MPXM2102AS datasheet(523/670 Pages) MOTOROLA

523 / 670 page

AN1636

3–377

Motorola Sensor Device Data

www.motorola.com/semiconductors

IMPROVEMENT ON OFFSET ERROR

In the following calculations, we will illustrate how auto–zero

will improve the offset error contribution. We will use the

MPXV4006G interfaced to an 8–bit A/D as an example. When

auto–zero is performed, the offset errors are reduced and the

resulting offset errors are replaced with the error (due to

resolution) of the A/D. We can categorize the offset error

contributions into temperature and calibration errors.

Temperature Coefficient of Offset Error

The offset error due to temperature is due to Temperature

Coefficient of Offset, or TCO. This parameter is the rate of

change of the offset when the sensor is subject to

temperature. It is defined as:

TCO = (

∆VOFF/∆T).

The MPXV4006G has a temperature coefficient of offset

(normalized with the span at 25

°C) of:

∆TCO = (∆VOFF/∆T)/VFS@25°C = 0.06% FS/°C.

As an example, if the sensor is subjected to temperature

range between 10

°C and 60°C, the error due to TCO is:

∆TCO = (0.06% FS/C)*(60°C * 10°C) = "3.0% FS.

Offset Calibration Errors

Even though the offset is laser trimmed, offset can shift due

to packaging stresses, aging and external mechanical

stresses due to mounting and orientation. This results in offset

calibration error. For example, the MPXV4006G data sheet

shows this as:

VOFF MIN = 0.100 V,

VOFF TYPICAL = 0.225 V and VOFF MAX = 0.430 V.

We can then calculate the offset calibration error with

respect to the full scale span as:

∆VOFF MIN,MAX =

(VOFF TYPICAL *VOFF MIN,MAX)/VFS.

This results in the following offset calibration error,

∆VOFF MIN = 2.7% FS and

∆VOFF MAX = 4.5% FS.

A/D Error

As mentioned above, we can reduce offset errors

(calibration and TCO) when we perform auto–zero. These

errors are replaced with the A/D error (due to its resolution),

∆OFFSETAUTOZERO = ∆TCO + ∆OFFSET = ∆A/D.

Typically, a sensor is interfaced to an 8–bit A/D. With the A/D

reference tied to VRH = 5 V and VRL = 0 V, the A/D can resolve

19.6 mV/bit. For example, the MXPV4006G has a sensitivity

of 7.5 mV/mmH20, the resolution is therefore,

A/DRESOLUTION = 19.6 mV/bit)/(7.5 mV/mmH20)

= 2.6 mmH20/bit.

Assuming +/

*1 LSB error, the error due to digitization and

the resulting offset error is,

∆A/D = ∆OFFSETAUTOZERO = 2.6 mmH20/612 mmH20

=+/

* 0.4% FS.

It can be seen that with increasing A/D resolution, offset

errors can be further reduced. For example, with a 10–bit A/D,

the resulting offset error contribution is only 0.1% FS when

auto–zero is performed.

If auto–zero is to be performed only once and offset

correction data is stored in non–volatile memory, the TCO

offset error and calibration error will not be corrected

if the

sensor later experiences a wide temperature range or later

experience an offset shift. However, if auto–zero is performed

at the operating temperature, TCO error will be compensated

although subsequent offset calibration error will not be

compensated. It is therefore best to auto–zero as often as

possible in order to dynamically compensate the system for

offset errors.

CONCLUSION

Auto–zero can be used to reduce offset errors in a sensor

system. This technique can easily be implemented when an

integrated pressure sensor is interfaced to an A/D and a

microcontroller. With a few lines of code, the offset errors are

effectively reduced; the resulting offset error reduction is

limited only by the resolution of the A/D.

Freescale Semiconductor, Inc.

For More Information On This Product,

Go to: www.freescale.com

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