SX Series
Pressure sensors
6/10
July 2008 / 052
www.sensortechnics.com
( )[ ( )]
IO
RB
IO
RB
( )
a)
VB =
α (VS + IO R2)
•
•
b)
=
(1 -
α)+
1-
α
c)
α
=
•
•
d)
= 3360 ppm/°C,
=+750ppm/°C
e)
IO=
The design steps are straight forward:
1) Knowing VS and the desired bridge
voltage VB, solve equation (b) for α.
2) Now, solve equation (c) for R2,
letting RB = 4650Ω.
3) Solve equation (a) for IO.
4) Find R1 or its nearest 1% tolerance
value from equation (e).
Table II gives specific 1% resistor values in
ohms, for several popular system voltages.
For best results, the resistors should be 1%
metal film with a low temperature coefficient.
Table II. Selected R values vs VS for
figure IV
VS
VB
R1(Ω)R2(Ω)
5V
3V
147
11.0k
6V
4V
105
9.53k
9V
6V
68.1
9.53k
12V
9V
43.2
8.25k
15V
10V
41.2
9.53k
Amplifier design
There are hundreds of instrumentation
amplifier designs, and the intent here will
be to briefly describe one circuit which:
•
does not load the bridge
•
involves minimal components
•
provides excellent performance
Amplifier adjustment procedure
1. Without pressure applied,
(a) Short points A and B together as
shown in Figure V. Adjust the 1 k
common-mode rejection (CMRR)
pot until the voltage at test point (Tp)
Vx is equal to the voltage at test
point (Tp) VR.
This is easily accomplished by
placing a digital voltmeter between
these test points and adjusting for
0.000.
VB
RB
IO
VS
VB
RB
IO
VB
( ) ( )
67.7 mV
R1
RB
R2 + RB
( )
a) VB=VS-4φ
b) VB
VB
VS
φ
c)
= -2500 ppm/°C for silicon diodes
Figure II. Equations
For example, solving equation (b) for VB/
VB when
VS = 6.0 V
φ = 0.7 V
Yields:
= 2188 ppm/°C
Since the sensor’s span changes with tem-
perature at -2150 ppm/°C, this technique will
typically result in an overall negative TC of
38 ppm/C. This error is acceptable in most
applications.
For operation with VS above 6V, it is recom-
mended to use the transistor or constant
current compensation technique.
2.
Transistor compensation network
Figure III uses a single transistor to simulate
a diode string, with the equations as shown.
The values shown in Table I were found to
give excellent results over 0°C to 70°C.
Again, if precision temperature compen-
sation is required for each device, the fixed
value resistors shown for R1 in Table I can
be replaced by a 3.24k resistor in series with
a 1k pot. Then, each devices temperature
compensation can be individually adjusted.
Figure III. Transistor/Resistor
span TC compensation
-4
( )
( )
(
)
φ
φ
( )
φ
φ
=
VB
VB
•
a) VB = VS - α φ
b)
= -
x
-
α
c)
α
=1
+
d)
φ
φ
Table I. Selected R values vs VS for
figure III
VS
R1 (Ω)R2 (Ω)
5V
3.32k
1.43k
9V
4.02k
806
12V
4.22k
604
3.
Constant current excitation
(Figure IV)
The circuits shown in Figures II and III,
although simple and inexpensive, have one
drawback in that the voltage across the
bridge is determined by the compensation
network. That is, the compensation network
is determined and what voltage is “leftover"
is across the bridge. The circuit of Figure
IV solves this problem and allows the bridge
voltage to be independently selected. In
Figure IV, the bridge is driven from a
constant current source, the LM334, which
has a very well known and repeatable
temperature coefficient of +3300 ppm/°C.
This temperature coefficient (TC), in
conjunction with the TC of the bridge resis-
tance, is too high to compensate the
sensitivity TC, hence resistor R2 is added
to reduce the total circuit TC.
The basic design steps for this method
of temperature compensation are shown
below. However, please refer to SenSym’s
Application Note SSAN-16 for details on the
temperature compensation technique.
Figure IV. Constant current span TC
Compensation
( )
( ) (
)
VB
φα
VB
φ
VS
φ
R1
R2
=
-2500 ppm/°C
( )
•
APPLICATION INFORMATION (cont.)
•
•
••
Vietnamese ▼
English
Chinese
German
Japanese
Russian
Korean
Spanish
French
Italian
Portuguese
Polish
Vietnamese
Indian
Mexican
British
New Zealand
