CURRENT CONVERTER IC
V
CC
=
V
S
−
I
OUT
⋅
R
L
AM402
AM402 is basically made up of three function blocks (see Figure 1):
1. The amplification of the high-precision
instrumentation amplifier
as the input stage is adjustable and thus
makes applications for a number of input signals and sensors possible. Gain
G
IA
is set via the two exter-
nal resistors
R
1
and
R
2
. When selecting the resistors, the sum of
R
1
+
R
2
given in the
Boundary Conditions
must be heeded. When configuring the instrumentation amplifier, the user should ensure that the input
signal has the correct polarity.
2. At the
voltage-controlled current output
an offset current can be set at the output with the help of the
internal voltage reference across external resistors
R
3
and
R
4
(see the
Description of Applications,
begin-
ning on page 7). Output current
I
OUT
is provided by external transistor
T
1
which is driven by the output
(IOUT) of the IC. One particular feature of AM402 is that the output current is switched–off if overvolt-
age occurs on the input side of the device. Another safety feature included in AM402 is the integrated
power-down function with excessive temperature. With this, the output current is switched off if the IC
gets too warm.
3. The
adjustable reference voltage source
supplies sensors or other external components with voltage of 5
or 10V (VSET =
N.C.
or
VSET
=
GND).
Additionally, any voltage value between 4.5 and 10V can be set
via an external voltage divider. Please note, that Capacitor
C
1
(ceramic) must also be connected even
when the voltage reference is not used.
Initial Operation of AM402
To compensate the offset of the output current for the first time, the input must be short-circuited (V
IN
= 0).
In doing so, it should be ensured that the input pins of the instrumentation amplifier have the voltage poten-
tials given in the
Electrical Specifications
(input voltage range). The short circuit at the input produces an
output current
I
OUT
=
I
SET
with
I
SET
(
V
IN
=
0
)
=
V
REF
R
4
⋅
2
R
0
R
3
+
R
4
The adjustment of the output current range depends on the choice of external resistors
R
1
and
R
2
. The maxi-
mum output current is defined by the general transfer function of the IC. The following equation is given for
the output current
I
OUT
:
G
I
OUT
=
V
IN IA
+
I
SET
R
0
The gain factor of the instrumentation amplifier
G
IA
=
1
+
R
1
R
2
is determined by the input voltage
V
IN
and
the maximum output current
I
OUTmax
.
The minimum supply voltage is dependent on the value of the reference voltage. The following applies:
V
CC
≥
V
REF
+
1V
.
R
L
[Ω]
V
CCmin
=
6V
The choice of supply voltage
V
S
also de-
pends on the load resistor
R
L
used by the
application. The following inequation de-
termines the minimum supply voltage:
V
S
≥
I
OUTmax
R
L
+
V
CCmin
.
R
L
≤
V
S
−
V
CCmin
I
OUTmax
R
Lmax
=
500Ω
I
OUTmax
= 20mA
500
300
Operating Area
0
0
6
12
16
24
35
V
S
[V]
The resulting operating range is given in
Figure 4. Example calculations and typical
values for the external components can be
found in the example application shown in
the
Applications
from page 7 onwards.
Figure 4
analog microelectronics
April 99
5/8
AME [ ANALOG MICROELECTRONICS ]
