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Motorola Sensor Device Data
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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 ifthe
sensor later experiences a wide temperature range or later
experience an offset shift. However, if auto–zero is performed
atthe 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.
F
Freescale Semiconductor, Inc.
n
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