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Motorola Sensor Device Data
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THE METHODOLOGY TO OPTIMIZE
PERFORMANCE
The methodology starts with defining all the known
parameters. The parameters with an asterisk (*) are specified
at 25
°
C.
Resolution
MaxFSS (*)
MinFSS (*)
TCVFSS (*)
MaxSensOff (*)
MinSensOff (*)
TCVoff
Vlo
Vhi
VREF
Vtol
MinTemp
Maxtemp
= Desired system resolution
= Maximum full–scale voltage span of
=
the pressure sensor
= Minimum full–scale voltage span of
=
the pressure sensor
= The maximum temperature coefficient
=
of the sensor’s full–scale voltage span
= The maximum zero pressure offset
=
voltage of the pressure sensor
= The minimum zero pressure offset
=
voltage of the pressure sensor
= The sensor’s maximum temperature
=
coefficient of offset voltage
= The low saturation level of the amplifier
=
or low reference voltage of an A/D
=
(whichever is most limiting case)
= The high saturation level of the
=
amplifier or the high reference voltage
=
of an A/D (whichever is most limiting
=
case)
= The reference voltage for positive dc
=
voltage level shifting
= The voltage regulator tolerance
= The application’s minimum operating
=
temperature
= The application’s maximum operating
=
temperature
These parameters are either chosen for the application
(e.g., system resolution) or can be determined from the
sensor’s data sheet. Tables 1 and 2 provide the necessary
information for the design examples presented here.
Note:The data in Tables 1 and 2 are scaled for a 5 V supply
voltage, whereas the MPX10 and MPX2010 data sheets are
specified at a 3 V and 10 V supply voltage, respectively.
The following steps outline the methodology that will be
applied to the MPX10 in the first design example and then
applied to the MPX2010 in the second design example.
1. Determine/choose the required Resolution for the
system.
2. Calculate the number of steps required for the chosen
resolution. The resolution determines the number of
steps into which the pressure signal needs to be broken
[see Figure 3 where an 8–bit A/D (255 steps of
resolution) is assumed]. A conservative approach to
determining this number of steps is to assume that with
an A/D, the digital quantization of the pressure signal can
be plus or minus one step. Therefore, assume that it
takes twice the number of steps previously determined to
resolve a given minimum incremental pressure. The
number of steps for the chosen resolution is
Number of Steps
2
·
100
Resolution
The scaling factor of 100 in the numerator converts the
resolution from a percentage to a decimal fraction.
A/D HIGH REFERENCE
A/D LOW REFERENCE
STEP 255
STEP 0
STEP 127
A
Figure 3. The 255 Digital Steps of an 8–Bit A/D
3. Calculate the minimum amplified sensor span (defined as
the Minimum Required Span — see Figure 4) required for
this resolution requirement. Using an 8–bit A/D with a 5 V
window where one step equals 19.6 mV (for the nominal
regulator voltage), the minimum amplified sensor span is
(Number of Steps)
·
(19.6 mV)
Minimum Required Span
A/D HIGH REFERENCE
A/D LOW REFERENCE
ZERO PRESSURE
OFFSET VOLTAGE
FULL–SCALE
OUTPUT VOLTAGE
MINIMUM
REQUIRED
SPAN
MAXIMUM
SPAN
A/D’S
DYNAMIC
RANGE
Figure 4. The Minimum Required Span for the
Required Resolution and the Maximum Span
Due to Sensor Span Variations
4. Calculate the amplifier’s gain. The gain must be large
enough to achieve, over the entire distribution of sensor
spans, the Minimum Required Span. Therefore, this gain
is calculated using the smallest pressure sensor voltage
span, MinFSS. By using the worst case smallest pres-
sure sensor voltage span to calculate the gain, the
Minimum Required Span (the minimum span that will
achieve the resolution requirement) is guaranteed for the
entire distribution of sensor spans. The worst case
minimum full–scale sensor span will occur at the hottest
temperature, Maxtemp, in the application (not exceeding
the operating temperature of the sensor), since the span
decreases with increasing temperature (TCVFSS is
negative).
Gain
Minimum Required Span
[MinFSS]
[1
The term [1 + TCVFSS
(Maxtemp – 25)] is the temperature
effect on the span.
TCVFSS
(Maxtemp–25)]
F
Freescale Semiconductor, Inc.
n
.