AD7705/AD7706
+5V
Smart Transmitters
V
DD
Another area where the low power, single supply, three-wire
interface capabilities is of benefit is in smart transmitters. Here,
the entire smart transmitter must operate from the 4 mA to
20 mA loop. Tolerances in the loop mean that the amount of
current available to power the transmitter is as low as 3.5 mA.
The AD7705 consumes only 320 µA, leaving at least 3 mA
available for the rest of the transmitter. Figure 25 shows a block
diagram of a smart transmitter which includes the AD7705. The
AD7705 with its dual input channel is ideally suited to systems
requiring an auxiliary channel whose measured variable is used
to correct that of the primary channel.
400A
REF IN(+)
6.25k⍀
REF IN(–)
MCLK IN
R
L1
AD7705
R
AIN1(+)
AIN1(–)
L2
RTD
MCLK OUT
R
L3
L4
RESET
DRDY
R
GND
DOUT DIN
SCLK
CS
Figure 24. RTD Measurement Using the AD7705
MAIN TRANSMITTER ASSEMBLY
ISOLATION
BARRIER
ISOLATED SUPPLY
DN25D
2.2F 0.1F
V
V
CC
BOOST
COMP
CC
0.01F
V
100k⍀
DD
REF IN
4.7F
4mA
TO
20mA
REF OUT1
MICROCONTROLLER UNIT
DRIVE
REF OUT2
REF IN
1k⍀
•PID
SENSORS
RTD
•RANGE SETTING
•CALIBRATION
•LINEARIZATION
•OUTPUT CONTROL
•SERIAL COMMUNICATION
•HART PROTOCOL
1000pF
4.7F
AD7705
mV
⍀
TC
AD421
LOOP
RTN
MCLK IN
C1 C2 C3
COM
COM
MCLK OUT
0.01F
0.0033F
GND
ISOLATED GROUND
0.01F
Figure 25. Smart Transmitter Using the AD7705
–30–
REV. A
ADI [ ADI ]
