AD5421_11 AD [Analog Devices], AD5421_11 Datasheet - Page 20

• Manufacturer Part Number: AD5421_11
• Description: 16-Bit, Serial Input, Loop-Powered, 4 mA to 20 mA DAC
• Manufacturer: AD [Analog Devices]

AD5421
Loop Voltage Fault
There are two loop voltage alert bits in the fault register:
V
COM pins falls below 0.6 V (corresponding to a 12 V loop
supply value), the V
the voltage returns above 0.7 V. Similarly, if the voltage between
the V
6 V loop supply value), the V
when the voltage returns above 0.4 V. If the V
the fault register, the FAULT pin is set to logic high.
Figure 40 illustrates how a resistor divider enables the monitor-
ing of the loop supply with the V
resistor divider consists of a 1 MΩ and a 19 MΩ resistor that
provide a 20:1 ratio, allowing the 2.5 V input range of the V
pin to monitor loop supplies up to 50 V. With a 20:1 divider ratio,
the preset V
generate loop supply faults according to their stated values. If
another divider ratio is used, the fault bits generate faults at values
that are not equal to 6 V and 12 V.
EXTERNAL CURRENT SETTING RESISTOR
The 24 kΩ resistor R
output voltage to a current, which is then mirrored with a gain
of 221 to the LOOP− pin. The stability of the loop current over
temperature is dependent on the temperature coefficient of R
Table 1 and Table 2 outline the performance specifications of
the AD5421 with both the internal R
24 kΩ R
justed error of better than 0.126% FSR can be expected. Using
an external resistor gives improved performance of 0.048% FSR.
This specification assumes an ideal resistor; the actual performance
depends on the absolute value and temperature coefficient of
the resistor used. For more information, see the Determining
the Expected Total Error section.
LOOP
LOOP
12V and V
SET
and COM pins falls below 0.3 V (corresponding to a
resistor. Using the internal R
Figure 40. Resistor Divider Connection at V
LOOP
AD5421
REG
COM
6V and V
LOOP
LOOP–
V
IN
LOOP
LOOP
SET
6V. If the voltage between the V
, shown in Figure 1, converts the DAC
12V bit is set; this bit is cleared when
LOOP
19MΩ
1MΩ
LOOP
12V alert bits of the fault register
LOOP
6V bit is set; this bit is cleared
SET
input. The recommended
resistor and an external,
SET
resistor, a total unad-
R
LOOP
V
L
LOOP
LOOP
6V bit is set in
Pin
LOOP
and
LOOP
Rev. 0 | Page 20 of 32
SET
.
LOOP CURRENT RANGE SELECTION
To select the loop current range, connect the RANGE0
and RANGE1 pins to the COM and DV
in Table 9.
Table 9. Selecting the Loop Current Range
RANGE1 Pin
COM
COM
DV
DV
CONNECTION TO LOOP POWER SUPPLY
The AD5421 is powered from the 4 mA to 20 mA current loop.
Typically, the power supply is located far from the transmitter
device and has a value of 24 V. The AD5421 can be connected
directly to the loop power supply and can tolerate a voltage up
to a maximum of 52 V (see Figure 41).
Figure 41 shows how the AD5421 is connected directly to the
loop power supply. An alternative power connection is shown
in Figure 42, which shows a depletion mode N-channel MOSFET
connected between the AD5421 and the loop power supply. The
use of this device keeps the voltage drop across the AD5421 at
approximately 12 V, limiting the worst-case on-chip power dissi-
pation to 288 mW (12 V × 24 mA = 288 mW). If the AD5421 is
connected directly to the loop supply as shown in Figure 41, the
potential worst-case on-chip power dissipation for a 24 V loop
power supply is 576 mW (24 V × 24 mA = 576 mW). The power
dissipation changes in proportion to the loop power supply voltage.
DD
DD
Figure 41. Direct Connection of the AD5421 to Loop Power Supply
Figure 42. MOSFET Connecting the AD5421 to Loop Power Supply
BSP129
DN2540
AD5421
REG
AD5421
COM
REG
COM
LOOP–
DRIVE
IN
RANGE0 Pin
COM
DV
COM
DV
LOOP–
DRIVE
T1
200kΩ
IN
DD
DD
DD
Loop Current Range
4 mA to 20 mA
3.8 mA to 21 mA
3.2 mA to 24 mA
3.8 mA to 21 mA
R
pins, as shown
V
L
R
LOOP
V
L
LOOP