AN1668

Sensors

4

Freescale Semiconductor

nulled out by auto-zeroing, these errors must be accounted for

when setting the system gain (refer to AN1556 for more

details). This forces the total span of the system to be smaller,

because we must reserve an extra headroom from the total

span to account for amplifier and A/D variations (i.e., amp. sat.

voltage, power supply variation, A/D quantization error, and

gain errors). If these errors are not accounted for, it could, for

example, result in non-linearity errors if the sensor span or

offset error causes the amplified output of the sensor to reach

the saturation voltage of the amplifier.

As an example, a MPX2010 sensor system is designed

RMS error. The system uses a +5.0 V ±5% linear regulated

power supply, a MC33272 dual op-amp and a 1% resistors.

Table 3 shows the resulting specification and component

values for the system based on MPX2010 sensor.

Note that the error due to system resolution is higher for the

MPX2010 solution (± 2 bit A/D accuracy). This is because the

MPX2010 span is limited as discussed above. Also, this

accuracy assumes that the amplifier does not induce

significant errors. As noted MPXV4006G sensor has better

overall accuracy. The system resolution is very good because

of its large span (4.6 V versus 3.0 V typical).

SUMMARY

Several washing machine solutions were examined. The

MPX10/12 solution can be expensive in terms of additional

support circuitry and the added time and labor involved during

the calibration procedure. The MPX2010 is good alternative

for high volume manufacturing because is already calibrated.

With this solution, however, the system amplifier design must

be chosen and designed carefully in order to minimize the

system error. This is a consideration when deciding to

implement a high accuracy solution with the MPX2010

because the cost of the system will go up.

The MPXV4006G solution is geared towards high volume

manufacturing because trimming, compensation and

amplification is already on board. Besides the system

simplicity and using less component, the resolution and

overall accuracy of this solution is better than the MPX2010

solution. In some cases, less components can actually

improve the reliability and manufacturability the system.

REFERENCES

[1] Benchmark of Washing Machine Mechanical Sensor,

Jack Rondoni, Freescale Semiconductor, Inc. Internal

Document.

[2] Mechanical Sensor Characterization, Ador Reodique,

Freescale Internal Document.

[3] AN1551 Low Pressure Sensing with the MPX2010

Pressure Sensor, Jeff Baum, Freescale Application Note.

[4] AN1636 Implementing Auto-Zero for Integrated Pressure

Sensors, Ador Reodique, Freescale Application Note.

[5] AN1556 Designing Sensor Performance Specifications

for MCU-based Systems, Eric Jacobsen and Jeff Baum,

Freescale Application Note.

Table 3. MPX2010 Sensor System Values

MPX2010 Sensor Design

Parameter

Description

Value

Units

Reg Power Supply

5

V

Differential Gain

Gain

433

V/V

Vout_FS

Full Scale Span

3.02

V

Offset Reference

0.66

V

Parts List

U1A,U1B

MC33272 Op-amp

R1

Gain Resistor

39.2K

Ω

R2

Gain Resistor

90.9

Ω

R3

Gain Resistor

909

Ω

R4

Gain Resistor

392K

Ω

R + S1

Level Shift Resistor

1K

Ω

R + S2

Level Shift Resistor

150

Ω

X1

MPX2010

Table 4. Performance Comparison between MPX2010 and MPXV4006G Solution

Error Contribution

MPX2010 Solution Error

MPXV4006G Solution Error

Max Sensor Error

7.19433.0018

System Resolution (A/D + Amplification)

1.30

8

0.80

5

System Error (Sensor + A/D + Amplification)

7.3

44

3.10

19

System Error with Auto-Zero

4.6

28

t3

t19