MPXM2102AS datasheet(542/670 Pages) MOTOROLA

542 / 670 page

AN1950

3–396

Motorola Sensor Device Data

www.motorola.com/semiconductors

Table 1: MPXM2010D OPERATING CHARACTERISTICS (VS = 10 Vdc, TA = 25°C unless otherwise noted, P1 > P2)

Characteristic

Symbol

Min

Typ

Max

Unit

Pressure Range(1)

POP

—

kPa

Supply Voltage(2)

—

Vdc

Supply Current

—

6.0

—

mAdc

Full Scale Span(3)

VFSS

Offset(4)

Voff

–1.0

—

1.0

Sensitivity

∆V/∆P

—

2.5

—

mV/kPa

Linearity(5)

—

–1.0

—

1.0

%VFSS

Amplifier Selection and Amplifier Induced Errors

The sensor output needs to be amplified before being

inputted directly to the microcontroller through an 8–bit A/D

input pin. To determine the amplification requirements, the

pressure sensor output characteristics and the 0–5V input

range for the A/D converter had to be considered.

The amplification circuit uses three op–amps to add an

offset and convert the differential output of the MPXM2010GS

sensor to a ground–referenced, single–ended voltage in the

range of 0 – 5V.

The pressure sensor has a possible offset of +/– 1mV at the

minimum rated pressure. To avoid a nonlinear response when

a pressure sensor chosen for the system has a negative offset

(Voff), we have added a 5mV offset to the positive sensor

output signal. This offset will remain the same regardless of

the sensor output. Any additional offset that the sensor or

op–amp introduce is compensated for by software routines

that are invoked when the initial system calibration is done.

To determine the gain required for the system, the maximum

output voltage from the sensor for this application had to be

determined. The maximum output voltage from the sensor is

approximately 12.5mV with a 5V supply since the full–scale

output of the sensor changes linearly with supply voltage. This

system will have a maximum pressure of 4kPa at 40cm of

water. At a 5V supply, we will have a maximum sensor output

of 5mV at 4kPa of pressure. To amplify the maximum sensor

output to 5.0V, the following gain is needed:

Gain = (Max Output needed) / (Max Sensor Output and

Initial Offset)

= 5.0V / (.005V + .005) = 500

The gain for the system was set for 500 to avoid railing from

possible offsets from the pressure sensor or the op–amp.

The Voltage Outputs from the sensor are each connected

to a non–inverting input of an op–amp. Each op–amp circuit

has the same resistor ratio. The amplified voltage signal from

the negative sensor lead is VA. The resulting voltage is

calculated as follows:

VA= (1+R8/R6) * V4

= (1+10/1000) * V4

= (1.001) * V4

The amplified voltage signal from the positive sensor lead

is VB. This amplification adds a small gain to ensure that the

positive lead, V2, is always greater than the voltage output

from the negative sensor lead, V4. This ensures the linearity

of the differential voltage signal.

VB = (1+R7/R5) * V2 – (R7/R5) * Vcc

= (1+10/1000) * V2 + (10/1000)*(5V)

= (1.001) * V2 + .005V

The difference between the positive sensor voltage, VB, and

the negative sensor voltage, VA is calculated and amplified

with a resulting by a gain of 500.

VC= (R12/R11) * (VB – VA)

= (500K/1K) * (VB – VA)

= 500 * (VB – VA)

The output voltage, Vc, is connected to a voltage follower.

Therefore, the resulting voltage, Vc, is passed to an A/D pin

of the microcontroller.

The range of the A/D converter is 0 to 255 counts. However,

the A/D Values that the system can achieve are dependent on

the maximum and minimum system output values:

Count = (Vout – VRL) / ( VRH – VRL) x 255

where VXdcr = Transducer Output Voltage

Vrh = Maximum A/D voltage

Vlh = Minimum A/D voltage

Count (0mm H20) = (2.5 – 0) / (5.0 – 0) x 255 = 127

Count (40mm H20) = (5.0 – 0) / (5.0 – 0) x 255 = 255

Total # counts = 255 – 127 = 127 counts.

The resolution of the system is determined by the mm of

water that is represented by each A/D count. As calculate

above, the system has a span of 226 counts to represent water

level up to and including 40cm. Therefore, the resolution is:

Resolution = mm of water / Total # counts

= 400mm/127 counts = 3.1 mm per A/D count

Freescale Semiconductor, Inc.

For More Information On This Product,

Go to: www.freescale.com

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