AM417 – Ratiometric instrumentation

amplifier with adjustable output stage

Analog Microelectronics GmbH

An der Fahrt 13, D – 55124 Mainz

July 2008 –Rev 3.1- Page 15/18

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

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

Internet: http://www.analogmicro.de

Email: info@analogmicro.de

In order to achieve a maximum a sensing element output signal it is best to increase set bridge

supply current IIB by the TCS Factor. Gain GOUT can also be increased by the same factor if, for

example, maximum bridge current IIBmax = 1.25mA or if the maximum voltage at pin 2 (IB) is

overshot during an increase.

EXAMPLE

Example 4: TCS compensation of a piezoresistive pressure sensing element [2]

• TCS = -0.0019/°C

• VCC = 5V

• RBR = 3KΩ

• TCR = +0.0028/°C

• Temperature range: -20°C – 80°C

Bridge supply current IIB is selected according to the following. Assuming that the maximum

operating temperature of the circuit is 80°C, the maximum bridge resistance is calculated using

Equation (16):

(

)

()

Ω

=

°

−

°

⋅

°

+

⋅

Ω

=

k

C

C

C

k

R

RB

46

,

3

25

80

/

0028

,

0

1

3

max

With a bridge current of IIB = 0.8mA, at 80°C and VCC = 5V, pin 2 (IB) has a potential of:

V

V

mA

k

V

IB

27

.

3

5

.

0

8

.

0

46

.

3

=

+

⋅

Ω

=

Applying Equation (17):

RTCS = 6.33KΩ

Using Equation (19) the following is calculated for T0:

TCS

Factor = 1.47

If bridge current IIB is now increased by a factor of TCSFactor, the result is a new amended bridge

current of:

IIBnew = 1.18mA

The original output signal of the sensing element is thus reinstated following TCS compensation.

Output stage gain GOUT could also be increased by a factor of TCSFactor by adjusting resistors R3

and R4 according to Equation (2).