infinityis 16 Posted February 25, 2012 Author Share Posted February 25, 2012 Actually, thermocouples generate a small voltage due to the dissimilar metals. So instead of a resistor divider, you just need measure it directly. Well, almost directly...as shown in the TI link I posted above, you want to put resistors in series with the lines and couple them with capacitors. The reason you'd need/want to do this is because even though the thermocouple generates a voltage, it cannot source much current. The ADC draws a small amount of current to measure the voltage, but it is more current than the thermocouple can source. Steady state, the capacitor voltage will settle at the thermocouple voltage. With capacitors near the ADC, the current used to sample comes out of the capacitors which are resting at the thermocouple voltage. The resistors between the capacitors and the thermocouple serve to limit (actually almost completely eliminates) the surge of ADC sampling current which comes from the thermocouple. And thus we have a basic low-pass RC filter. Other temperature sensing elements (such as an RTD, resistive temperature detector) do have a resistance that varies with temperature, so you can use a resistor divider circuit to simply measure the voltage. There are two things to be aware of for high precision temperature measurements with RTDs, however. First, you will want to use two wires to apply the voltage across the resistance and two separate wires connected very close to the RTD to measure the voltage (such that the sensing lines don't have to carry much/any current and thus you eliminate resistance contributions in the measurement lines). Second, the act of measuring continuously (forcing continuous DC current through the RTD) actually causes the RTD to heat up slightly, affecting the measurement; this can be mitigated through some clever low-current AC coupling as shown here. GeekDoc 1 Quote Link to post Share on other sites
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