CN0229 Datasheet, PDF (2/6 Pages) Analog Devices – Industrial Current

CN-0229
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CIRCUIT FUNCTION AND BENEFITS
The circuit shown in Figure 1 is a multichannel, flexible, analog
output solution with only two analog components and meets
most requirements for multichannel I/O cards, programmable
logic controllers (PLCs), and distributed control systems (DCSs)
applications. The AD5686R quad, 16-bit nanoDAC+ with rail-
to-rail buffered outputs combined with four of the AD5750-2
industrial current/voltage output drivers provide all the typical
output current and voltage ranges with 16-bit resolution, no
missing codes, 0.05% linearity, and less than 0.1% output error.
An ultralow drift (2 ppm/°C typical), 2.5 V voltage reference with
high drive capability (up to ±5 mA) is integrated in the AD5686R
and provides the reference voltage for both the AD5686R and
the AD5750-2. This guarantees low noise, high accuracy, and
low temperature drift for the circuit.
The ADuM1301 and ADuM5400 provide 2500 V rms isolation
both on power, and all the necessary signals between the analog
signal chain and the host controller.
For multichannel I/O card applications that need more than
4 channels, several AD5686Rs can be connected in a daisy chain,
and no additional external digital I/O circuits are required. This
minimizes the cost, especially for high channel count isolated
applications.
The circuit also contains key features for industrial applications,
such as on-chip output fault detection, packet error checking (PEC)
by the CRC, flexible power-up options, and ESD protection (4 kV
for the AD5686R, human body model and 3 kV for the AD5750-2,
human body model), making it an ideal choice for robust industrial
control systems. No external precision resistors or calibration
routines are needed to maintain consistent performance in mass
production, thereby making it ideal for PLC or DCS modules.
CIRCUIT DESCRIPTION
The AD5750/AD5750-1/AD5750-2 are single-channel, low cost,
precision voltage/current output drivers developed to meet the
requirements of industrial process control applications. The output
voltage range can be programmed for the standard output ranges
for PLC and DCS applications: 0 V to 5 V, 0 V to 10 V, −5 V to
+5 V, and −10 V to +10 V. A 20% overrange setting is also provided
for the standard ranges, giving the following options: 0 V to
+6 V, 0 V to +12 V, −6 V to +6 V, and −12 V to +12 V.
Circuit Note
The current output, which is provided on a separate pin, can
be programmed for the ranges of +4 mA to +20 mA, 0 mA to
+20 mA, −20 mA to +20 mA, 0 mA to +24 mA, and −24 mA
to +24 mA. The unipolar ranges have a 2% overrange setting.
Because the AD5750/AD5750-1/AD5750-2 current outputs can
either source or sink current, they can interface to a wide variety of
sensors or actuators. The voltage and current output pins can be
connected together to configure the system as a single-channel
output if desired.
Generally, the current output circuit needs at least one precision
resistor for current sensing. The current accuracy and temperature
drift characteristic of the circuit depends partly on the resistor as
well as the reference. The AD5750/AD5750-1/AD5750-2 integrates
the high precision, low drift resistor and still allows the use of an
external resistor, if desired. As a method of improving the stability
of the output current over temperature, an external low drift
resistor can be connected to the REXT1 and REXT2 pins of the
AD5750/AD5750-1/AD5750-2, instead of the internal resistor.
The external resistor is selected via the input shift register. If the
external resistor option is not used, leave the REXT1 and
REXT2 pins floating.
The AD5686R is a quad 16-bit nanoDAC+ with rail-to-rail voltage-
buffered outputs integrated with an on-chip 2 ppm/°C typical
(5 ppm/°C maximum), 2.5 V reference. The on-chip reference
can drive the reference input of all four AD5750-2s, has a low
output impedance of 0.05 Ω, and can source and sink up to 5 mA.
The AD5686R incorporates a power-on reset circuit to ensure
that the DAC output powers up to 0 V and remains there until a
valid write command takes place.
The interface between the AD5686R DAC and the AD5750-2
driver is simple and requires no external voltage reference or
precision resistors. The output voltage range of the AD5686R is
0 V to 2.5 V, which matches the input range of the AD5750-2.
In addition, the reference output voltage of the AD5686R is 2.5 V,
which precisely matches the reference input requirement of the
AD5750-2.
The ADuM1301 is a triple-channel digital isolator. The ADuM5400
is a quad-channel digital isolator with an integrated, isolated,
dc-to-dc converter. They are both based on iCoupler® technology
and are used to provide isolation between the signal chain and
the system microcontroller, with an isolation rating of 2.5 kV rms.
The ADuM5400 provides an isolated 5 V power supply for the
5 V circuits in the secondary side.
Devices for PLC and DCS applications generally need ESD
protection and overvoltage protection much higher than the
formal recommend specifications. The AD5686R and the
AD5750-2 have internal ESD protection diodes to each pin
that can prevent damage from a 4 kV (AD5686R) and 3 kV
(AD5750-2) transient (human body model). However, the
industrial control environment can subject I/O circuits to much
higher transients.
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