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

Phone: +49 (0)6131/91 073-0

Fax: +49 (0)6131/91 073-30

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 ISET = 0. In this case the output current will be always: IOUT≠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

∆IOUT = IOUTmax – IOUTmin:

0

6

8R

V

I

PIN

OUT

∆

=

∆

⇒

OUT

PIN

I

R

V

∆

=

∆

0

6

8

(11)

For an input current swing of

∆VIN = VINmax – VINmin 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:

7

6

7

6

R

R

R

V

V

G

IN

PIN

+

=

∆

∆

=

⇒

1

6

7

6

−

∆

∆

=

PIN

IN

V

V

R

R

(13)

From input offset VINmin the following output current is then obtained when ISET = 0:

()

0

7

6

7

min

min

8

1

R

R

R

R

V

V

I

IN

IN

OUT

⋅

+

⋅

=

(14)

Using the SET pin and Equation 2 the required minimum output current IOUTmin can then be set:

SET

IN

OUT

I

R

R

R

R

V

I

+

⋅

+

⋅

=

0

7

6

7

8

1

with

4

3

4

0

2

R

R

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 VIN = 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 R0 = 27Ω, a voltage swing is obtained at pin 6 of:

V

456

.

3

mA

16

27

8

8

0

6

3

=

⋅

Ω

⋅

=

∆

=

∆

=

∆

OUT

PIN

PIN

I

R

V

V