Hello,

I want to use an AD7124 ADC with a PRTD100 1/3 DIN 4 wires sensor. This sensor has an accuracy of 0,1°C and I want to keep this accuracy at the end of my measuring chain. I have some questions:

It looks like the accuracy of the excitation current sources is 4%. Is that right ?

I also want to know if it is possible to get the voltage of the Rref resistor. Maybe I can link two analog inputs at its end to retrieve this value on the output.

Finally, I don't really know how to choose my resistor reference value. If the current flowing through my sensor is 0.250mA, the voltage accross the sensor will move from 25mV to 34.5mV between 0-100°C with a pitch of 0.032mV for 0.1°C.

Thank you for your help !

Aaron

Hi, Aaron.

AD7124 is very well suited for high accuracy RTD measurements since all blocks needed in this type of applications are integrated which reduces the cost and complexity of the systems.

Below is an example computation on how can you achieve a very high resolution of less than 0.1degC using AD7124.

Example: pt100, 4-Wire, -200 degC to +650 degC

Max Resistance = 313.72 ohms (650 degC)

Iexc = 250uA

Voltage across RTD = 250uA x 313.72 = 78.43 mV

PGA=32 => 32 x 65.89mV = 2.50976V

Rref = 2.50976 / 250 uA = 10.04 K

Vref = 10.2K x 250uA = 2.55 V

RMS Noise = 260 nV at 25 sps (1600nV p-p) – Low Power (Post Filter)

P-P Resolution: log2(78.43mV/(1600nV) = 15.9 bits

Resolution = 850 degC /2^15.9 = 0.01degC

Yes, you are correct about the initial accuracy of the excitation sources, however errors in excitation currents can be easily removed in ratio metric configurations. In ratio metric, the RTD measurements is referred to a voltage across a precision reference resistor that is driven with the same current source. We also have an available reference circuit for 4 wire RTD measurements using AD7124 in ratio metric configurations. Please see link below. You can use this as your reference or as it is and you only need to change the component values based on your target design and specifications.

http://www.analog.com/media/en/reference-design-documentation/reference-designs/CN0381.pdf

Thanks,

Jellenie