AM417 – Ratiometric instrumentation

amplifier with adjustable output stage

Analog Microelectronics GmbH

An der Fahrt 13, D – 55124 Mainz

July 2008 –Rev 3.1- Page 7/18

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

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

Internet: http://www.analogmicro.de

Email: info@analogmicro.de

where VTHRFSH is current limitation threshold.

Should no current limit be necessary, the T1 transistor emitter can be directly connected up to pin

VCC (R2 = 0). Good thermal coupling between T1 and the IC reduces the temperature drift of

output current IOUT, thus raising the quality of the current limit.

The output stage is not protected against reverse polarity. Reverse polarity of VCC referenced to

ground can be realized using a simple additional circuit, see [3].

CALIBRATION WITH A RESISTOR BRIDGE CIRCUIT

Setting the output span

The output signal span can be set using gain GOUT of the output stage (see Equation 2):

IA

OUTME

SPAN

OUT

G

V

V

G

⋅

=

(5)

where

VSPAN = VOUTmax – VOUT min and VOUTME is the output voltage of the sensing element.

Setting the output offset

In a Wheatstone bridge circuit, such as those frequently used with piezoresistive sensors, the offset

of the output voltage

VOUTmin must be calibrated depending on the required degree of accuracy and

with reference to the offset of both the sensing element and the IC. To this end, a compensating

1

7

AM417

C

1

V

OUT

8

V

S

T

1

R

2

C

2

R

3

R

4

6

C

3

Ground

R

9 R

2

3

OP

IA

I

IB

Output-

stage

R

1

4

5

R

B1

R

B2

R

B3

R

B4

R

o

V

IN -

V

IN +

Figure 2: Measuring a constant-current sensing element using a Wheat-

VOUTME - = negative

bridge output signal

VOUTME + = positive

bridge output signal

VOUTME + - VOUTME - =

VOUTME

VOUTME + = VIN+

VOUTME - = VIN-

VIN+ - VIN_ = VIN