CURRENT CONVERTER IC

AM402

analog microelectronics

April 99

5/8

L

OUT

S

CC

R

I

V

V

⋅

−

=

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 GIA is set via the two exter-

nal resistors R1 and R2. When selecting the resistors, the sum of R1 + R2 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 R3 and R4 (see the Description of Applications, begin-

ning on page 7). Output current IOUT is provided by external transistor T1 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 C1 (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 (VIN = 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 IOUT = ISET with

()

4

3

4

0

2

0

R

R

R

R

V

V

I

REF

IN

SET

+

⋅

=

=

The adjustment of the output current range depends on the choice of external resistors R1 and R2. The maxi-

mum output current is defined by the general transfer function of the IC. The following equation is given for

the output current IOUT:

SET

IA

IN

OUT

I

R

G

V

I

+

=

0

The gain factor of the instrumentation amplifier

2

1

1

R

R

GIA

+

=

is determined by the input voltage VIN and

the maximum output current IOUTmax.

The minimum supply voltage is dependent on the value of the reference voltage. The following applies:

V

1

+

≥

REF

CC

V

V

.

The choice of supply voltage VS also de-

pends on the load resistor RL used by the

application. The following inequation de-

termines the minimum supply voltage:

CCmin

L

OUTmax

S

V

R

I

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.

V

S [V]

R

L [Ω]

6

35

V

CCmin

= 6V

R

Lmax

= 500

Ω

I

OUTmax = 20mA

16

R

VV

I

L

SCCmin

OUTmax

≤

−

24

12

500

Operating Area

0

300

0

Figure 4