AD627

–16–

REV. A

A 4 mA-to-20 mA Single Supply Receiver

Figure 46 shows how a signal from a 4 mA-to-20 mA transducer

can be interfaced to the AD

µC812, a 12-bit ADC with an em-

bedded microcontroller. The signal from a 4 mA-to-20 mA

transducer is single ended. This initially suggests the need for

a simple shunt resistor, to convert the current to a voltage at the

high impedance analog input of the converter. However, any

line resistance in the return path (to the transducer) will add a

current dependent offset error. So the current must be sensed

differentially.

In this example, a 24.9

Ω shunt resistor generates a maximum

differential input voltage to the AD627 of between 100 mV (for

4 mA in) and 500 mV (for 20 mA in). With no gain resistor

present, the AD627 amplifies the 500 mV input voltage by a

factor of 5, to 2.5 V, the full-scale input voltage of the ADC.

The zero current of 4 mA corresponds to a code of 819 and the

LSB size is 4.9 mA.

A Thermocouple Amplifier

Because the common-mode input range of the AD627 extends

0.1 V below ground, it is possible to measure small differential

signals which have low, or no, common mode component. Fig-

ure 47 shows a thermocouple application where one side of the

J-type thermocouple is grounded.

AD627

0.1 F

24.9

G = 5

REF

4–20mA

4–20mA

TRANSDUCER

AGND

DGND

AIN 0–7

+5V

AVDD

+5V

DVDD

AD C812

+5V

LINE

IMPEDANCE

0.1 F

0.1 F

MicroConverter is a trademark of Analog Devices, Inc.

Figure 46. A 4 mA-to-20 mA Receiver Circuit

Over a temperature range from –200

°C to +200°C, the J-type

thermocouple delivers a voltage ranging from –7.890 mV to

2.1 k

Ω) and a voltage on the AD627 REF pin of 2 V, results in

the AD627’s output voltage ranging from 1.110 V to 3.077 V

relative to ground. For a different input range or different volt-

age on the REF pin, it is important to check that the voltage on

internal node A1 (see Figure 34) is not driven below ground).

This can be checked using the equations in the section entitled

Input Range Limitations in Single Supply Applications.

2.1k

AD627

0.1 F

+5V

J-TYPE

THERMOCOUPLE

2V

REF

Figure 47. Amplifying Bipolar Signals with Low Common-

Mode Voltage

8-Lead Plastic DIP

(N-8)

8

14

5

0.430 (10.92)

0.348 (8.84)

0.280 (7.11)

0.240 (6.10)

PIN 1

SEATING

PLANE

0.022 (0.558)

0.014 (0.356)

0.060 (1.52)

0.015 (0.38)

0.210 (5.33)

MAX

0.130

(3.30)

MIN

0.070 (1.77)

0.045 (1.15)

0.100

(2.54)

BSC

0.160 (4.06)

0.115 (2.93)

0.325 (8.25)

0.300 (7.62)

0.015 (0.381)

0.008 (0.204)

0.195 (4.95)

0.115 (2.93)

OUTLINE DIMENSIONS

Dimensions shown in inches and (mm).

8-Lead SOIC

(SO-8)

0.0098 (0.25)

0.0075 (0.19)

0.0500 (1.27)

0.0160 (0.41)

8

0

0.0196 (0.50)

0.0099 (0.25)

45

85

4

1

0.1968 (5.00)

0.1890 (4.80)

0.2440 (6.20)

0.2284 (5.80)

PIN 1

0.1574 (4.00)

0.1497 (3.80)

0.0500 (1.27)

BSC

0.0688 (1.75)

0.0532 (1.35)

SEATING

PLANE

0.0098 (0.25)

0.0040 (0.10)

0.0192 (0.49)

0.0138 (0.35)