PI converter

Here is a copy (minus the diagrams) of Microchip Technology, Inc., AN695 product. This may help you understand the electronic circuits.

INTRODUCTION

Pressure measurement devices can be classified into

two groups: those where pressure is the only source of

power and those that require electrical excitation. The

mechanical style devices that are only excited by pressure,

such as bellows, diaphragms, bourdons, tubes or

manometers, are usually suitable for purely mechanical

systems. With these devices a change in pressure

will initiate a mechanical reaction, such as a change in

the position of mechanical arm or the level of liquid in a

tube.

Electrically excited pressure sensors are most synergistic

with the microcontroller environment. These style

of sensors can be piezoresistive, Linear Variable Differential

Transformers (LVDT), or capacitive sensors.

Most typically, the piezoresistive sensor is used when

measuring pressure.

This application note will concentrate on the signal conditioning

path of the piezoresistive sensing element

from sensor to microcontroller. It will show how the

electrical output of this sensor can be gained, filtered

and digitized in order to ready it for the microcontroller's

calibration routines. This theoretical discussion will be

followed with a specific pressure sensing design that is

specifically designed to measure barometric pressure.

PIEZORESISTIVE PRESSURE

SENSORS

The piezoresistive is a solid state, monolithic sensor

that is fabricated using silicon processing. Piezo means

pressure, resistance means opposition to a DC current

flow. Since piezoresistive pressure sensors are fabricated

on a wafer, 300 to 500 sensors can be produced

per wafer. Since these wafers generate a large number

of sensors they are available on the market at a

reduced cost as compared to mechanical sensors.

Figure 1:

The resistive wheatstone bridge configuration can have one variable element (a.), two elements that vary

with excitation (b.) or four elements (c.). The piezoresistive pressure sensing element is usually a four element bridge

and is constructed in silicon (d.).

Author: Bonnie Baker

Microchip Technology Inc.

RS1 RS1

RS1

RS2

RS2

RS2

RS4

RS4 RS3 RS4

RS3

RS3

Voltage or

Current

Excitation

Voltage or

Current

Excitation

Voltage or

Current

Excitation

VOUT+

VOUTVOUT+

VOUTVOUT+

VOUT-

(a.) single element bridge (b.) two element bridge

(c.) four element or full bridge (d.) single side of a sandwiched

piezoelectric pressure