AD627B Analog Devices, AD627B Datasheet - Page 16

• Manufacturer Part Number: AD627B
• Description: Micropower/ Single and Dual Supply Rail-to-Rail Instrumentation Amplifier
• Manufacturer: Analog Devices

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 C812, 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
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
0.210 (5.33)
0.160 (4.06)
0.115 (2.93)
PIN 1
0.022 (0.558)
0.014 (0.356)
MAX
8
0.430 (10.92)
1
0.348 (8.84)
0.100
(2.54)
BSC
MicroConverter is a trademark of Analog Devices, Inc.
8-Lead Plastic DIP
0.070 (1.77)
0.045 (1.15)
TRANSDUCER
5
4
4–20mA
shunt resistor generates a maximum
0.060 (1.52)
0.015 (0.38)
0.280 (7.11)
0.240 (6.10)
(N-8)
0.130
(3.30)
MIN
SEATING
PLANE
IMPEDANCE
0.325 (8.25)
0.300 (7.62)
LINE
Figure 46. A 4 mA-to-20 mA Receiver Circuit
0.015 (0.381)
0.008 (0.204)
Dimensions shown in inches and (mm).
4–20mA
0.195 (4.95)
0.115 (2.93)
OUTLINE DIMENSIONS
24.9
–16–
G = 5
Over a temperature range from –200 C to +200 C, the J-type
thermocouple delivers a voltage ranging from –7.890 mV to
10.777 mV. A programmed gain on the AD627 of 100 (R
2.1 k ) 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.
Figure 47. Amplifying Bipolar Signals with Low Common-
Mode Voltage
0.0098 (0.25)
0.0040 (0.10)
THERMOCOUPLE
AD627
0.1574 (4.00)
0.1497 (3.80)
+5V
0.1 F
SEATING
PIN 1
REF
PLANE
J-TYPE
0.1968 (5.00)
0.1890 (4.80)
0.0500 (1.27)
8
1
BSC
AIN 0–7
0.0192 (0.49)
0.0138 (0.35)
V
REF
5
4
AVDD
AGND
8-Lead SOIC
0.2440 (6.20)
0.2284 (5.80)
+5V
0.0688 (1.75)
0.0532 (1.35)
0.1 F
MicroConverter
(SO-8)
AD C812
DGND
DVDD
0.0098 (0.25)
0.0075 (0.19)
+5V
2.1k
0.1 F
R
G
TM
AD627
8
0
+5V
0.0196 (0.50)
0.0099 (0.25)
0.0500 (1.27)
0.0160 (0.41)
0.1 F
REF
REV. A
V
2V
G
OUT
45
=