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TMP007
SBOS685C – APRIL 2014 – REVISED JULY 2015
Feature Description (continued)
7.3.3 Thermopile Principles and Operation
The TMP007 senses radiation by absorbing the radiation on a hot junction. The thermopile then generates a
voltage proportional to the temperature difference between the hot junction, Thot, and the cold junction, Tcold.
Thermopile
EHOT ß TDIE 4
EOBJ ß TOBJ 4
+
Thot
Thot
Thot
Thot
Tcold
Heat Absorbor
Cold Junction
Figure 21. Principle of Thermopile Operation
The cold junction is thermally grounded to the die, and is effectively TDIE, the die temperature. In thermal
equilibrium, the hot junction is determined by the object temperature, TOBJ. The energy emitted by the object,
EOBJ, minus the energy radiated by the die, EDIE, determines the temperature of the hot junction. The output
voltage, VOUT, is therefore determined by the relationship shown in Equation 2:
VOUT = VSENSOR = C ´ (THOT - TCOLD ) µ (TOBJ 4 - TDIE 4)
where
• C is a constant depending on the design of the sensing element.
(2)
Note that the sensor voltage is related to both the object temperature and the die temperature. A fundamental
characteristic of all thermopiles is that they measure temperature differentials, not absolute temperatures. The
TMP007 contains a highly-accurate, internal temperature sensor to measure TDIE. Knowing TDIE and VSENSOR
enables the TMP007 to estimate TOBJ. For each 250-ms conversion cycle, the TMP007 measures a value for
VSENSOR and for TDIE, calculates TOBJ, and then places the values in the respective registers.
Bits CR2 to CR0 determine the number of local and sensor analog-to-digital converter (ADC) results to average
before the object temperature is calculated.
After power-on reset (POR), the TMP007 starts in four conversions per second (mode 010). In general, for a
mode with N conversions, the local temperature, TDIE, result is updated at the end of the Nth ADC conversion
with the value shown in Equation 3:
TDIE
=
1
N
å
N X=1
Local
Temp
ConversionX
(3)
Similarly, the sensor voltage result is updated at the end of the Nth sensor ADC conversion with the value shown
in Equation 4:
å 1 N
VSENSOR
=
Sensor
N X=1
ConversionX
(4)
These results are then used in the object temperature calculation by the math engine, which updates the object
temperature result register. The total conversion time and averages per conversion can be optimized to select
the best combination of update rate versus noise for an application. Additionally, low-power conversion mode is
available. In CR settings 101, 110, and 111, the device inserts a standby time before the beginning of the next
conversion or conversions.
The method and requirements for estimating TOBJ are described in the next section.
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