AM417 – Ratiometric instrumentation

amplifier with adjustable output stage

Analog Microelectronics GmbH

An der Fahrt 13, D – 55124 Mainz

July 2008 –Rev 3.1- Page 8/18

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

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

Internet: http://www.analogmicro.de

Email: info@analogmicro.de

resistor RO is inserted into the measuring bridge (see Figure 2). By using this compensating resistor

the instrumentation amplifier input voltage

∆VIN is set in such a way, that output voltage VOUTmin has

a value of 0.5V, for example. The voltage drop

VRO across resistor RO is given by:













+

−

























+

−

+

−

∆

=

4

3

3

2

1

2

4

3

4

1

/

B

B

B

B

B

B

B

B

B

BR

IN

RO

R

R

R

R

R

R

R

R

R

V

V

V

(6)

where VBR is the voltage drop across the entire sensing element, RBR the total bridge resistance and

RB1,2,3,4 the individual bridge resistors. Assuming that the four separate bridge resistors have the

same value, the following approximation formula is valid:

VRO = 2∆VIN

(7)

∆VIN is the voltage to be set at the input of the instrumentation amplifier where there are no offsets.

OUT

IA

OUT

SYS

OUT

IN

G

G

V

G

V

V

⋅

=

=

∆

min

min

(8)

Taking the offset of the sensing element (VOSME) and that of the IC (VOSIC) into account

(VOSIC = VOSIA + 0.1VOSOUT, where VOSIA is the instrumentation amplifier offset and VOSOUT the

output stage offset), the adjustable voltage is calculated as:

∆VIN’ = ∆VIN - VOSIC - VOSME

(9)

From (9) and (8) it follows that:

OSME

OSIC

OUT

IA

OUT

IN

V

V

G

G

V

V

−

−

⋅

=

∆

min

´

(10)

Applying (7) and (10), the necessary voltage drop across RO required to calibrate the offset of the

output voltage VOUTmin is expressed thus:













−

−

⋅

⋅

=

OSME

OSIC

OUT

IA

OUT

RO

V

V

G

G

V

V

min

2

(11)

On condition, the sensing element offset is low referenced to the sensing element output voltage

(VOSME < 10 VOUTME), the resistor RO is calculated with sufficient accuracy as:

IB

RO

O

I

V

R

⋅

=

2

(12)

Applying (11) and the condition, that the voltage drop across Ro may only be positive, the

maximum compensatable offset is computed thus:

OUT

IA

OUT

OSME

OSIC

G

G

V

V

V

⋅

≤

+

min

(13)