Hi all!!!
I want to use ADS1247 for temperature sensing with PT100.
Either 3 wire or 4 wire circuit is fine, but please specify what is the advantage or disadvantage using any of them...?
In the document slyp160(http://www.ti.com/lit/ml/slyp160/slyp160.pdf) page18, the PT100 is used with the bridge circuit, Is it really required to use a bridge circuit with PT100..?
Thanks and regards
-Anand
Hello Anand,
The ADS1247 is perfectly suited for use with PT100 RTDs. Three-wire or four-wire connections are simply ways of improving the accuracy of a resistance measurement by cancelling the resistance of the wires used to make the measurement. This is an issue because resistance measurements are performed by using wires to force a current through the resistance and measuring the resulting voltage drop. However, the wires themselves also have a resistance (called lead resistance), so the voltage drop that develops across the wires themselves can negatively affect the accuracy of the measurement.
Three-wire and four-wire connections correct for this by measuring the voltage drop with additional wires (called sense wires) through which no current flows, so no additional voltage drop occurs. The "Operating Principle" section in the following article does a great job of explaining this concept: Four-wire Sensing. In general, a three-wire RTD connection is sufficient since the voltage drop from the "positive" force wire is the same as the voltage drop from the "negative" force wire or common wire.
A bridge circuit was used in the document you linked because the instrumentation amplifier used (INA114) did not have built-in current sources available. The ADS1247 has two built-in, well-matched excitation current sources which can be digitally programmed to certain values (50μA, 100μA, 250μA, etc.) so the additional bridge circuit is not required. If you follow the example of Figure 85 on the ADS1247 data sheet (shown below) you can get excellent performance with a three-wire PT-100 RTD.
Best regards,
Ian Williams
Linear Applications Engineer
Precision Linear and Sensing Products
Hi Anand,
In addition to the methods that Ian and Bruce suggest, here are some easy ways to create current sources to excite your RTD. Voltage excitation results in an inherently non-linear response because the excitation current will change through the RTD over the span or the RTD temperature measurement as the RTD resistance changes. The circuits shown in the figure below can be used to create 100uA current sources and current sinks. Please let us know if you're interested in one of these circuits and we can help tailor it to your desired current output. If you choose to use one of the circuits in Bruce Trump's nice article then the non-linear behaviour of the voltage excitation is not an issue and cancelled with the circuit.
Regards,
Collin Wells
Precision Linear Applications
Hi Anand,
The RTD nonlinearity will account for around 0.7% error over your temperature span in addition to the nonlinearity due to the voltage excitation.
4-wire measurements are not practical when using a voltage excitation source unless you're willing to measure the current flowing through the RTD which I'm going to guess you're not.
See below for examples of 3-wire measurements with current and voltage excitation. The "Rl" resistors represent the resistance due to the wiring and/or connections of the RTD to your circuit. The 3-wire measurement with current sources enables a complete cancellation of the Rl resistors. The 3-wire voltage measurement is better than a 2-wire circuit but does not completely cancel the lead resistors.
If you're curious as to how to set the resistors I suggest you download a copy of TINA-TI ( http://www.ti.com/tool/tina-ti ) and start having fun measuring the currents and voltages!
Regards,
Collin Wells
Precision Linear Applications
