form but fully simulated, a (nearly) point-source heater emits a pulse a few seconds long. The pulse diffuses into the surrounding soil medium. A thermometer at a few cm distance (the LMT70 attached to a thin metal disk of cm size) warms according to a function T(t) and reaches a MaxT ~ 1 C at t = tens of seconds after the pulse begins. The T(t) values decline after MaxT VERY slowly. The LAST t for measurements may be at about half an hour. From a measure of the height of T(t) the experimenter can derive an estimate of the thermal conductivity k of the soil. The thermal conductivity is known to increase monotonically (crudely linearly) with the soil water content, so an estimate of k can be used to control when irrigation should be turned ON or OFF.
    That scenario reveals that the "typical accuracy at the temperature ranges 20C to 42C" is NOT relevant. Rather, what matters is one's ability to follow the values of T(t) in its rise to ~ 1 C and its later fall to the starting temperature of the medium. That ability requires that the ADC have enough meaningful bits to resolve a change in the thermometer reading to ~ 0.01 C.
    In a PTAT circuit that voltage change is about 10 mV times the ratio of 2 resistor values, which in SNAA267, pg 6, is called R2/R1.
    Can the ratio R2/R1 be much more than 1 - or do noise reasons prevent it?
    Thus the requirement on the ADC following the LMT70 is R2/R1 * 10 mV > the least meaningful step size (in bits) of the ADC.
    The fact that the voltage change made by the LMT70 is so small may require choosing a thermistor as the T-transducer. The non-linearity of the thermistor is NOT a challenge when MaxT is ~ 1 C.
    Charles KenKnight
    <chaskenk@yahoo.com>
  • 29 July 2016
    Hello,

    Following "EPCOS. 2011. SMD NTC Thermistors." [Google "thermistor aging"]
    The temperature coefficient [Section 3.1.3] of the resistance is defined
    as the relative change in resistance referred to the change in temperature.
    Alpha = 1/R * (-dR/dT) (5)
    = B/T^2
    Then -dR/R = Alpha * dT. Since B is a temperature ~ 4000 K, Alpha ~ 4000 /
    300^2 ~ 1/22 /deg C or /K.
    For a constant current of 1E-4 A through a thermistor with R25 = 1E4 Ohms,
    i.e., with 1 V across the thermistor, the output changes by dV if its
    temperature changes by 1 deg C, i.e., by about 1/22 V, i.e., the sensitivity
    is about 44 mV/deg C.
    The R25 value of the thermistor does not matter if the designed driving
    current is shifted so that a designed 1 V is maintained across the
    thermistor and is the nominal voltage presented to the ADC circuit.
    By comparison, the IC LMT70 mentioned by Kuglestadt (2015), using n = 3
    [so Ln(n) = 1.099] and 2*R1/R2 ~ 6, then across R1 the change dV ~
    1.099*6*0.08614 mV/deg C = 0.568 mV/deg C.
    Thus the thermistor signal can be ~ 78x larger than the LMT70. Given the
    fact that the signal from a thermistor can be arranged to be a voltage drop
    of about 44 mV from the nominal thermistor output of 1 V, the requirement to
    "resolve" 1% of 44 mV becomes 1/(2^n) < 0.44 mV / 1 V ~ 4.4E-4. Then n >
    11.1 answers how many bits must be "meaningful" in the ADC that interrogates
    the thermistor. Since a 16-bit ADC is both common and cheap, a 16-bit ADC
    watching a 10KOhm thermistor is a secure answer for this problem.

    Charles KenKnight, PhD
    <chaskenk@yahoo.com>

  • The LMT70 average output slope is roughly -5.18mV/C. The R1/R2 that you speak of as shown in the Kuglestadt app note are not available externally to the user. The output slope of the LMT70 is fixed. In the LMT70 datasheet there is a simple second order equation that can be used, that is if linear interpolation of the lookup table values provided in the LMT70 data sheet are not preferred. This equation does not contain any logarithmic terms whatsoever (such as in the Stienhart/Hart equations) thus much easier to implement in a microcontroller. Since accuracy is not an issue in your application and you are only looking for resolution I would suggest another device the LM94023 as it comes in the same package as the LMT70 and would provide a similar response time but has an -8.2mV/C output slope. If you are willing to go to a different package check out the LM61 it has an output slope of 15.6mV/C. You are correct as the thermistor still has superior sensitivity but for a limited temperature range. Over a temperature range that is greater than 70-80 C the analog sensors become a superior solution.

    Since you are trying to measure moisture in the ground take care to make sure that all of the thermistor contacts are moisture and dirt proof - as leakage currents in the thermistor solution can cause wrong temperature values. The analog temperature sensors on the other hand have low output impedance thus they are not sensitive to connector leakage without sacrificing self heating (as would be the case with the thermistor). Also take care that you do not overheat the thermistor, so I caution you to check the power dissipation at your operating temperature.

    Take care,
  • To: Temperature Sensors forum
    Date: 23 Aug 2016
    Subject: TI Application Report SNAA267 mentions the transducer LMT70
    I review the concept of what is needed to make a soil moisture sensor using a heat pulse. A ring around a hollow insulating cylinder (about 6-mm radius) is heated by a few Watts for a few seconds - say 40 W-s. The disk glued to the end of the cylinder has either a thermistor or a small-footprint IC glued at its end. Thus soil does NOT enter the hollow cylinder. The glued-on disk at a distance r~15 mm along the cylinder responds to the soil temperature rise T(t) caused by the heating pulse. The rise in T(t) reaches MaxT (about 1 deg C) after about 10 s. After that T(t) is a SLOW (power-law in t) decay back to the before-heating temperature T0 over half an hour or more. To start a new measurement before an hour or so, a special analysis of the data will be required because a given T(t) is superimposed on all the earlier ones.
    Denton warned about the long-term temperature rise in a self-heated thermistor transducer. It is VERY small in this environment where moist soil contacts the entire disk-shaped seal at the end of the cylinder. In a possible design for a thermistor transducer (see Bishop 2000 SLOA052), a constant current of about 100 uA enters a thermistor of, say, 10 kOhm resistance (at 25 deg C) and develops a steady heating power q of 1E-4 W, which causes a steady temperature rise in the highly conductive disk-seal of q/(4*pi*k*r). In mks units that rise ~ 1E-4 W/(12 * 3 W/m-K * 0.005 m) = 5E-4 deg C. Any changes in that rise in thermistor temperature is necessarily slower than 100 s, i.e., changing more slowly than 5E-6 deg C/s. The signal T(t) that is to be measured rises about 1E-1 deg C/s, i.e., faster by 200,000x than the rate fixed by conduction into soil by the self-heating thermistor transducer that would be glued to the disk-shaped seal.
    The SLOW T(t) decay affords thousands of samples of T(t) from which one may establish the magnitude (Scale) of T(t) with small uncertainty (S.E./Average), particularly if the Gaussian measurement noise in the T(t) samples is small. In a stream of digital numbers the accuracy in the Scale of T(t) improves if the RESOLUTION of the T(t) samples decreases in magnitude. The temperature rise T(t) relative to T0 is described by Scale * Z(t), where Z(t) is the complimentary error function erfc(r/sqrt(4*w*t)) and w=k/C is the diffusivity of the (moist) soil, k is its thermal conductivity, and C is its heat capacity per unit volume. The theory for these statements is in Carslaw & Jaeger (1959, p. 261), a standard reference for heating transients in solids.
    A few messages from the TI E2E Forum on temperature sensors have confused the words "accuracy" and "resolution." I hope that the above specifics will clarify the (rather peculiar) system characteristics involved.
    I seek the answer to 2 simple questions.
    1) Can a Voltage Output IC sensor followed by a suitable analog front end, then a micro-controller, achieve a RESOLUTION of 0.01 deg C at a sampling rate on the order of 1 Hz in this constrained change-of-temperature environment? Those two values could be relaxed somewhat, if needed.
    2) Is there a TI circuit for an IC transducer that, in your opinion, could achieve this goal - or be fine-tuned to achieve it?
    If (1) is "Yes", how do I go about learning about (2)?
    If (1) is "No", then I am sure that a thermistor transducer circuit can be (probably has been) devised that will work.
    However, a thermistor has the unpleasant feature of non-linearity. Even though MaxT will be less than 1 deg C so that the Scale of the T(t) function can be derived without doing a full calculation of T(t) for each voltage sample, the value for Scale must be corrected after each "measurement" for the current operating point of the sensor, namely T0. That can be done with a simple calculation, but it requires that T0 be obtained with modest ACCURACY, say, +/-0.5 deg C.

    Charles KenKnight, PhD
    <chaskenk@yahoo.com>

  • To: Temperature Sensors forum
    Date: 30 Aug 2016
    Subject: TI Application Report SNAA267 mentions the transducer LMT70
    My posting here on 23 Aug stated the peculiar requirements of a temperature sensor desired for control of irrigation. The transducer is located on the air-only side of a metallic disk in moist soil. The disk seals the end of a hollow insulating cylinder from which a ring-shaped heater sends out a few-second heat pulse from its location a few cm from the cylinder end.
    What is odd about this application is that the temperature rise T(t) at the transducer arrives after 10+ s, attains a MaxT of only about 1 C [a condition that keeps moisture from moving away from the heater], and lingers for nearly an hour because the decay of T(t) is nearly a power law. Thus sampling T(t) faster than 1 Hz is a waste, whereas most electronic temperature sensors today respond to a T(t) much faster than that. For the best estimate of how a moist soil conducts heat, the measurements of T(t) that are crucial are those between 10 and 2000 s.
    A small complication is that the ambient temperature of the measurement, T0, drifts on a time scale of half a day by a few C to 20 C over months.
    Suppose that the transducer is the CMOS IC LMT70 named in the title for this forum. Two conditions need to be met: 1) For one T(t) pulse the resolution of the ADC that follows must resolve about 0.01 C meaningfully; 2) The system should accept to operate according to condition (1) without operator interference even though T(0) changes (slowly) by about 20 C.
    Given the slope for the LMT70, dV/dT = -5.19 mV/C, condition (2) requires that the system (notably the ADC) operates normally if the output voltage from the LMT70 shifts by 100 mV. Yet in one "measurement," condition (1) requires that a temperature shift of 0.01 C, i.e., for voltage shifts of only 0.01*5120 uV = 50 uV, the ADC must output a distinct code.
    The ADS1146, ADS1147, and ADS1148 devices are precision 16-bit analog-to-digital converters (ADCs) that include many integrated features to decrease
    system cost and component count for sensor measurement applications. The devices feature a low-noise, programmable gain amplifier, a precision delta-sigma ADC with a digital filter that settles in a single cycle, and an internal oscillator. The ADS1146 device supports one differential input, e.g., a LMT70 transducer, while the ADS1147 and ADS1148 devices support 2 and 4 differential inputs - something not needed for the soil moisture sensor.
    According to Table 1, pg 15 of the ADS1146 description (SBAS453G, the 2016 revision), at 5 samples per sec and the low gain settings of 1 (2), the equivalent input noise is 62 uV (31 uV), which is encouragingly close to the above condition (1) of 50 uV that is set by a desired resolution of 0.01 C. According to LMT70 data presented by Michael Wong in his description (tidubt8, pg 3) of its use in a skin temperature application, its accuracy (i.e., deviation from the straight line with slope -5.19 mV/C) is 0.13 C between 20 and 42 C, which is encouraging for condition (2).
    A reader more expert than I am is needed to discuss any special modifications that would be needed to use the ADS1146 with a LMT90 transducer to achieve a system that meets the conditions set forth here.
    Charles KenKnight, PhD
    <chaskenk@yahoo.com>

  • To: Temperature Sensors forum
    Date: 31 Aug 2016
    Subject: TI Application Report SNAA267 mentions the transducer LMT70
    My posting on 30 Aug stated the peculiar requirements of a temperature sensor desired for control of irrigation. It used data on the transducer LMT70 to show a likely match to 2 desired conditions if it is coupled to a ADS1146 device, which has a programmable gain before an ADC. That amplifier would probably be used at 1 V/V, so it would be used simply to attain a lower output impedance, i.e., larger output current sent to the ADC.
    This posting notes that the LMT87 transducer certainly satisfies the desired conditions because the slope of the response rises from -5.19 mV/C to -13.6 mV/C. In addition its built-in amplifiers used in push-pull enable an output current of 7 mA without extra parts. Further, its two possible housings have effectively 3 leads. One housing (the TO-92) has 3 leads mounted in parallel from one side. That arrangement permits the LMT87 to be glued on the metallic disk that is needed across the end of the hollow cylinder. Then the ADC and the needed power supplies can be near the heater ring around the cylinder, where those heat sources will not affect the LMT87.
    The data document for the LMT87 (at www.ti.com/lmt87-ca) shows it would be used with a generic ADC in Fig. 12 on pg 12. Details of the circuit controlling the ADC samples are omitted.
    Fig. 12 shows a low-pass filter before the sampling switch. A Table on pg. 12 suggests values of a series resistor that would be needed if the capacitor value becomes large, e.g., 800 Ohms with a 1 uF capacitor. That RC time constant is 800 us, which cuts off noise below 1250 Hz. My previous post pointed out that sampling faster than 2 Hz is not needed for the application. Thus larger R and C values are needed, e.g., larger by 25x each to reach sampling at 2 Hz.
    A reader more expert than I am is needed to suggest a specific ADS system.
    Charles KenKnight, PhD
    <chaskenk@yahoo.com>

  • Hi Charles,

    I recommend posting your ADS question on the precision data converters forum: e2e.ti.com/.../73

    -Michael Wong