UNIVERSAL AMPLIFIER IC

AM400

analog microelectronics

October 2005

Analog Microelectronics GmbH

Phone:

+49 (0)6131/91 073 – 0

10/21

An der Fahrt 13, D – 55124 Mainz

Fax:

+49 (0)6131/91 073 – 30

Rev.: 4.2

Internet: http://www.analogmicro.de

Email:

info@analogmicro.de

OPERATING AM400

General information on 2- and 3-wire applications

In 3-wire operation (cf. Figure 5, for example) the ground of the IC (pin GND) is connected up to

and pin VCC to pin RS+.

In 2-wire operation the IC ground is "virtual" (floating), as with a constant load resistance the

applies to 2-wire operation:

()

L

IN

OUT

S

CC

R

V

I

V

V

−

=

(5)

The reason for this is that in 2-wire operation the IC is connected in series to the actual load resistor

In 3-wire operation Equation 5 no longer applies as the IC ground is connected to the ground of the

system. For 3-wire operation the supply voltage can be expressed thus:

S

CC

V

V

=

(6)

In a 2-wire setup the power consumption of the overall system (AM400 and all external

R

L

V

S

2-wire system

Signal source and

conditioning IC

GND Ground

≠

V

Ground

R

L

3-wire system

Signal source and

conditioning IC

GND Ground

=

V

Ground GND

=

GND

V

CC

I

OUT

I

OUT

VV

CC

S

=

Figure 3: The difference between 2- and 3-wire operation