Datasheet

AD421
–12–
REV. C
DV
DD
AV
DD
REF IN
CS
DATA OUT
SCLK
DATA IN
AGND
DGND
MCLK IN
MCLK OUT
AD7714/
AD7715
ANALOG
TO
DIGITAL
CONVERTER
SENSORS
RTD
mV ⍀ TC
REF OUT1
BOOST
V
CC
LV
COMP
DRIVE
LOOP
RTN
REF OUT2
REF IN
CLOCK
LATCH
DATA
COM
C1 C2
C3
LOOP
POWER
0.01␮F
DN25D
2.2␮F
3.3V
1.25V
4.7␮F
AMBIENT
TEMP
SENSOR
AD421
MICROCONTROLLER
V
CC
GND
WAVEFORM
SHAPER
BANDPASS
FILTER
HART
MODEM
BELL 202
0.1␮F
0.01␮F
1k⍀
1000pF
C
C
*
HT20C12/20C15
4.7␮F100k⍀
*FOR SELECTION OF C
C
, REFER TO AN-534
Figure 17. AD421 in HART Transmitter Application
across R2. The ratio of R1 to R2 determines the current that
flows in the load resistor R
L
. I
L
= [1 + R1/R2] × I
PROG
, where I
L
is the current that flows in the load resistor R
L
and I
PROG
is the
current programmed to the AD421. R1 and R2 are external to
the AD421 and will need to be matched resistors to obtain a
highly accurate current source.
Current Source
Figure 18 shows an application circuit for the AD421 being
used as a current source. The current programmed to the
AD421 (4 mA to 20 mA) will develop a voltage across R1.
This same voltage due to negative feedback will be generated
LOOP
RTN
R2
R
L
R1
C1 C2
C3
COM
V
CC
LV
2.2␮F
COM
DRIVE
COMP
BOOST
0.01␮F
0.01␮F
0.0033␮F
LATCH
CLOCK
DATA
REF IN
REF OUT2
REF OUT1
4.7␮F
COM
AD421
+5V
10k⍀
10k⍀
10k⍀
CLOCK
LATCH
DATA
V
S
RETURN
5 VOLT
REGULATOR
OUT IN
V
S
COM
Figure 18. AD421 in Programmable Current Source/Sink