(Eq. 4)

where

After that, temperature is converted into 4–20mA loop current using equation Eq. 5:

DACcode = 4mAcode + (20mAcode - 4mAcode)/(Tmax - Tmin) × (T - Tmin)

(Eq. 5)

Thus, loop current is proportional to the temperature (T).

The MAX11213 ADC allows auto compensation of the sensor wire resistance by measuring that resistance during the calibration

procedure and continuously subtracting that value from each conversion result. The wire resistance compensation technique is a one-step

process called system offset calibration.

In normal operation the ADC measures the voltage drop between points 2 and 3 of connector J1, see Figure 2. For the offset calibration,

the MAX4729 switches into the NO (normally opened) position and the ADC measures the voltage between points 1 and 2 of connector

J1. In other words, the ADC measures wire resistance and automatically stores that value into the System Offset Calibration register.

Figure 2. Sensor's wire measurement using the MAX4729 mux.

The central control unit (CCU) or programmable logic controller (PLC) can convert the loop current back to the temperature using Eq. 6:

(Eq. 6)

where

Tmax is the maximum measured temperature in °C

Tmin is the minimum measured temperature in °C

The Novato (MAXREFDES16#) smart sensor transmitter board is loaded with the firmware that supports any RTD, from PT100 to PT1000.

Selection of the right sensor can be done during calibration.

Novato Calibration Procedure and Software

Calibration of the sensor is easy with the supporting Novato calibration software and USB-to-UART programming adapter, Figure 3.

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