Datasheet

Industrial V/I Converter and Protector IC
AM462
Analog Microelectronics GmbH
An der Fahrt 13, D – 55124 Mainz Oktober 2007 – Rev.2.5 -Page 15/20
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Internet:
http://www.analogmicro.de
Email: info@analogmicro
.de
Application for an input signal with an offset
It is not uncommon for input signals to have an offset (e.g. of 0.5V for a 0,5...4.5V input signal or
1V for a 1..6V input signal). For signals such as these an offset current is generated at the IC output
also when I
SET
= 0. In this case the output current will be always: I
OUT
≠0. The circuit can then be
dimensioned as described in the following.
According to Equation (2) the following applies for a required current swing at the output of
∆I
OUT
= I
OUTmax
– I
OUTmin
:
0
6
8R
V
I
PIN
OUT
∆
=∆
⇒
OUTPIN
IRV
∆
=
∆
06
8 (11)
For an input current swing of ∆V
IN
= V
INmax
– V
INmin
the necessary gain is calculated as:
IN
PIN
V
V
G
∆
∆
=
6
(12)
If G < 1, the input signal can be routed directly to pin 6 (INDAI) via a voltage divider without OP1
having to be used (see Figure 9). With this circuitry the following results:
76
76
RR
R
V
V
G
IN
PIN
+
=
∆
∆
=
⇒ 1
67
6
−
∆
∆
=
PIN
IN
V
V
R
R
(13)
From input offset V
INmin
the following output current is then obtained when I
SET
= 0:
()
076
7
minmin
8
1
RRR
R
VVI
ININOUT
⋅
+
⋅= (14)
Using the SET pin and Equation 2 the required minimum output current I
OUTmin
can then be set:
SETINOUT
I
RRR
R
VI +⋅
+
⋅=
076
7
8
1
with
43
4
0
2 RR
R
R
V
I
REF
SET
+
⋅=
(15)
Example: 4..20mA Voltage-To-Current Converter with Input Signal Offset (3-wire
application)
To convert a signal of V
IN
= 0.5...4.5V the external components are to be dimensioned in such a
way that the output current has a range of 4...20mA. The circuitry is shown in Figure 9. OP1 is used
as input stage to have a high ohmic input and to profite from internal overvoltage protection.
OP2 is not used here. It is, however, available to the user as an additional OP. The pin 1 and 2 has
to be connected.
With reference to Equation (11) and with R
0
= 27Ω, a voltage swing is obtained at pin 6 of:
V456.3mA162788
063
=
⋅
Ω
⋅
=
∆=∆=∆
OUTPINPIN
IRVV