Hi

I have implemented source code for a MCU to driver the AD7794/7795 and able to configure and measure various voltage and H bridge.

Now I looking into RTD measurement which involve current source. I have scanned the AD website and to my surprise, they say only about feature of RTD measurement but say nothing on how measure RTD with two different current source nor equation.

Can anyone direct me to relevent material or book detailing how to configure AD7794/5 in step and measure RTD and provide equation as necessary how to calculate the RTD. I'm not interested in feature, I'm more interested in practical implementation.

AD7793 diagram shown two IOUT connection where AD7794/95 diagram shown only one IOUT connection (which I have implemented), why they differ?,

Can anyone demonstrate "how" to implement them and what step based on one current source?

R.

Hi,

the application circuit shown in the AD7793 datasheet shows the connections for a 3-wire RTD. The AD7794 datasheet includes an application circuit for flowmeters. The diagram also shows how a thermistor can be connected for temperature compensation. The connections for a 2-wire RTD would be similar.

For 2-wire and 4-wire RTDs, a single excitation current is used. The lead resistance will add an error in a 2-wire RTD. With a 4-wire RTD, the error is minimised. With a 3-wire RTD, again the lead resistance adds an error. However, in this case, the error can be compensated for using either a second excitation current oy by measuring the lead resistance.

You mention that you are using a 4-wire RTD. In this case, a single excitation current can be used. The value of the excitation current to use depends on the max resistance of the RTD (what is the max temperature that you want to measure). Knowing this, you can then select a precision resistor which will be used as the reference resistor in the circuit. The value of this resistor is selected so that (Iexc x max RTD resistance) x gain = Iexc x Rref where Rref is the value of the precision resistor and gain is the gain to which the AD7793 PGA is set.

To measure the temperature, the AD7793 measures the voltage generated across the RTD. You can then calculate the resistance of the RTD. The datasheet for the RTD shows the relationship between the resistance and the temperature. RTDs are non linear. So, for best results, a linearisation routine should be included in your post-processing.

Regards,

Mary.