VOLTAGE/CURRENT CONVERTER

AM422

analog microelectronics

April 99

5/10

has to be connected between the resistors R5 and RL. In this case the IC supply voltage VCC depends

on the voltage VS and the load resistor RL and can be calculated:

VV

I

R

CC

S

OUT

L

=−

⋅

(1)

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 R1 and R2. 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 RSET, R3 and R4. The resistors fix the output offset current, which depends on the reference

voltage and corresponds to a minimal output current. The output current IOUT is supplied by the

external transistor T1, 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 VIN. The transfer function of the AM422 is then

()

( )

( )

IV

V

I

V

I

V

OUTSET

IN

SETSET

IN

IN

,

=+

(2)

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 VIN = 0V. The output offset current

becomes

()

()

IV

R

V

VV

R

RR

R

RR

R

SET

IN

SET

REF

REF

SET

SET

==

⋅

−







 =⋅

+−

++

0

1

2

2

00

43

34

()

(3)

With R3 = R4 and simplifications the set resistor RSET is

R

RR I

V

SET

SET

REF

≈

4

04

(4)

The output current range has to be set with the transfer function of the IC and can be calculated by

∆II

I

V

R

R

RR

OUT

OUTmax

SET

INmax

=−

=

+













2

0

2

12

(5)

The relationship of R1/R2 becomes

()

R

R

V

RI

I

INmax

OUTmax

SET

1

2

0

2

1

=

−

−

(6)

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

∆IOUT

∆

∆

I

V

R

OUT

PIN

=

4

0

2

⇒∆

∆

VR

I

PIN

OUT

40

2

=

(7)