AD7705/AD7706
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.
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
MAIN TRANSMITTER ASSEMBLY
ISOLATION
BARRIER
ISOLATED SUPPLY
DN25D
2.2μF 0.1μF
V
V
CC
BOOST
COMP
CC
V
0.01μF
100kΩ
DD
REF IN
4.7μF
4mA
TO
20mA
REF OUT1
REF OUT2
REF IN
MICROCONTROLLER UNIT
DRIVE
1kΩ
•PID
SENSORS
•RANGE SETTING
•CALIBRATION
•LINEARIZATION
•OUTPUT CONTROL
•SERIAL COMMUNICATION
•HART PROTOCOL
1000pF
RTD
mV
Ω
AD7705
4.7μF
AD421
LOOP
RTN
MCLK IN
TC
C1 C2 C3
COM
COM
MCLK OUT
0.01μF
0.0033μF
GND
ISOLATED GROUND
0.01μF
Figure 28. Smart Transmitter Using the AD7705
Rev. C | Page 40 of 44
ADI [ ADI ]
