Datasheet

AD8546
Rev. A | Page 20 of 24
With a zero-scale input, a current of V
REF
/R
NULL
flows through
R. This creates a current flowing through the sense resistor,
I
SENSE
, determined by the following equation:
I
SENSE, MIN
= (V
REF
× R)/(R
NULL
× R
SENSE
)
With a full-scale input voltage, current flowing through R is
increased by the full-scale change in V
IN
/R
SPAN
. This creates an
increase in the current flowing through the sense resistor.
I
SENSE, DELTA
= (Full-Scale Change in V
IN
× R)/(R
SPAN
× R
SENSE
)
Therefore
I
SENSE, MAX
= I
SENSE, MIN
+ I
SENSE, DELTA
When R >> R
SENSE
, the current through the load resistor at the
receiver side is almost equivalent to I
SENSE
.
Figure 71 shows a design for a full-scale input voltage of 5 V. At
0 V of input, loop current is 3.5 mA, and at a full-scale input of
5 V, the loop current is 21 mA. This allows software calibration
to fine-tune the current loop to the 4 mA to 20 mA range.
The AD8546 and ADR125 together consume only 160 µA
quiescent current, making 3.34 mA current available to power
additional signal conditioning circuitry or to power a bridge
circuit.
R
L
100Ω
V
DD
18V
C2
10µF
C3
0.1µF
C1
390pF
C4
0.1µF
R4
3.3kΩ
Q1
D1
4mA
TO
20mA
R3
1.2kΩ
R
NULL
1MΩ
1%
V
REF
R
SPAN
200kΩ
1%
V
IN
0V TO 5V
R1
68kΩ
1%
R2
2kΩ
1%
NOTES
1. R1 + R2 = R´.
1/2
AD8546
C5
10µF
R
SENSE
100Ω
1%
09585-072
V
OUT
GND
ADR125
V
IN
Figure 71. 4 mA to 20 mA Current Loop Transmitter