AM417 – Ratiometric instrumentation

amplifier with adjustable output stage

Analog Microelectronics GmbH

An der Fahrt 13, D – 55124 Mainz

July 2008 –Rev 3.1- Page 14/18

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

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

Internet: http://www.analogmicro.de

Email: info@analogmicro.de

The output signal of a piezoresistive sensing element is accrued from:

BR

IB

BR

OUTME

R

I

P

S

V

P

S

V

⋅

⋅

⋅

=

⋅

⋅

=

(14)

S

is the sensor sensitivity of the sensing element and P is the applied pressure. Sensor sensitivity S

and bridge resistor RBR are the dominant temperature-dependent variables in Equation 14. The

following applies:

(

)

()

0

0

1

T

T

TCS

S

S

−

⋅

+

⋅

=

(15)

(

)

()

o

BR

BR

T

T

TCR

R

R

−

⋅

+

⋅

=

1

0

(16)

S0 is the basic value of the sensitivity and RBRO the basic value of the bridge resistance at T0 (usually

room temperature). T is the actual temperature.

TCS

and TCR are the linear temperature coefficients of sensitivity and bridge resistance. Typical

values are:

TCS

= -0.0019/°C and TCR = +0.0028/°C [3].

Good temperature compensation of sensing element output signal VOUTME would be automatically

achieved if both temperature coefficients had the same value. If both are different, however, an

attempt is made to equalize them. This is done by adding an additional compensatory TCS resistor

RTCS which is inserted parallel to the sensing element (see Figure 4). The TCR value of the entire

system is thus amended so that it is the same as TCS of the sensing element.

In the temperature compensation of the sensing element output signal described above the

following applies to the compensatory TCS resistor:

TCS

TCR

TCS

R

R

BR

TCS

+

−

⋅

=

(17)

As part of the set bridge supply current IIB´ flows through the shunt resistor RTCS the circuit output

signal is reduced after TCS compensation according to the following equation:

()

BR

TCS

TCS

IB

IB

R

R

R

I

I

+

=

'

(18)

In order to reinstate the original output signal of the circuitry the circuit gain must be increased by

the reciprocal ratio:

(

)

TCS

BR

TCS

IB

IB

R

R

R

I

I

TCSFactor

+

=

=

'

(19)