High Precision Instrumentation Amplifier

AM447

analog microelectronics

April 2000

5/6

FUNCTIONAL DESCRIPTION

The IC AM447 is an integrated high precision amplifier for low bridge output signals. Basically the

AM447 is composed of 2 functional sections as shown in Figure 1:

1. A high accuracy instrumentation amplifier (IA) allows amplification with a high signal-to-noise

offset voltage of the instrumentation amplifier to nearly zero. But offset compensation over tem-

thermore, these resistors have to be placed together very close. It is also necessary to use similar

metals for the connection of the sensor bridge and the AM447 to avoid thermocouple effects. The

internal gain of the IA is fixed to the value

102

=

INT

G

IA is given by the following equation:

()

VG

V

V

V

IAOUT

INT

IN

IN

VREF

=⋅

−

+

+−

2. An output stage de–couples the IA and thus improves the performance of the AM447. The gain

defined by

G

R

R

ADJ

G

G

=+

1

2

1

()

(

)

{

rs

transduce

kalibrated

for

adjustment

Offset

adjustment

Span

VREF

IN

IN

ADJ

INT

VREF

IN

IN

OUT

V

V

V

G

G

V

V

V

G

V

+

−

⋅

=

+

−

=

−

+

−

+

4

4

4

43

4

4

4

42

1

The AM447 is suited for two types of transducers. The IC is designed for usage with non-

for sensor systems with calibrated transducers. The remaining offset of the transducers can be cali-

CC

O

O

O

VREF

V

R

R

R

V

2

1

2

+

=

The entire sensor systems realised with the different types of AM447 and only a few external com-

ponents are shown in Figures 2 and 3.

Offset calibration of the instrumentation amplifier

The offset compensation has to be handled with care because the entire system performance de-

pends on it. Please note, that this offset adjustment doesn't include the bridge offset.

The offset compensation has to be done in the following order:

• T =°

25 C and V

• VVV

IN

IN

REF

+−

==

= 25

.V