AD8546ARMZ-R7 datasheet(20/24 Pages) AD

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AD8546/AD8548

Data Sheet

Rev. B | Page 20 of 24

Note that 100 kΩ resistors are used in series with the input of

the op amp. If smaller resistor values are used, the supply current

of the system increases much more. For more information about

using op amps as comparators, see the AN-849 Application Note,

Using Op Amps as Comparators.

4 mA TO 20 mA PROCESS CONTROL CURRENT

LOOP TRANSMITTER

A 2-wire current transmitter is often used in distributed control

systems and process control applications to transmit analog signals

between sensors and process controllers. Figure 70 shows a 4 mA

to 20 mA current loop transmitter.

100Ω

VDD

18V

10µF

0.1µF

390pF

0.1µF

3.3kΩ

4mA

20mA

1.2kΩ

RNULL

1MΩ

VREF

RSPAN

200kΩ

VIN

0V TO 5V

68kΩ

2kΩ

NOTES

1. R1 + R2 = R´.

1/2

AD8546

10µF

RSENSE

100Ω

VOUT

GND

ADR125

VIN

Figure 70. 4 mA to 20 mA Current Loop Transmitter

The transmitter is powered directly from the control loop

power supply, and the current in the loop carries signal from

4 mA to 20 mA. Thus, 4 mA establishes the baseline current

budget within which the circuit must operate.

The AD8546 is an excellent choice due to its low supply current

of 33 μA per amplifier over temperature and supply voltage. The

current transmitter controls the current flowing in the loop, where

a zero-scale input signal is represented by 4 mA of current and a

full-scale input signal is represented by 20 mA. The transmitter

also floats from the control loop power supply, VDD, whereas signal

ground is in the receiver. The loop current is measured at the load

resistor, RL, at the receiver side.

With a zero-scale input, a current of VREF/RNULL flows through

R. This creates a current, ISENSE, that flows through the sense

resistor, as determined by the following equation:

ISENSE,MIN = (VREF × R)/(RNULL × RSENSE)

With a full-scale input voltage, current flowing through R is

increased by the full-scale change in VIN/RSPAN. This creates an

increase in the current flowing through the sense resistor.

ISENSE,DELTA = (Full-Scale Change in VIN × R)/(RSPAN × RSENSE)

Therefore,

ISENSE,MAX = ISENSE,MIN + ISENSE,DELTA

When R >> RSENSE, the current through the load resistor at the

receiver side is almost equivalent to ISENSE.

Figure 70 shows a design for a full-scale input voltage of 5 V. At

0 V of input, the loop current is 3.5 mA, and at a full-scale input

of 5 V, the loop current is 21 mA. This allows software calibration

to fine-tune the current loop to the 4 mA to 20 mA range.

Together, the AD8546 and the ADR125 consume quiescent

current of only 160 µA, making 3.34 mA current available to

power additional signal conditioning circuitry or to power a

bridge circuit.

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