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Sensors Mag

Designing Intelligent 4–20 mA Transducers

August 1, 2006 By: Brendan Cronin Sensors

Intelligent transmitters are only as good as their components. Here's how ADCs, and other low-power devices, come into play.


The resistance of an RTD varies with temperature. Typical elements used for RTDs are nickel, copper, and platinum, with 100 Ω and 1000 Ω platinum (PT100 or PT1000) RTDs being the most common. RTDs are useful for measuring temperatures from –200°C to 800°C and have a near-linear response over this temperature range. Figure 3 shows how to interface a ΔΣ ADC, such as the Analog Devices AD7793, to a commonly used 3-wire PT100 RTD.

Figure 3. Interfacing the AD7793 ADC to a PT100 RTD
Figure 3. Interfacing the AD7793 ADC to a PT100 RTD
 

In this 3-wire configuration, the lead resistances (RL1, RL2, and RL3) will cause errors if only one current source (IOUT1) is used, because the excitation current will flow through RL1, developing a voltage error between AIN1(+) and AIN1(–), the positive and negative terminals of the differential analog input ADC channel. The second RTD current source (IOUT2) is used to compensate for the error introduced by the excitation current flowing through RL1. While the absolute accuracy of each current source is not important, good matching of the two current sources is essential. The second RTD current flows through lead resistance RL2. Assuming RL1 and RL2 are equal and IOUT1 and IOUT2 match, the error voltage across RL2 cancels the error voltage across RL1, and no error voltage is developed between AIN1(+) and AIN1(–). The ADC in this example has differential analog inputs and accepts a differential reference, allowing the implementation of a ratiometric configuration. The ADC's reference voltage is also generated using the matched current sources; it is developed across the precision resistor RREF, and is applied to the differential reference inputs of the ADC. This scheme ensures that the analog input voltage span remains ratiometric to the reference voltage. Any errors in the analog input voltage due to temperature drift of the RTD current source are compensated for by the variation in the reference voltage.

 

Isolators

 

If the sensor resides in a harsh industrial environment, safety measures require both intrinsically safe operation and an isolation barrier to prevent ground loops. Isolation devices are used to protect against high voltages or currents caused by line surges or ground loops, which can occur in any system that has multiple paths to ground. System grounds that are separated by long cables will not be at the same potential, resulting in ground current between the two systems. Without isolation, this current could introduce noise, degrade measurements, or even destroy system components.

For smart transmitter systems, the isolation barrier can be placed between the microcontroller and the digitized sensor data. Data can be transferred across this barrier by magnetic couplers, which are a good alternative to using optocouplers, and offer the following advantages:

  • 1. Lower power, resulting in reduced heat dissipation
  • 2. More reliable (no LED aging or temperature sensitivity)
  • 3. Faster data rates
  • 4. Multiple channels on one device, resulting in significant space reduction
  • 5. Higher DC accuracy

 

Digital isolators, such as the quad-channel Analog Devices ADuM1401, simplify 3-wire serial interface communications between the ADC and the controller and encompass an additional channel to incorporate data readback functionality (Figure 4).

>Figure 4. Functional block diagram of the ADuM1401 digital isolator (3/1 channel directionality)
>Figure 4. Functional block diagram of the ADuM1401 digital isolator (3/1 channel directionality)
 

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