3–421
Motorola Sensor Device Data
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For example, if the evaluation board is being used under the
following conditions:
Vs = 3V
G1 = 101
G2 = 6
MPX2010 zero pressure offset = 0.3mV
At this supply voltage, VOFFSET can be calculated to be
6.7% x 3V = 0.2V. The voltage V, due simply to the zero
pressure sensor offset voltage of 0.3mV, can be calculated
from equation (1):
V1 = 0.3mV * 101 + 0.2V = 0.23V
The voltage after the second gain stage comes from
equation (2),
V2 = 6 x 0.23V = 1.38 V.
Therefore, before any pressure is applied to the sensor, a
1.38V DC signal will appear at V. Since the supply voltage is
3V, the available signal for actual pressure is 1.62 V. With a
total gain of G1 x G2 = 606, the largest raw pressure signal that
can be accurately measured would be 1.62V/606 = 2.67 mV.
For the MPX2010 family operating at Vs = 3V, this
corresponds to roughly 3.5 kPa.
The board lends itself well to system integration via an A/D
converter and microprocessor. For particular applications,
general knowledge of the expected pressure signal can aid in
choosing the proper customized gain. This will avoid op-amp
saturation and will also ensure that the full-scale output signal
is suitable for A/D conversion. To take another example,
suppose that a particular application has the following
constraints:
Supply Voltage, Vs = 5.0 V,
(thus VOFFSET = 6.7% x 5 = 0.335 V)
Sensor zero–pressure offset voltage, VZP = 0.3mV
Expected Pressure range = 0—2 kPa,
(corresponds to VSENSOR-MAX = 2.5mV @ 5V)
Desired maximum output range, V2MAX = 2V
(assume VMIN = 2V, V2MAX = 4V for reasonable A/D
resolution)
By manipulating equations (1) and (2) it can be shown that,
V2MAX = GT x VSENSOR–MAX
where GT is the total gain, equal to G1G2.
Thus GT = 2V/2.5mV = 800
To find G1 and G2, evaluate V2MIN at the zero pressure
condition.
V2MIN = G2 V1MIN,
But V1MIN = G1 VZP + VOFFSET
Thus V2MIN = GT VZP + G2 VOFFSET
Solving for G2, G2 = (V2MIN – GT VZP)/ VOFFSET
numerically, G2 = (2V — (800x.0003V))/.335V
G2 = 5.2, and G1 = GT /G2 = 152
BOARD LAYOUT & CONTENT
The low-pressure evaluation board has been designed
using standard components. The only item that requires
careful selection is the operation amplifier IC. Because the
selected gain may be relatively high as in the previous
example, it is essential that this device have a low offset
voltage. A device with a typical voltage offset of 35 mV has
been selected. Even with a gain of 1500, this will result in a
52mV offset. Table 3 is a parts list for the board layout shown
in Figure1.
Table 3. Parts List
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Ref.
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Qty.
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Vcc Cap
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Description
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Motorola
Generic
Vendor
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MPX2010
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10 Kpa
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C1
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2nd stage cap
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1 uF
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D1
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LED
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1/4 W Resistor
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Devices
R2,R3,
R5,R6
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1/4 W Resistor
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1/4 W Resistor
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R9
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Bourns
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Potentiometer
1/4 W Resistor
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1K
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R12
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1/4 W Resistor
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3 Pos Connector
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2K
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7
Jumper
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Phoenix
JP1 –
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MKDS1
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