3–420
Motorola Sensor Device Data
For More Information On This Product,
Go to: www.freescale.com
Table 1 shows the jumper settings for both analog and
switches outputs.
Table 1. Output Jumper Settings
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For the switch output option, it is desirable to apply some
hysteresis on the output signal to make it relatively immune to
potential noise that may be present in the voltage signal as it
reaches and passes the threshold value. This is
accomplished with feedback resistor R10. From basic
op-amp theory, it can be shown that the amount of hysteresis
is computed as follows:
Output
JP4
JP5
JP6
JP7
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Analog
Switch
Out
In
In
Out
In
Out
Out
In
VH = Vout *[1–(10 / ( R10 + R pot-eff))]
Where:
– VH is the output voltage attenuation, due to hystere-
sis, in volts
– Vout is the output voltage (railed hi or low)
– R10 is the feedback resistor, = 50K
– Rpot-eff is the effective potentiometer resistance
VH may vary depending on the particular value of the
potentiometer.
Figure 5a. Output Transition without Hysteresis
Figure 5b. Output Transition with Hysteresis
To take an example, suppose that the supply voltage, Vs is
5 volts, and the threshold is set to 60 percent of Vs, or 3 volts.
This corresponds to one leg of the 1K potentiometer set to
0.4K while the other is set to 0.6K. Thus the effective pot
resistance is 0.4K // 0.6K = 0.24K.
Therefore,
VH = 5V* [1– (50K/(50K + 0.24K))] = 24 mV.
Under these conditions, V signals passing through the
threshold will not cause Vout to oscillate between Vs and
Ground as long as noise and signal variations in V are less
than 24mV during the transition. Figure 5. Illustrates the
benefit of having a hysteresis feedback resistor.
GAIN CUSTOMIZATION
The low-pressure evaluation board comes with default
gains for both G1 and G2. G1 is factory set at 101, while G2
is set to 1. Jumpers JP1, JP2 and JP3 physically connect the
resistors that produce these default gains. Three resistor
sockets (R11, R41 and R51) are provided in parallel with R1,
R4 and R5, respectively. By removing jumpers JP1,JP2 and
JP3, and soldering different resistor values in the appropriate
sockets, different gain values can be achieved. The limit on
the largest overall gain that can be used is determined by
op–amp saturation. Thus if gain values are chosen such that
the output would be larger than the supply voltage, then the
op–amp would saturate, and the pressure would not be
accurately reflected. Table 2 outlines the jumper settings for
customizing the gain.
Table 2. Resistor and Jumper Settings for Gain
Customization
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Set
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Set
load
load
load
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load
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User
Set
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User
Set
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no
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load
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load
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load
Out
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Out
R11=R41
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load
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Out
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DESIGN CONSIDERATIONS
Since the evaluation board is primarily intended for
low–pressure gage and differential applications, large gain
values can be utilized for pressures less than 1.0 kPa. For
example if G1 is set to 101, and G2 set to 6, then the total gain
is 606.
Inherent in the MPX2010 family of pressure sensors is a
zero-pressure offset voltage, which can be up to 1 mV. This
offset is amplified by the circuit and appears as a DC offset at
Vout with no pressure applied. The op-amp also has a voltage
offset specification, though for the recommended op-amp this
value is small and does not contribute significantly to the Vout
offset.
F
Freescale Semiconductor, Inc.
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