VOLTAGE/CURRENT CONVERTER
AM422
has to be connected between the resistors
R
5
and
R
L
. In this case the IC supply voltage
V
CC
depends
on the voltage
V
S
and the load resistor
R
L
and can be calculated:
V
CC
=
V
S
−
I
OUT
⋅
R
L
Basically the IC AM422 is composed of 3 functional sections as they shown in Figure 1:
1. An
operational amplifier input
which allows the adjustment of the output current range with the
two external resistors
R
1
and
R
2
. With the variation of the resistors the input voltage range and
the output current range can be set.
2. A
voltage controlled current output,
with a wide current range is adjustable with external resis-
tors
R
SET
,
R
3
and
R
4
. The resistors fix the output offset current, which depends on the reference
voltage and corresponds to a minimal output current. The output current
I
OUT
is supplied by the
external transistor
T
1
, driven by the output of the IC pin 3 (OUT).
3. A
programmable voltage reference
(VSET =
N.C.
or
VSET
=
GND)
can be used as an excitation
for constant voltage sensors or as supply for other external devices.
The output current is based on two partial currents: an adjustable offset current and a current relying
on the input signal
V
IN
. The transfer function of the AM422 is then
I
OUT
(
V
SET
,
V
IN
)
=
I
SET
(
V
SET
)
+
I
IN
(
V
IN
)
(2)
(1)
For the adjustment of the AM422 two cases have to be differentiated. For input voltage ranges
without an offset voltage (0...5V, 0...10V) the adjustment of the output current range is as follows:
The minimum output current has to be set with an input voltage
V
IN
= 0V. The output offset current
becomes
I
SET
(
V
IN
=
0
)
=
V
V
(
R
+
R
SET
)
−
R
3
1
⋅
V
SET
−
REF
=
REF
⋅
4
R
0
2
R
0
2
(
R
3
+
R
4
+
R
SET
)
(3)
With
R
3
=
R
4
and simplifications the set resistor
R
SET
is
R
SET
≈
4
R
0
R
4
I
SET
V
REF
V
INmax
R
2
2
R
0
R
1
+
R
2
(4)
The output current range has to be set with the transfer function of the IC and can be calculated by
∆I
OUT
=
I
OUTmax
−
I
SET
=
The relationship of
R
1
/R
2
becomes
R
1
V
INmax
=
−
1
R
2
2
R
0
(
I
OUTmax
−
I
SET
)
(6)
(5)
The adjustment of the IC using an input offset voltage (for example 0.5...4.5V) can be described as
follows. For a desired change of the output current of
∆I
OUT
∆
I
OUT
=
∆
V
PIN
4
2
R
0
⇒
∆
V
PIN
4
=
2
R
0
∆
I
OUT
(7)
analog microelectronics
April 99
5/10
AME [ ANALOG MICROELECTRONICS ]
