3–206
Motorola Sensor Device Data
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The result is twice a differential conversion. As
demonstrated below, all errors from the instrument amplifier
are cancelled. Other averaging techniques may be used to
improve the result, but the appropriated algorithm is always
determined by the maximum bandwidth of the input signal
and the required accuracy of the system.
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1
2
4
3
PRESSURE
SENSOR
SYSTEM
MPX2100AP
+ 5 V
I/O
0 V
V1
Rf
Rg
Rf
V2
MC68HC05B6
CH1
P
CH2
VRH
VDD
VRL
VSS
Figure 9. Two Channel Input and One Output Port Are Used by the MCU
MC33078
+
–
SYSTEM CALCULATION
Sensor out 2
Vs2 = a (P) + of2
Sensor out 4
Vs4 = b (P) + of4
Amplifier out 1
V1 = Avd (Vs2 + OF1)
Amplifier out 2
V2 = Avd (Vs4 + OF2)
Inverting of the amplifier input
V1x = Avd (Vs4 + OF1)
V2x = Avd (Vs2 + OF2)
Delta = V1–V2
= Avd * (Vs2 of OF1) – Avd * (Vs4 + OF2)
1st differential result
Deltax = V2x–V1x
= Avd * (Vs2 + OF2) – Vdc * (Vs4 + OF1)
2nd differential result
Adding of the two differential results
VoutV = Delta + Deltax
= Avd*Vs2 + Avd*OF2 + Avd*OF2 – Avd*OF1
+ Avd*OF1 – Avd*OF2 + Avd*OF2 – Avd*OF1
= 2 * Avd * (Vs2 –Vs4)
= 2 * Avd * [(a (P) + of2) – (b (P) + of4)]
= 2 * Avd * [V(P) + Voffset]
There is a full cancellation of the amplifier offset OF1 and
OF2. The addition of the two differential results V1–V2 and
V2x–V1X produce a virtual output voltage VoutV which
becomes the applied input voltage to the A/D converter. The
result of the conversion is expressed in the number of counts
or bits by the ratiometric formula shown below:
count = VoutV*
255
VRH–VRL
255 is the maximum number of counts provided by the A/D
converter and VRH–VRL is the reference voltage of the
ratiometric A/D converter which is commonly tied to the 5.0 V
supply voltage of the MCU.
When the tolerance of the full scale pressure has to be in the
range of
±
2.5%, the offset of the pressure sensor may be
neglected. That means the system does not require any
calibration procedure.
The equation of the system transfer is then:
count = 2
*
Avd
*
V(P)
*
51/V where:
Avd is the differential–mode gain of the instrument amplifier
which is calculated using the equation (1). Then with Rf = 510
k
and Rg = 9.1 k
Avd = 113.
The maximum counts available in the MCU register at the
Full Scale Pressure is:
count (Full Scale) = 2
*
113
*
0.02 V
*
51/V = 230
knowing that the MPX2100AP pressure sensor provides
20 mV at 5.0 excitation voltage and 100 kPa full scale
pressure.
The system resolution is 100 kPa/230 that give 0.43 kPa per
count.
MC68HC05B6
VRH
CH1
P
CH2
VRL
VSS
VDD
+ 5 V
0 V
I/O
FINE
CAL.
Figure 10. Full Scale Output Calibration Using the
Reference Voltage VRH–VRL
F
Freescale Semiconductor, Inc.
n
.