Hello! I want to interface PT500/1000 Sensor with 24-bit AD7714. Please tell me how to do the circuit designing to have a stable accuracy of 0.0001 degree Celsius. Is this accuracy achievable with AD7714? What current source i should prefer? What should be my OP-AMP Gain? Any other external components required? Reply as soon as possible.

Hi, deep1990.

Apologies for not looking this soon enough. We have overlooked your query and it is only just lately that we found out this has been unattended.

An RTD will not achieve an accuracy of 0.0001 degC. An RTD can achieve an accuracy of approximately 0.01 degC. So, an RTD may not be an option for the application. Below is some information on selecting gain, external components, etc which you may find useful.

Firstly, the AD7793 or AD7794 are 24-bit ADCs which include the current sources. An external current source is not required if one of these ADCs is chosen. The AD7793 datasheet shows a typical circuit for a 3-wire RTD.

The gain to use depends on whether the RTD is 2, 3 or 4 wire. It also depends on the value of the excitation current. In all cases, an external precision resistor should be used to generate the reference. Other considerations

1) What is the temp range being measured by the RTD. From this, the min/max resistance values of the RTD can be calculated.

If the RTD is a 4-wire PT100 and we select the 420 uA current source from the AD7793/4 (the two 210 uA currents are directed to the same pin), then the RTD generates a voltage of 0.042 V at zero degC. Let assume the max temperature being measured is 600 degC. At 600 degC, the RTD has a resistance of 313.71 ohms (from RTD datasheet). Therefore, the max voltage being generated by the RTD is 313.6 x 420 uA = 0.1318 V.

If the PGA gain is set to 16, then the voltage from the RTD is gained by 16 within the ADC to generate a max voltage of 0.1318 x 16 = 2.108 V. The value for the reference resistor is chosen so that a signal of 2.108 V can be accommodated by the ADC. If a 5.11 K resistor is used to generate the reference voltage, the reference voltage is 5.11 K x 420 uA = 2.4162 V which is acceptable.

The AD7793/4 excitation currents have an output compliance of 0.65V when the 210 uA currents are used. This means that the voltage at the IOUT pin cannot exceed AVDD - 0.65 V. With the above components, the max voltage at the IOUT pin is 2.4162+ 0.1318 = 2.278 V which is acceptable if the power supply is 3 V or higher.

The ADC noise is dependent on the output data rate along with the gain. At a gain of 16 and an output data rate of 16.7 Hz, the ADC’s rms noise is 180 nV rms. The p-p value is 180 x 6.6 = 1188 nV = 1.188 uV.

For an ideal pt100, a one degC change in temperature causes a change in resistance of 0.385 ohms. With a 420 uA current, the voltage per degC is 0.385 x 420 uA = 161.7 uV. So, the theoretical accuracy is 161.7uV/1.188 uV = 136. Therefore, the accuracy if degC is 1/134 = 0.007 degC. This is assuming that the RTD is linear.

In practice, an RTD is not fully linear and it will also add some noise. A linearization routine needs to be implemented in the uC. There are several sites on the internet that discuss this. With linearization, the best performance that can be achieved from an RTD is approximately 0.01degC.

Regards,

Johnny