Datasheet

AD7705/AD7706
Rev. C | Page 40 of 44
SMART TRANSMITTERS
Another application where the low power, single-supply, 3-wire
interface capabilities of the AD7705/AD7706 are beneficial is in
smart transmitters.
Figure 28 shows a block diagram of a smart
transmitter using the AD7705. Because a smart transmitter
must operate from a 4 to 20 mA loop, tolerances in the loop
mean that the current available to power the transmitter can be
as low as 3.5 mA. The AD7705 consumes only 320 μA, leaving
at least 3 mA available for the rest of the transmitter. The
AD7705, with its dual-input channel, is ideally suited for
systems requiring an auxiliary channel whose measured
variable is used to correct that of the primary channel.
V
DD
GND
REF IN
ISOLATED SUPPLY
4.7μF
100kΩ
MCLK IN
MCLK OUT
AD7705
ISOLATED GROUND
MICROCONTROLLER UNIT
•PID
•RANGE SETTING
•CALIBRATION
•LINEARIZATION
•OUTPUT CONTROL
•SERIAL COMMUNICATION
•HART PROTOCOL
V
CC
COM
V
CC
BOOST
REF OUT1
REF OUT2
REF IN
AD421
C1 C2 C3 COM
LOOP
RTN
COMP
DRIVE
4.7μF
0.01μF
0.1μF2.2μF
DN25D
1kΩ
1000pF
0.01μF
0.01μF
0.0033μF
ISOLATION
BARRIER
SENSORS
RTD
mV
Ω
TC
MAIN TRANSMITTER ASSEMBLY
4mA
TO
20mA
01166-028
Figure 28. Smart Transmitter Using the AD7705