MPXM2102AS datasheet(354/670 Pages) MOTOROLA

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Motorola Sensor Device Data

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AN1100

Analog to Digital Converter Resolution

Extension Using a Motorola Pressure Sensor

PURPOSE

This paper describes a simple method to gain more than

8–bits of resolution with an 8–bit A/D. The electronic design is

relatively simple and uses standard components.

PRINCIPLE

Consider a requirement to measure pressure up to

200 kPa. Using a pressure sensor and an amplifier, this

pressure can be converted to an analog voltage output. This

analog voltage can then be converted to a digital value and

used by the microprocessor as shown in Figure 1.

If we assume for this circuit that 200 kPa results in a +4.5 V

output, the sensitivity of our system is:/

+ 4.5 V 200kPa

(1)

+ 0.0225 V kPa

+ 22.5 mV kPa

If an 8–bit A/D is used with 0 and 5 Volt low and high

references, respectively, then the resolution would be:

+ 5V

28–1

+ 5V 255

(2)

+ 0.01961 V

Rv + 19.60 mV per bit

This corresponds to a pressure resolution of:

P + 5V

19.60 mV bit)

22.5 mV kPa

(3)

+ 0.871 kPa per bit

Assume a resolution of at least 0.1 kPa/bit is needed. This

would require an A/D with at least 12 bits ( 212 = 4096 steps).

One can artificially increase the A/D resolution as described

below.

Refer to Figure 1 and assume a pressure of 124 kPa is to

be measured. With this system, the input signal to the A/D

should read (assuming no offset voltage error):

Vm(measured) + 4.5 (Papp) x (S)(4)

+ (124 kPa) x 22.5 mV kPa

+ 2790 mV,

where Papp is the pressure applied to the sensor.

Due to the resolution of the A/D, the microprocessor

receives the following conversion:

+ (2790 mV) 19.60 mV bit

(5)

+ 142.35

+ 142 (truncated to integer)

The calculated voltage for this stored value is:

Vc (calculated) + (142 count) x 19.60mV

(6)

count)

+ 2783 mV

The microprocessor will output the stored value M to the

D/A. The corresponding voltage at the analog output of the

D/A, for an 8–bit D/A with same references, will be 2783 mV.

The calculated pressure corresponding to this voltage

would be:

Pc (calculated) + (2783 mV)

22.5 mV kPa

(7)

123.7 kPa

Thus, the error would be:

+ Papp–Pc

(8)

+ 124 kPa–123.7 kPa

+ 0.3 kPa

This is greater than the 0.1 kPa resolution requirement.

Figure 1. Block Diagram

A/D

MPU

OUTPUT

CIRCUITRY

MOTOROLA

SEMICONDUCTOR APPLICATION NOTE

REV 1

Freescale Semiconductor, Inc.

For More Information On This Product,

Go to: www.freescale.com

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