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參數資料
| 型號： | SX15GD2 |
| 英文描述： | Fast Recovery Rectifier Diodes |
| 中文描述： | DRUCKSENSOR |
| 文件頁數： | 5/10頁 |
| 文件大?。?/td> | 122K |
| 代理商： | SX15GD2 |
SX - Series
PRESSURE SENSORS
March 1998/052
Aubinger Weg 27, 82178 Puchheim, Germany
Phone 0049 - (0) 89 80 08 30, Fax 0049 - (0) 89 8 00 83 33
http://www.sensortechnics.com
APPLICATION INFORMATION
General
The SX family of pressure sensors
functions as a Wheatstone bridge.
When pressure is applied to the device (see
Figure I) the resistors in the arms of the
bridge change by an amount
.
Figure I. Button Sensor Bridge
Schematic
The resulting differential output voltage
V0, is easily shown to be VO= VB x
.
Since the change in resistance is directly
proportional to pressure, VO can be written
as:
VO = S x P x VB ± VOS
Where: VO is the output voltage in mV
S is the sensitivity in mV/V per psi
P is the pressure in psi
VB is the bridge voltage in volts.
VOS is the offset error (the differential
output voltage when the applied pressure
is zero). The offset voltage presents little
problem in most applications, since it can
easily be corrected for in the amplifier
circuitry, or corrected digitally if a
microprocessor is used in the system.
(1)
Temperature Effects
In this discussion, for simplicity of notation,
the change of a variable with temperature
will be designated with a dot () over the
variable. For example,
change in sensitivity
change in temperature
S
=
δ
S
δ
T
From equation (1), and ignoring the VOS
term, it in seen that for a given constant
pressure, the output voltage change, as a
function of temperature*, is:
VO = SPVB
Thus, in order for output voltage to be
independent of temperature, the voltage
across the bridge, VB, must change with
temperature in the "opposite direction” from
the sensitivity change with temperature.
From the typical curves for the temperature
dependence of span (span = S x P x VB),
(2)
=
it can be seen that the sensitivity change
with temperature is slightly non-linear and
can be correlated very well with an
equation of he form:
S = SO[(1 - TD) +
ρ
TD2]
where TD is the temperature difference
between 25°C and the temperature of inte-
rest, SO is the sensitivity at 25°C, and beta
() and rho (
ρ
) are correlation constants.
Fortunately, between 0°C and 70°C the
change in sensitivity with tem-perature is
quite linear, and excellent results can be
obtained over this temperature range by
ignoring the second-order temperature
dependent term. Operating outside the 0°C
and 70°C temperature range will require a
more rigorous mathematical approach and
the use of non-linear compensating cir-
cuitry, if accuracy of better than ±1% is re-
quired. Because the majority of SX appli-
cations fall within the 0°C to 70°C operating
temperature range, the discussion and
circuit designs given here will ignore the
non-linear effects.
Thus:
(3)
S = SO (1 - TD)
Substituting equation (4) into equation (1)
and ignoring VOS, it can be shown that the
necessary bridge voltage, VB, will be of
the form:
=VBO [(1 - TD + (TD)2+...)]
(4)
VBO
(1-TD)
where VBO is the bridge voltage at 25°C.
This equation is again non-linear.
However, for the temperature range of
interest, and since is small (0.215%/°C
from the electrical tables), the above
expression can be approximated by:
VB=VBO [1 +TD]
with less than 1% error. Thus to com-
pensate for a negative 2150 ppm/°C
sensitivity change with temperature, the
bridge voltage should increase with
temperature at a rate of +2150 ppm/°C.
The above value of bridge voltage
change will be used in the circuit
discussions that follow. That is to say, the
required change in terms of ppm/°C is:
= +2050 ppm/°C
( )
The bridge input resistance*, RB also
changes with temperature and is quite linear
in the temperature range of interest. The
bridge resistance has a temperature
coefficient of typically:
= +750 ppm/°C
( )
This term enters into several compensation
VB =
circuit equations, particularly when the
bridge excitation is from a constant current
source.
To summarize, the following list indicates
how the sensor variables can be accommo-
dated
Full-scale span from device to device.
Make the gain adjustment in the op amp
circuitry
Temperature coefficient of span:
1) temperature compensate the bridge
or
2) temperature compensate the op amp
gain
Offset voltage:
Adjustment in op amp circuitry
Offset voltage temperature coefficient:
Usually can be ignored. For more precise
design requirements, contact the factory
for information on how to compensate
for this term.
Bridge Compensation Circuits
Although thermistors can be used to
temperature compensate the bridge (and
in fact will be required for extended tempe-
rature operation), they are inherently non-
linear, difficult to use in volume production,
and more expensive than the circuit appro-
aches shown here, which use inexpensive
semiconductor devices The circuits shown
have been designed to incorporate a mini-
mum number of adjustments and allow
interchangeability of devices with little
variation from device to device.
In general, equations for the bridge voltage
and its change with temperature are given
to enable the user to modify or adjust the
circuitry as required.
1.Diode String
(Figure II)
For systems using 6V supplies, this method
of compensating for the effects of span
over temperature is the lowest cost solution
The diodes are small signal silicon diodes,
such as 1N914 or 1N4148, and do not have
to be matched.
Figure II. Diode String Span
Compensation
VB
RB
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