AD421 Analog Devices, AD421 Datasheet - Page 13

• Manufacturer Part Number: AD421
• Description: Loop-Powered 4-20 mA DAC
• Manufacturer: Analog Devices

Specifications of AD421

• Resolution (bits): 16bit
• Dac Update Rate: 125SPS
• Dac Settling Time: 8ms
• Max Pos Supply (v): +5.05V
• Single-supply: Yes
• Dac Type: Current Out
• Dac Input Format: Ser

AD421
Battery Backup
Figure 19 shows an application circuit for the AD421 where the
DN25D
IN3611
micro and memory sections of the circuitry are protected against
losing data if the loop is broken. The backup circuit switches
IRFF9113
0.1 F
4.7 F
IN3611
from V
to battery voltage without a glitch when V
power is
CC
CC
V
LOOP
V
CC
lost. The IRFF9113 acts as a current source during normal
100k
2.2 F
MICRO/
operation and provides a constant charging current to the supercap
MEMORY
or Nicad. The loss of V
drops the IRFF9113’s gate voltage to
CC
V
SUPERCAP
CC
GND
zero volts, which allows the battery or supercaps current to flow
DRIVE
through the MOSFETs channel and integral body diode to
AD421*
provide power for the micro and memory sections. To calibrate
LOOP
RTN
this circuit, connect an ammeter in series with the battery or
COM
supercap. Then with V
and the load present adjust the 100 kΩ
CC
*ADDITIONAL CIRCUITRY OMITTED FOR CLARITY
potentiometer for the battery charging current recommended by
Figure 19. Battery Backup Circuit
the battery or supercap manufacturer.
Nonrechargeable batteries should not be used in this application
due to danger of explosion.
REV. C
–13–