DL200 Datasheet, PDF (350/670 Pages) Motorola, Inc – Sensor

AN1097
Freescale Semiconductor, Inc.
PRESSURE SENSOR CHARACTERISTICS
OP AMP CHARACTERISTICS
Figure 2 shows the differential output voltage of the
MPX2100 series at +25°C. The dispersion of the output
voltage determines the best tolerance that the system may
achieve without undertaking a calibration procedure, if any
other elements or parameters in the chain do not introduce
additional errors.
Vout (mV)
20
VS = 5 Vdc
TA = 25°C
For systems with only one power supply, the instrument
amplifier configuration shown in Figure 4 is a good solution to
monitor the output of a resistive transducer bridge.
The instrument amplifier does provide an excellent CMRR
and a symmetrical buffered high input impedance at both
non–inverting and inverting terminals. It minimizes the
number of the external passive components used to set the
gain of the amplifier. Also, it is easy to compensate the
temperature variation of the Full Scale Output of the Pressure
Sensor by implementing resistors “Rf” having a negative
coefficient temperature of –250 PPM/°C.
The differential–mode voltage gain of the instrument
amplifier is:
10
FULL–SCALE
5
OFFSET
0
–5
P
0
20
40
60
80
100 ^(kPa)
Figure 2. Spread of the Output Voltage versus the
Applied Pressure at 25°C
The effects of temperature on the full scale output and offset
are shown in Figure 3. It is interesting to notice that the offset
variation is greater than the full scale output and both have a
positive temperature coefficient respectively of +8.0
µV/degree and +5.0 µV excitation voltage. That means that
the full scale variation may be compensated by modifying the
gain somewhere in the chain amplifier by components
arranged to produce a negative TC of 250 PPM/°C. The dark
area of Figure 3 shows the trend of the compensation which
improves the full scale value over the temperature range. In
the area of 40 kPa, the compensation acts in the ratio of
40/100 of the value of the offset temperature coefficient.
Vout (f) ∆T
+85°C
–15°C
POSITIVE
FULL SCALE
VARIATION
OFFSET VARIATION
0
20
40
60
P
80
100
(kPa)
Avd
=
V1–V2
Vs2–Vs4
=
1 + ^{2 Rf}
Rg
(1)
+Vs
+
3
4
2
1
Rg ^{ÉÉÉÉÉÉ–} _{ÉÉÉÉ}^Rf _{ÉÉÉÉ}
V1
–
V2
+
0V
Figure 4. One Power Supply to Excite the Bridge
and to Develop a Differential Output Voltage
The major source of errors introduced by the op amp is
offset voltages which may be positive or negative, and the
input bias current which develops a drop voltage ∆V through
the feedback resistance Rf. When the op amp input is
composed of PNP transistors, the whole characteristic of the
transfer function is shifted below the DC component voltage
value set by the Pressure Sensor as shown in Figure 5.
The gain of the instrument amplifier is calculated carefully
to avoid a saturation of the output voltage, and to provide the
maximum of differential output voltage available for the A/D
Converter. The maximum output swing voltage of the
amplifiers is also dependent on the bias current which creates
a ∆V voltage on the feedback resistance Rf and on the Full
Scale output voltage of the pressure sensor.
Figure 3. Output Voltage versus Temperature. The
Dark Area Shows the Trend of the Compensation
3–204
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