CN-0267 Datasheet, PDF (2/8 Pages) Analog Devices – Complete 4 mA to 20 mA Loop Powered Field Instrument with HART Interface

CN-0267
Circuit Note
CIRCUIT DESCRIPTION
Analog Front-End Interface
The ADuCM360 analog front-end incorporates dual, high
performance 24-bit sigma-delta (Σ-Δ) analog-to-digital converters
(ADCs). It also integrates programmable gain instrumentation
amplifiers, a precision band-gap reference, programmable current
sources, a flexible multiplexer, and many other features. It allows a
direct interface to multiple analog sensors, such as pressure
sensor bridges, resistive temperature sensors, thermocouples,
and many other types of sensors used in the industry.
The circuit in Figure 1 shows an example connection for a primary
bridge type sensor and a secondary resistive temperature sensor;
however the ADuCM360 flexible front-end allows many other
configurations to accommodate any type of precision analog
sensor application.
Primary Sensor Input
The ADuCM360 on-chip ADC0 measures the field instrument
primary sensor, shown as a bridge transducer in Figure 1. The
sensor connects to the analog input pins, AIN0 and AIN1, via an
RC filter network for improved system electromagnetic immunity.
The common-mode filter bandwidth is approximately 16 kHz,
and the differential-mode bandwidth is 800 Hz.
The ADuCM360 VREF+ and VREF− voltage reference inputs
sense the bridge excitation voltage and enable the circuit to work in
a ratiometric mode, making the measurement independent of
the exact value of the sensor power supply voltage. The on-chip
ground switch can dynamically disconnect the bridge excitation
and save power when required by the application.
Secondary Sensor Input
The circuit uses a platinum (Pt) 100 Ω resistive temperature
device (RTD) as a secondary sensor. The RTD can sense the
temperature of the primary sensor and thus allow for temperature
compensation of the primary sensor if required.
The ADuCM360 programmable current source supplies the RTD
via the AIN4 pin. The ADC1 on the ADuCM360 measures the
voltage across the RTD using the AIN3 and AIN2 pins configured
as a differential input. The exact value of the current flowing
through the RTD is sensed by a precision resistor (RREF) and is
measured by the ADC1 using the AIN7 pin. The ADC1 uses the
on-chip, band-gap voltage reference.
Digital Data Processing, Algorithm, and Communications
All the field instrument digital functions are provided by the
ADuCM360 32-bit ARM Cortex™ M3 RISC processor, with
integrated 128 k bytes of nonvolatile flash/EE memory, 8 k bytes
of SRAM, and an 11-channel direct memory access (DMA)
controller that supports wired (2× SPI, UART, I²C) communication
peripherals.
The demonstration software performs the initialization and
configuration, processes data from the analog inputs, controls
the analog output, and performs the HART communication.
Analog Output
The AD5421 integrates a low power precision 16-bit DAC with
a 4 mA to 20 mA, loop powered output driver and provides all
functions required for the field instrument analog output.
The AD5421 interfaces with the ADuCM360 controller via the
SPI interface.
The AD5421 also includes a range of diagnostic functions
related to the 4 mA to 20 mA loop. The auxiliary ADC can
measure the voltage across the instruments loop terminals via
the 20 MΩ/1 MΩ resistive divider connected to the VLOOP pin.
The ADC can also measure the chip temperature via the integrated
sensor. The ADuCM360 controller can configure and read all
the diagnostics of the AD5421, but the AD5421 can also operate
autonomously.
As an example, if the communication between the controller and
the AD5421 fails, the AD5421 automatically sets its analog
output to a 3.2 mA alarm current after a defined period. This
alarm current indicates to the host that the field instrument
failed to operate.
The software controls any change of the output current from
one value to another to prevent disturbance of the HART
communication. (See the Analog Rate of Change section).
HART Communication
The AD5700 integrates a complete HART FSK modem. The
modem is connected to the ADuCM360 controller via a standard
UART interface, complemented by request to send (RTS) and
carrier detect (CD) signals.
The HART output is scaled to the required amplitude by the
0.068 µF/0.22 µF capacitive divider and coupled to the AD5421
CIN pin, where it is combined with the DAC output to drive and
modulate the output current.
The HART input is coupled from LOOP+ via a simple passive
RC filter to the AD5700 ADC_IP pin. The RC filter works as
the first stage, band-pass filter for the HART demodulator and
also improves the system electromagnetic immunity, which is
important for robust applications working in harsh industrial
environments.
The AD5700 low power oscillator generates the clock for the
HART modem with a 3.8664 MHz external crystal connected
directly to the XTAL1 and XTAL2 pins.
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