EVAL-AD421EB Datasheet, PDF (1/7 Pages) Analog Devices – Evaluation Board for Loop Powered 4-20mA DAC

a
Evaluation Board for
Loop Powered 4-20mA DAC
EVAL-AD421EB
FEATURES
Loop Powered
5V, 3.3V and 3V Modes of Operation
Direct Hook-Up to Printer Port of PC
PC Software for Control of Loop Current
Included on the evaluation board, along with the AD421, are a
DN2535/DN2540, an N-Channel depletion mode FET which is
used as part of the regulator loop to regulate the supply for the
board from the externally supplied loop voltage applied between
the loop(+) and loop (-) inputs, digital buffers to buffer and
level shift the input signals from the printer port edge connector
and a number of passive components required to achieve
optimum performance from the AD421.
Interfacing to the AD421 board is provided via a 36-Pin
INTRODUCTION
This Application Note describes the evaluation board for the
Centronics Connector.
OPERATING THE AD421 EVALUATION BOARD
AD421, loop powered 4-20mA DAC. The AD421 is a
complete, loop-powered, digital to 4-20mA converter, designed
to meet the needs of smart transmitter manufacturers in the
Industrial Control industry. It provides a high precision, fully
integrated, low cost solution in a compact 16-pin package. The
AD421 is ideal for extending the resolution of smart 4-20mA
transmitters at very low cost.
Power Supplies
The evaluation board operates from a single power supply
connected between the LOOP PLUS and LOOP MINUS
terminals on the board. This loop power supply must be floating
with respect to the computer ground and have a minimum limit
of V_CC+2 V and a maximum limit of the breakdown voltage of
the external pass transistor. The pass transistor used on this
The AD421 includes a selectable regulator which can be used to
power all other devices in the transmitter. This regulator
provides either a +5 V, +3.3 V or +3 V regulated output
voltage. The part also contains +1.25 V and +2.5 V precision
references. The AD421 thus eliminates the need for a discrete
regulator and voltage reference. The only external components
required are a number of passive components and a pass
transistor to span large loop voltages. Full data on the AD421 is
board is a DN2535/DN2540, N-Channel Depletion mode
transistor available from Supertex inc.., this facilitates the use of
loop voltages up to 350V. This pass transistor in association
with the regulator loop of the AD421 regulates the supply
voltage for the AD421 itself and for the digital buffers on the
board. The regulated voltage is referenced to the COM pin on
the AD421. A link option on the board selects the operating
mode for the board to be either 5V, 3.3V or 3V.
available in the AD421 data sheet available from Analog
Devices and should be consulted in conjunction with this
Application Note when using the Evaluation Board.
The regulated voltage on the AD421 V_CC pin is decoupled with
10µF tantalum and 0.1µF ceramic disc capacitors.
The loop current can be measured in either the LOOP PLUS or
LOOP MINUS lines.
FUNCTIONAL BLOCK DIAGRAM
36-WAY
CENTRONICS
CONNECTOR
VCC
LEVEL
SHIFTERS/
BUFFERS
GND
DN2535/
DN2540
VCC
DRIVE
AD421
LOOP
RTN
COM
LOOP
PLUS
LOOP
MINUS
REV. A
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use, nor for any infringements of patents or other rights of third parties
which may result from its use. No license is granted by implication or
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