VOLTAGE/CURRENT CONVERTER
TYPICAL THREE–WIRE APPLICATION (0 – 5/10V)
R
3
V
REF
6
8
AM422
C
1
R
SET
5
AM422−1
V
SET
V
REF
2
V
S
R
4
G
=1
V
OFFSET
5/10 V Reference
1
R
0
V
V
IN
4
2
Amplifier
R
1
R
2
7
I
3
T
1
D
1
R
5
I
OUT
R
L
Ground
Figure 7
Used in a three–wire circuit (AM422–1) ground pin 7 (GND) is connected to
Ground
(Figure 7).
The relationship of
R
1
/R
2
, using equation 2, becomes
R
1
V
INmax
=
−
1
R
2
2
R
0
(
I
OUTmax
−
I
SET
)
The current
I
SET
is used to set the output current offset and can be calculated by using equation 3
I
SET
(
V
IN
=
0
)
=
V
REF
(
R
4
+
R
SET
)
−
R
3
⋅
R
0
2
(
R
3
+
R
4
+
R
SET
)
With
R
3
=
R
4
the set resistor
R
SET
becomes (equation 4)
R
SET
≈
4
R
0
R
4
I
SET
V
REF
Example 1:
Output current range 4...20mA
In this case the values of the external devices (V
IN
=
0
K
5V ,
V
REF
=
5V ) are as follows
R
0
= 25Ω
R
3
=
R
4
= 33kΩ
R
SET
≈
2.64kΩ
R
5
= 40Ω
R
1
/R
2
≈
5.25
R
L
= 0...500Ω
C
1
= 2.2µF
Example 2:
Output current range 0...20mA
In this case the values of the external devices (V
IN
=
0
K
10V ,
V
REF
=
5V ) are as follows
R
0
= 25Ω
R
3
=
R
4
= 33kΩ
R
SET
= 0Ω
R
5
= 40Ω
R
1
/R
2
≈
9
R
L
= 0...500Ω
C
1
= 2.2µF
analog microelectronics
April 99
8/10
AME [ ANALOG MICROELECTRONICS ]
