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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

Centronics Connector.

OPERATING THE AD421 EVALUATION BOARD

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

the external pass transistor. The pass transistor used on this

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.

The regulated voltage on the AD421 V

10

µF tantalum and 0.1µF ceramic disc capacitors.

The loop current can be measured in either the LOOP PLUS or

LOOP MINUS lines.

INTRODUCTION

This Application Note describes the evaluation board for the

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.

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

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.

FUNCTIONAL BLOCK DIAGRAM

36-WAY

CENTRONICS

CONNECTOR

LEVEL

SHIFTERS/

BUFFERS

AD421

V

CC

V

CC

COM

DRIVE

LOOP

RTN

DN2535/

DN2540

LOOP

PLUS

LOOP

MINUS

GND

REV. A

Information furnished by Analog Devices is believed to be accurate and

reliable. However, no responsibility is assumed by Analog Devices for its

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

otherwise under any patent or patent rights of Analog Devices.