MXA2500U Просмотр технического описания (PDF) - Unspecified

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MXA2500U

Ultra Low Noise, ±1 g Dual Axis Accelerometer with Analog Outputs

Unspecified 

60

45

30

15

0

-15

-30

-45

-60

10

100

Frequency - Hz

1000

Figure 6: Amplitude Frequency Response

COMPENSATION FOR ZERO G OFFSET CHANGE

OVER TEMPERATURE

In applications where a stable zero g offset is required, and

where the AC coupling external filter described earlier can

not be used, analog or digital temperature compensation

can be applied. The compensation requires individual

calibration because the magnitude of the zero g offset

change over temperature is different for each unit. To

compensate the change, a calibrated temperature dependent

signal equal in magnitude but with opposite polarity is

added to the accelerometer output. The circuit in Figure 7

shows a circuit example applying an analog linear

compensation technique. In this circuit the accelerometer

temperature sensor output is added to or subtracted from

the accelerometer output. The calibration sequence is: start

at room temperature with the 100K pot set so that its wiper

is at Vref. Next, soak the accelerometer at the expected

extreme temperature and observe the direction of the

change. Then set the switch to the non-inverting input if the

change is negative or vice versa. Finally, adjust the 100K

pot while monitoring the circuit output, until the zero g

offset change is removed.

Vref

10K

10K

SW SPDT

Tout

Aoutx or y 10K

100K

100K

100K

100K

+5V

+

-

100K

100K

Aoutx or y

zero g drift

compensated

Figure 7: Zero g Offset Temperature Compensation Circuit

Various digital compensation techniques can be applied

using a similar concept. Digital techniques can provide

better compensation because they can compensate for non-

linear zero g offset vs. temperature. A micro-controller or

micro-processor would perform the compensation. The

acceleration signal and the temperature signal would be

digitized using an analog to digital converter. Like in the

analog compensation, the first step is to test and

characterize the zero g change. The purpose of the

characterization is to create a look up table or to estimate a

mathematical representation of the change. For example,

the change could be characterized by an equation of the

form:

Change = a * Temperature 2 + b * Temperature + c

where a,b,c are unique constants for each accelerometer.

In normal operation the processor calculates the output:

Compensated Output = Acceleration – Change.

For a more detail discussion of temperature compensation

reference MEMSIC application note #AN-00MX-002.

TEMPERATURE OUTPUT NOISE REDUCTION

It is recommended that a simple RC low pass filter is used

when measuring the temperature output. Temperature

output is typically a very slow changing signal, so a very

low frequency filter eliminates erroneous readings that may

result from the presence of higher frequency noise. A

simple filter is shown in Figure 8.

8.2K

TOUT

MEMSIC

Accel.

0.1uF

Filtered TOUT

Figure 8: Temperature Output Noise Reduction

MEMSIC MXA2500U Rev A

Page 7 of 8

05/02

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