Datasheet TMP03, TMP04 (Analog Devices) - 5

TMP03/TMP04 Table I. Counter Size and Clock Frequency Effects on Quantization Error Maximum Maximum Maximum Quantization Quantization Count Available Temp Required Frequency Error (25 C) Error (77 F)
4096 125°C 94 kHz 0.284°C 0.512°F 8192 125°C 188 kHz 0.142°C 0.256°F 16384 125°C 376 kHz 0.071°C 0.128°F
Optimizing Counter Characteristics
with no load. In the TO-92 package mounted in free air, this Counter resolution, clock rate, and the resultant temperature accounts for a temperature increase due to self-heating of decode error that occurs using a counter scheme may be deter- ∆T = PDISS × θJA = 4.5 mW × 162°C/W = 0.73°C (1.3°F) mined from the following calculations: For a free-standing surface-mount TSSOP package, the tem- 1. T1 is nominally 10 ms, and compared to T2 is relatively perature increase due to self-heating would be insensitive to temperature changes. A useful worst-case assumption is that T1 will never exceed 12 ms over the ∆T = PDISS × θJA = 4.5 mW × 240°C/W = 1.08°C (1.9°F) specified temperature range. In addition, power is dissipated by the digital output which is T1 max = 12 ms capable of sinking 800 µA continuous (TMP04). Under full load, the output may dissipate Substituting this value for T1 in the formula, temperature (°C) = 235 – ([T1/T2] × 400), yields a maximum value of   T2 of 44 ms at 125°C. Rearranging the formula allows the P = 0.6 V ( ) 0.8 mA ( ) T2 DISS  T1+ T 2 maximum value of T2 to be calculated at any maximum operating temperature: For example, with T2 = 20 ms and T1 = 10 ms, the power T2 (Temp) = (T1max × 400)/(235 – Temp) in seconds dissipation due to the digital output is approximately 0.32 mW with a 0.8 mA load. In a free-standing TSSOP package, this 2. We now need to calculate the maximum clock frequency we accounts for a temperature increase due to output self-heating can apply to the gated counter so it will not overflow during of T2 time measurement. The maximum frequency is calculated using: ∆T = PDISS × ΘJA = 0.32 mW × 240°C/W = 0.08°C (0.14°F) Frequency (max) = Counter Size/ (T2 at maximum This temperature increase adds directly to that from the quies- temperature) cent dissipation and affects the accuracy of the TMP03 relative to the true ambient temperature. Alternatively, when the same Substituting in the equation using a 12-bit counter gives, package has been bonded to a large plate or other thermal mass Fmax = 4096/44 ms ⯝ 94 kHz. (effectively a large heatsink) to measure its temperature, the 3. Now we can calculate the temperature resolution, or quanti- total self-heating error would be reduced to approximately zation error, provided by the counter at the chosen clock ∆T = PDISS × ΘJC = (4.5 mW + 0.32 mW) × 43°C/W = 0.21°C (0.37°F) frequency and temperature of interest. Again, using a 12-bit counter being clocked at 90 kHz (to allow for ~5% tempera-
Calibration
ture over-range), the temperature resolution at 25°C is The TMP03 and TMP04 are laser-trimmed for accuracy and calculated from: linearity during manufacture and, in most cases, no further adjustments are required. However, some improvement in per- Quantization Error (°C) = 400 × ([Count1/Count2] – formance can be gained by additional system calibration. To [Count1 – 1]/[Count2 + 1]) perform a single-point calibration at room temperature, measure Quantization Error (°F) = 720 × ([Count1/Count2] – the TMP03 output, record the actual measurement tempera- [Count1 – 1]/[Count2 + 1]) ture, and modify the offset constant (normally 235; see the Output Encoding section) as follows: where, Count1 = T1max × Frequency, and Count2 = T2 (Temp) × Frequency. At 25°C this gives a resolution of Offset Constant = 235 + (TOBSERVED – TTMP03OUTPUT) better than 0.3°C. Note that the temperature resolution A more complicated 2-point calibration is also possible. This calculated from these equations improves as temperature involves measuring the TMP03 output at two temperatures, increases. Higher temperature resolution will be obtained by Temp1 and Temp2, and modifying the slope constant (normally employing larger counters as shown in Table I. The internal 400) as follows: quantization error of the TMP03 sets a theoretical minimum resolution of approximately 0.1°C at 25°C. Slope Constant = Temp 2 − Temp1
Self-Heating Effects
 T1 @ Temp1   The temperature measurement accuracy of the TMP03 may be  T 2 @ Temp1 − T1 @ Temp2  T 2 @ Temp2 degraded in some applications due to self-heating. Errors intro- duced are from the quiescent dissipation, and power dissipated where T1 and T2 are the output high and output low times, by the digital output. The magnitude of these temperature er- respectively. rors is dependent on the thermal conductivity of the TMP03 package, the mounting technique, and effects of airflow. Static dissipation in the TMP03 is typically 4.5 mW operating at 5 V REV. B –5– Document Outline RevHistory_TMP03_04.pdf REVISION HISTORY