Adding a digital interface to accurate 4-20mA loop

With an eye on factory automation, Maxim has released a reference design for a precision 4-20mA temperature transmitter with added data capability.

The whole thing, branded Novato, is loop-powered, and sends data back to the central office using the HART (highway addressable remote transducer) communication protocol buried in the analogue temperature signal.

“Temperature is among the most widely measured parameters in industrial process control and automation applications. This transmitter enables low-power temperature measurement from -200 to +850°C with accuracy better than +/-0.1% or +/-1.0°C,” said Maxim.

The design works with 2, 3, or 4-wire resistance temperature detectors – any type from PT100 to PT1000.

The HART functions set is developed by AB Tech Solution, a firm specialised in industrial automation. In this design, the HART stack runs on the Renesas RL78/G13 microcontroller and “supports all universal and common practice commands”, said Maxim.

The signal chain contains the MAX11213 16bit ΔΣ ADC with programmable gain and GPIO, the RL78/G13 with 128kbyte flash, and the 4-20mA loop transmitter built on the MAX5216 and MAX9620. Auto-calibration is available through the MAX4729 analogue switch. MAX15006 and MAX6133 provide power and a reference voltage, while the DS8500 is a one-chip HART modem.

Within the block labelled ‘voltage controlled current source’ is simply a mosfet with a source resistor (in addition to the 10Ω). Overload limiting comes from an npn transistor whose Vbe is the resistor voltage, pulling down the gate with its collector.

The firm has a more detailed description of the precision current interface on line in a paper titled “High-Performance, High-Accuracy 4-20mA Current-Loop Transmitter Meets Toughest Industrial Requirements.” This includes a full circuit diagram of the interface (left),

A hardware development kit is available which fits into a DIN size B (<40mm dia) enclosure.

This can be calibrated to match its sensor, but only via a custom made USB-to-UART adapter which “is not provided with Novato board and has to be requested separately”, said Maxim.

Why HART?

The HART protocol was originally developed and is currently managed by the HART Communication Foundation (HCF) and is based on Bell 202 modem technology.

Its main value is that its modem signals have an average voltage of zero, and do not therefore upset the precision of dc analogue sent from the sensor to the central office. So HART-compliant field devices can replace standard field devices.

HART devices must supply a device description (DD) file which describes the device to the controller. The HART Communication Foundation maintains a compliance test suite for conformance and interoperability. Once the test suite is passed, the device is registered with the HCF and can utilise the logo and marketing resources of the HCF.

Novato at a glance

<3.2mA current consumption

12V to 40V supply voltage range

2, 3, or 4wire RTD sensor input

200µA RTD excitation current

Auto compensation of wire resistance for 3-wire RTD

-200 to +850°C measured

-40 to +85°C operation

0.03°C resolution

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

<0.1% span error or +/-1.0°C over operating range

4–20mA current loop output

10Ω to 1kΩ loop load HART requires 230Ω minimum load

<3.2mA NAMUR sensor fail indicator

20.5 to >24mA (programmable) NAMUR sensor fail

30mA ±5mA loop limiter:

HART protocol compatible

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via Yuichun