MPXM2102AS datasheet(464/670 Pages) MOTOROLA

464 / 670 page

3–318

Motorola Sensor Device Data

www.motorola.com/semiconductors

AN1525

The A-B-C'sof Signal-Conditioning Amplifier

Design for Sensor Applications

Prepared by: Eric Jacobsen and Jeff Baum

Sensor Applications Engineering

Motorola Signal Products Division

Phoenix, AZ

INTRODUCTION

Although

fully

signal–conditioned,

calibrated,

and

temperature compensated monolithic sensor IC’s are

commercially available today, there are many applications

where the flexibility of designing custom signal–conditioning

is of great benefit. Perhaps the need for a versatile low–level

sensor output is best illustrated by considering two particular

cases that frequently occur: (1) the user is in a prototyping

phase of development and needs the ability to make changes

rapidly to the overall transfer function of the combined

sensor/amplifier subsystem, (2) the specific desired transfer

function does not exist in a fully signal–conditioned,

precision–trimmed sensor product (e.g., a signal–conditioned

device is precision trimmed over a different pressure range

than that of the application of interest). In such cases, it is

obvious that there will always be a need for low–level,

nonsignal–conditioned sensors. Given this need, there is also

a need for sensor interface amplifier circuits that can signal

condition the “raw” sensor output to a usable level. These

circuits should also be user friendly, simple, and cost effective.

Today’s unamplified solid–state sensors typically have an

output voltage of tens of millivolts (Motorola’s basic 10 kPa

pressure sensor, MPX10, has a typical full–scale output of

58 mV, when powered with a 5 V supply). Therefore, a gain

stage is needed to obtain a signal large enough for additional

processing.

This

additional

processing

may

include

digitization by a microcontroller’s analog to digital (A/D)

converter,

input to a comparator, etc. Although the

signal–conditioning circuits described here are applicable to

low–level, differential–voltage output sensors in general, the

focus of this paper will be on interfacing pressure sensors to

amplifier circuits.

This paper presents a basic two operational–amplifier

signal–conditioning

circuit

that

provides

the

desired

characteristics of an instrumentation amplifier interface:

• High input impedance

• Low output impedance

• Differential to single–ended conversion of the pressure

sensor signal

• High gain capability

For this two op–amp circuit, additional modifications to the

circuit allow (1) gain adjustment without compromising

common mode rejection and (2) both positive and negative dc

level shifts of the zero pressure offset. Varying the gain and

offset is desirable since full–scale span and zero pressure

offset voltages of pressure sensors will vary somewhat from

unit to unit. Thus, a variable gain is desirable to fine tune the

sensor’s full–scale span, and a positive or negative dc level

shift (offset adjustment) of the pressure sensor signal is

needed to translate the pressure sensor’s signal–conditioned

output span to a specific level (e.g., within the high and low

reference voltages of an A/D converter).

For the two op–amp gain stage, this paper will present the

derivation of the transfer function and simplified transfer

function for pressure sensor applications, the derivation and

explanation of the gain stage with a gain adjust feature, and

the derivation and explanation of the gain stage with the dc

level shift modification.

Adding another amplifier stage provides an alternative

method of creating a negative dc voltage level shift. This

stage is cascaded with the output from the two op–amp stage

(

Note: gain of the two op–amp stage will be reduced due to

additional gain provided by the second amplifier stage). For

this three op–amp stage, the derivation of the transfer

function, simplified transfer function, and the explanation of

the negative dc level shift feature will be presented.

GENERAL NOTE ON OFFSET ADJUSTMENT

Pressure sensor interface circuits may require either a

positive or a negative dc level shift to adjust the zero pressure

offset voltage. As described above, if the signal–conditioned

pressure sensor voltage is input to an A/D, the sensor’s

output dynamic range must be positioned within the high and

low reference voltages of the A/D; i.e., the zero pressure

offset voltage must be greater than (or equal to) the low

reference voltage and the full–scale pressure voltage must

be less than (or equal to) the high reference voltage (see

Figure 1). Otherwise, voltages above the high reference will

be digitally converted as 255 decimal (for 8–bit A/D), and

voltages below the low reference will be converted as 0. This

creates a nonlinearity in the analog–to–digital conversion.

MOTOROLA

SEMICONDUCTOR APPLICATION NOTE

Freescale Semiconductor, Inc.

For More Information On This Product,

Go to: www.freescale.com

MPXM2102AS Datasheet(PDF) 464 Page - Motorola, Inc

MPXM2102AS Datasheet(HTML) 464 Page - Motorola, Inc

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 ...More

Datasheet Download

Link URL