MPXM2102AS datasheet(453/670 Pages) MOTOROLA

453 / 670 page

AN1517

3–307

Motorola Sensor Device Data

www.motorola.com/semiconductors

THE SIGNAL CONDITIONING

The amplifier circuitry, shown in Figure 1, is composed of

two op–amps. This interface circuit has a much lower

component

count

than

conventional

quad

amp

instrumentation amplifiers. The two op amp design offers the

high input impedance, low output impedance, and high gain

desired for a transducer interface, while performing a

differential to single–ended conversion. The gain is set by the

following equation:

GAIN

+ 1 ) R6

where R6

+ R3 and R4 + R5.

For this specific design, the gain is set to 201 by setting

R6 = 20 k

Ω and R5 = 100 Ω. Using these values and setting

R6 = R3 and R4 = R5 gives the desired gain without loading

the reference voltage divider formed by R1 and Roff. The offset

voltage is set via this voltage divider by choosing the value of

Roff. This enables the user to adjust the offset for each

application’s requirements.

Figure 1. Pressure Switch Schematic

Amplifier Stage

Comparator Stage

12.1 k

Ω

100

Ω

100

Ω

20 k

Ω

Roff

MPX2100DP

Pressure Sensor

RTH

10 k

Ω

121 k

Ω

R10

24.3 k

Ω

10.0 k

Ω

R11

4.75 k

Ω

MMBT3904LT1

LM324D

VTH

CN1

Vout

GND

+5 V

20 k

Ω

3 4

THE COMPARISON STAGE

The comparison stage is the “heart” of the pressure switch

design. This stage converts the analog voltage output to a

digital output, as dictated by the comparator’s threshold. The

comparison stage has a few design issues which must be

addressed:

• The threshold for which the output switches must be pro-

grammable. The threshold is easily set by dividing the sup-

ply voltage with resistors R7 and RTH. In Figure 1, the

threshold is set at 2.5 V for R7 = RTH = 10 kΩ.

• A method for providing an appropriate amount of hystere-

sis should be available. Hysteresis prevents multiple tran-

sitions from occurring when slow varying signal inputs

oscillate about the threshold. The hysteresis can be set by

applying positive feedback. The amount of hysteresis is

determined by the value of the feedback resistor, RH (refer

to equations in the following section).

• It is ideal for the comparator’s logic level output to swing

from one supply rail to the other. In practice, this is not pos-

sible. Thus, the goal is to swing as high and low as possible

for a given set of supplies. This offers the greatest differ-

ence between logic states and will avoid having a micro-

controller read the switch level as being in an indeterminate

state.

• In order to be compatible with CMOS circuitry and to avoid

microcontroller timing delay errors, the comparator must

switch sufficiently fast.

• By using two comparators, a window comparator may be

implemented. The window comparator may be used to

monitor when the applied pressure is within a set range. By

adjusting the input thresholds, the window width can be

customized for a given application. As with the single

Freescale Semiconductor, Inc.

For More Information On This Product,

Go to: www.freescale.com

MPXM2102AS Datasheet(PDF) 453 Page - Motorola, Inc

MPXM2102AS Datasheet(HTML) 453 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 Descarga

Enlace URL