NJM4151
-
-
Ver.2012-10-24
5. Temperature to Frequency converter
Temperature is a physical variable that is most commonly measured. Therefore, the transmitter (transducer) is used.
However, it each has advantages and disadvantages. Sensitivity, measuring range, linearity, reproducibility, cost,
depends on the transducer. Monolithic transistor pair can be used as temperature sensors. Figure 14 shows the circuit.
The advantage of used this circuit linearity and repeatability. AD converter is used so that VFC displayed digital to
temperature. Technique of temperature measurement based on a constant characteristic of the transistor. One of dual
transistor acts as a temperature-sensitive element. Another dual transistor supplies a constant current to
temperature-sensitive portions. High precision amplifier stabilized the voltage difference between the base-emitter’s
voltage of temperature-sensitive portions. Also, it is amplified at good linearity. It is used to zero to the voltage drop of a
fixed I
1
R
E
, when the output voltage of the amplifier is 0°C. This extra voltage canceled out the absolute temperature. It is
necessary to obtain Celsius temperature, when need to read of the absolute temperature can be ignored.
V
BE
=KT/q * 1n * Ic/Is’
Except Ic/Is >> 1
K= Boltzmann constant (1.38062 * 10
-23
Joule/°K)
T= Absolute temperature (°K)
q= Charge of Electron (1.60219 * 10
-19
coulomb)
Is= Reverse saturation current (≅ 1.87 * 10
-19
µA)
Ic= Collector current
When running in the range of collector current is 1µA to 1mA, monolithic transistor pair is close to this formula. Transistor
pair is high β. It is matched pair has a low offset voltage. In addition, IS low, and β is also consistent. ∆VBE integrity is
within 1mV at a typical value. In addition, the drift is ±3µV / °C, the difference in β is less than 10%.
The differential input to a precision amplifier,
V
+IN
-V
-IN
=KT/q * 1n * I
1
R
E
+KT/q * 1n * I
2
/I
S2
=KT/q (1n * I
2
/I
1
+ 1n * I
S1
/I
S2
) - I
1
R
E
When Transistor pairs are consistent, IS2 IS1 is the same and the second term 1n1 is zero.
The above equation,
V
+IN
-V
-IN
=KT/q * 1n * I
2
/I
S2
- I
1
R
E
KT/q is 86.171µV / °C. It is 198.4µV / °K when ratio of I1 versus I2 are 10 versus 1. It subtracted 273.15°C when the
absolute - temperature want back to Celsius. It is 54.193mV, when Constant current (I1) and RE multiplied. In that case,
the amplifier's input voltage is changed to Celsius. 0.1V / °C is convenient as the scale factor. It generates a voltage of 0
to 7V at 0 to 70 °C ranges. High-precision amplifier input common mode rejection takes 0.5 to 0.6. Then take 504 as an
amplifier. The amplifier input bias current should be low in relation to the emitter current of transistor temperature sensitive
pair. This temperature measurement method can be applied to other applications. Sensitivity can be adjusted easily by
changing the gain of the amplifier or changing the ratio of the collector current. Linearity of output voltage and
temperature are within ±0.01% linearity in the range of 0 to +70°C at circuit of Figure 14. Application will be at large by
introducing microprocessors. It can change the sensitivity of the transducer at the computer, and program can also be
automated.
