Datasheet

VOLTAGE/CURRENT CONVERTER AM422
analog microelectronics
April 99
9/10
TYPICAL THREE–WIRE APPLICATION (0.5 – 4.5V)
Example 3: Output current range I
OUT
= 4...20mA, input voltage range V
IN
= 0.5...4.5V
The transfer function of the output current I
OUT
is (equation 2)
IIII
V
R
R
RR
OUT SET IN SET
IN
=+=+
+






2
0
2
12
For the 0.5 to 4.5V application (Figure 8) a change of the input voltage from ∆V
IN
= 4V should be
displayed over a change of the output current ∆I
OUT
= 16mA.
With the voltage change at pin 4 (∆V
PIN4
= 800mV, equation 7) the relationship of R
1
/R
2
, using
equation 8, becomes then
∆
∆
V
V
R
RR
PIN
IN
42
12
800
4
=
+
=
mV
V
⇒
R
R
VV
V
IN PIN
PIN
1
2
4
4
4
=
−
=
∆∆
∆
With that relation an offset current I
SET
from
II I I
SET OUTmin INmin OUTmin
=−=−
2mA = 2mA
has to be adjusted (equation 10). With R
3
= R
4
the set resistor R
SET
can be calculated (equation 4)
R
RRI
V
SET
SET
REF
≈
4
04
The values of the external devices than can be calculated as follows (V
REF
= 5V )
R
0
= 25Ω R
3
= R
4
= 33kΩ R
SET
≈ 1.32kΩ R
5
= 40Ω
R
1
/R
2
≈ 4 R
1
≈ 68kΩ R
2
≈ 18kΩ R
L
= 0...500Ω
C
1
= 2.2µF
C
1
R
4
R
3
V
REF
R
0
V
S
I
OUT
R
L
Ground
3
4
7
G = 1
Amplifier
2
1
V
REF
2
8
V
SET
V
OFFSET
R
1
R
2
R
SET
5
V
I
6
5/10 V Reference
AM422
−
1
R
5
D
1
T
1
V
IN
Sensor
0.5
−
4.5V
Figure 8