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Motorola Sensor Device Data
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Figure 1. Positioning the Sensor’s Full–Scale Span within the A/D’s or Amplifier’s Dynamic Range
A/D HIGH REFERENCE OR HIGH
SATURATION LEVEL OF AMPLIFIER
FULL–SCALE
OUTPUT VOLTAGE
A/D LOW REFERENCE OR LOW
SATURATION LEVEL OF AMPLIFIER
ZERO PRESSURE
OFFSET VOLTAGE
S
V
A
A similar requirement that warrants the use of a dc level
shift is the prevention of the pressure sensor’s voltage from
extending into the saturation regions of the operational
amplifiers. This also would cause a nonlinearity in the sensor
output measurements. For example, if an op–amp powered
with a single–ended 5 V supply saturates near the low rail of
the supply at 0.2 V, a positive dc level shift may be required
to position the zero pressure offset voltage at or above 0.2 V.
Likewise, if the same op–amp saturates near the high rail of
the supply at 4.8 V, a negative dc level shift may be required
to position the full–scale pressure voltage at or below 4.8 V.
It should be obvious that if the gain of the amplifiers is too
large, the span may be too large to be positioned within the
4.6 V window (regardless of ability to level shift dc offset). In
such a case, the gain must be decreased to reduce the span.
THE TWO OP–AMP GAIN STAGE
TRANSFER FUNCTION
The transfer function of the two op–amp signal–conditioning
stage, shown in Figure 2, can be determined using nodal
analysis at nodes 1 and 2. The analysis can be simplified by
calculating the transfer function for each of the signals with the
other two signals grounded (set to zero), and then employing
superposition to realize the overall transfer function. As shown
in Figure 2, VIN2 and VIN1 are the differential amplifier input
signals (with VIN2 > VIN1), and VREF is the positive dc level
adjust point. For a sensor with a small zero pressure offset
and operational amplifiers powered from a single–ended
supply, it may be necessary to add a positive dc level shift to
keep the operational amplifiers from saturating near zero
volts.
Figure 2. The Two Operational–Amplifier Gain Stage
VIN2
VIN1
VREF
NODE 1
R1
R2
R4
R3
VCC
NODE 2
U1
VO
′
VO
U1
F
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