MPXM2010D datasheet(2/24 Pages) FREESCALE

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AN1950

Sensors

Freescale Semiconductor

Amplifier Induced Errors

The sensor output needs to be amplified before being

inputted directly to the microcontroller through an eight-bit A/D

input pin. To determine the amplification requirements, the

pressure sensor output characteristics and the 0-5 V 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 0–5.0 V.

The pressure sensor has a possible offset of ±1 mV at the

minimum rated pressure. To avoid a nonlinear response when

a pressure sensor chosen for the system has a negative offset

(VOFF), we added a 5.0 mV offset to the positive sensor output

signal. This offset will remain the same regardless of the

sensor output. Any additional offset the sensor or op-amp

introduces is compensated for by software routines 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 12.5 mV with a 5.0 V supply since the

full-scale output of the sensor changes linearly with supply

voltage. This system will have a maximum pressure of 4 kPa

at 40 cm of water. At a 5.0 V supply, we will have a maximum

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

maximum sensor output to 5.0 V, the following gain is needed:

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

and Initial Offset) = 5.0 V / (0.005 V + 0.005) = 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)*(5.0 V)

= (1.001) * V2 + 0.005 V

The difference between the positive sensor voltage, VB,

and the negative sensor voltage, VA is calculated and

amplified with a resulting gain of 500.

VC = (R12/R11) * (VB – VA)

= (500 K/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 (0 mm H20) = (2.5 – 0) / (5.0 – 0) * 255 = 127

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

Total # counts = 255 – 127 = 127 counts.

The resolution of the system is determined by the mm of

water represented by each A/D count. As calculated above,

the system has a span of 226 counts to represent water level

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

Resolution = mm of water / Total # counts

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

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

Characteristic

Symbol

Min

Typ

Max

Unit

Pressure Range

POP

0—

kPa

Supply Voltage

—10

Vdc

Supply Current

—6.0

–

mAdc

Full Scale Span

VFSS

Offset

Voff

-1.0

—

1.0

Sensitivity

DV/DP

—

2.5

—

mV/kPa

Linearity

—

-1.0

—

1.0

%VFSS

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