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Maxim > Design Support > Technical Documents > Reference Schematics > A/D and D/A Conversion/Sampling Circuits > APP 5817

Maxim > Design Support > Technical Documents > Reference Schematics > Amplifiers > APP 5817

Maxim > Design Support > Technical Documents > Reference Schematics > Analog Switches and Multiplexers > APP 5817

Keywords: smart transducer, 4-20mA current loop, industrial process control, HART modem

REFERENCE SCHEMATIC 5817

Novato (MAXREFDES16#): 4-20mA Loop-Powered Temperature

Sensor with HART

By:

Yuriy Kurtsevoy, Strategic Applications

Engineer

Whitney Scott, Strategic Applications Engineer

Jan 30, 2014

Abstract:

The Novato reference design (MAXREFDES16#) is a 16-bit, high-accuracy, loop-powered temperature transducer that transmits

temperature information from a remote object to the central control unit over a 4–20mA current loop and using the highway addressable

remote transducer (HART) communication protocol.

Introduction

Temperature is one of the most widely measured parameters in industrial process control and automation. This reference design provides

a complete signal-chain solution that works with any type of RTDs, from PT100 to PT1000. The Novato PT100 2-wire, loop-powered smart

temperature transmitter guarantees a low-power, easy-to-use, reliable solution of temperature measurement from -200°C to +850°C with

accuracy better than 0.1% or 1.0°C, whichever is more accurate, over the entire operating range.

The Novato MAXREFDES16

smart sensor transmitter reference design features:

2-, 3-, or 4-wire RTD sensor input

-200°C to +850°C measured temperature range

-40°C to +85°C operating range

0.03°C temperature resolution

< 0.05% span error or ±0.2°C at +23°C (whichever is greater)

< 0.1% span error or ±1.0°C over operating range

12V to 40V supply voltage range

200µA RTD excitation current

Auto compensation of wire resistance for 3-wire RTD

4–20mA current loop output

10½ to 1k½ loop load

Highway addressable remote transducer (HART) communication protocol requires minimum load of 230½

Sensor burnout detection

NAMUR down scale: < 3.2mA

NAMUR up scale: programmable from 20.5mA to > 24mA

Loop current limiter: 30mA ±5mA

HART protocol compatible

Detailed Description

Block diagram of the Novato smart sensor transmitter is shown in Figure 1.

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Summary of content (10 pages)

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Maxim > Design Support > Technical Documents > Reference Schematics > A/D and D/A Conversion/Sampling Circuits > APP 5817 Maxim > Design Support > Technical Documents > Reference Schematics > Amplifiers > APP 5817 Maxim > Design Support > Technical Documents > Reference Schematics > Analog Switches and Multiplexers > APP 5817 Keywords: smart transducer, 4-20mA current loop, industrial process control, HART modem REFERENCE SCHEMATIC 5817 Novato (MAXREFDES16#): 4-20mA Loop-Powered Temperature Sensor with HA
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More detailed image. Figure 1. Novato (MAXREFDES16#) block diagram. The signal chain contains the MAX11213 (U1), 16-bit delta-sigma ADC with programmable gain and GPIO, the RL78/G13 Renesas' microcontroller with 128kB flash memory (U6), and the 4–20mA current loop transmitter built on the MAX5216 and MAX9620 (U2 and U3). The DS8500 modem (U7) provides digital interface utilizing HART protocol over 4–20mA current loop.
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T(°C) = (-A + Ã(A2 - 4B(1 - RRTD/R0)))/2B (Eq. 4) where RRTD = (ADCcode × RREF)/((216 - 1) × gain) 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).
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Figure 3. Novato USB-to-UART programming adapter. Figure 4. Novato smart sensor transmitter. The programming adapter is connected to a PC through the USB port, and to the Novato board J4 header with a 2-wire UART interface, Figure 4. Connect the Novato board to the 4–20mA current loop and supply the power according to the block diagram in Figure 1. Plug in the USB-to-UART programming adapter to the PC and start the Novato calibration software, Figure 5.
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Figure 5. Novato calibration GUI. The GUI is user friendly and easy to navigate. Just follow the tips in the Status box. Step 1: Disable main loop to allow communication to the sensor by selecting the Calibration Mode checkbox. Note all functions are disabled (grayed) until Calibration mode is enabled. Step 2: Select RTD. In the Calibration tab, clicking the "RTD Settings" button opens the RTD Settings window, Figure 6.
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Figure 6. RTD Settings pop up window. Use the appropriate settings from Table 1 and click the Write button. Click the Read button to verify the correct parameters are written. Table 1. RTD Parameters Parameter PT100 PT200 PT500 PT1000 R0 (Ω) 100 200 500 1000 Gain 16 16 8 4 Max. Temperature (°C) 850 630 630 630 Min. Temperature (°C) -200 -200 -200 -200 Step 3: Perform system offset calibration. Short J1.1, J1.2, J1.3, and J1.4 of the sensor input connector.
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Figure 7. 2-wire RTD connection. In the Debug ADC tab, select the "Convert 5sps" ADC Command from the pulldown menu, Figure 8. 6.1. Click the "Send" button. 6.2. Click the "Read" button in the ADC Registers group box. The ADC Registers are read back, Figure 8. 6.3. Click the "0%" button on Fluke 724 to select -200°C. 6.4. Repeat 6.1. and 6.2. The ADC Data register should read 0x50F or something close to that value. 6.5. Click the "100%" button on Fluke 724 to select +800°C. 6.6. Repeat 6.1 and 6.2.
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Figure 8. Debug ADC tab. Step 8: Save to Flash. Click the "Save To Flash" button in the Calibration tab to store calibration values into nonvolatile memory. Step 9: Disable Calibration Mode. Resume normal operation by deselecting the Calibration Mode checkbox.
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Note, the Novato calibration software only works with custom made USB-to-UART programming adapter. This adapter is not provided with Novato board and has to be requested separately. Firmware Files Firmware is developed using Renesas' e2 studio—complete development and debug environment based on the popular Eclipse CDT project. Use Renesas' E1 Emulator to download firmware through connector J3. The E1 Emulator also provides all the basic debugging function.
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Figure 10. Error (°C) change vs. temperature. The Novato design fits into standard head mounting DIN form B (< 40mm diameter) enclosure. Buy Reference Design Buy Direct: Novato (MAXREFDES16#) Or Order the Novato reference design (MAXREFDES16#) from your local Maxim representative.