
M
sure sensor evaluation. Note that the resistor values
and PGA gain obtained from this evaluation will repre-
sent a starting point. The final compensated transducer
will likely use slightly different values. The required sen-
sor information is shown in Table 1, and can be used to
obtain the values for the parameters shown in Table 2.
Selecting R
ISRC
R
ISRC
programs the nominal sensor excitation current
and is placed between ISRC and V
SS
. Use a variable
resistor with a nominal starting value of:
where Rb(T1) is the sensor input impedance at temper-
ature T1 (usually +25°C).
Selecting R
STC
R
STC
compensates the FSO TC errors and is placed
between BBUF and ISRC. Use a variable resistor with
a nominal starting value of the following:
This approximation works best for bulk, micromachined,
silicon piezoresistive sensors (PRTs). Negative values
for R
STC
indicate unexpected sensor behavior that can-
not be compensated by the MAX1450 without addition-
al external circuitry.
Selecting PGA Gain Setting
Calculate the ideal gain using the following formula,
and select the nearest gain setting from Table 3.
SensorFSO can be derived as follows:
where S is the sensor sensitivity at T1, V
BDRIVE
is the
sensor excitation voltage (initially 2.5V), and
P is the
maximum pressure differential.
SensorFSO
S x V
1.5mV/V psi x 2.5V x 10 psi
0.0375V
x P
BDRIVE
=
=
=
R
R
x 500ppm/ C
TCR
TCS
65k x 500ppm/ C
2600ppm/ C
2100ppm/ C
65k
STC
ISRC
≈
≈
=
R
13 x Rb(T1)
13(5k )
65k
ISRC
≈
≈
=
Low-Cost, 1%-Ac c urate S ignal Conditioner
for Piezoresistive S ensors
6
_______________________________________________________________________________________
Table 1. Sensor Information
Table 2.CompensationComponents/Values
PARAMETER
SENSOR
DESCRIPTION
TYPICAL
VALUE
Rb(T)
Input/Output Impedance
5k
at +25°C
TCR
Input/Output Impedance
Tempco
2600ppm/°C
S(T)
Sensitivity
1.5mV/V psi at
+25°C
TCS
Sensitivity Tempco
-2100ppm/°C
O(T)
Offset
12mV/V at
+25°C
OTC
Offset Tempco
-1030 ppm-
FSO/°C
S(p)
Sensitivity Linearity Error as
% FSO BSLF (Best Straight-
Line Fit)
0.1% FSO
BSLF
P
MIN
Minimum Input Pressure
0 PSI
P
MAX
Maximum Input Pressure
10 PSI
PARAMETER
DESCRIPTION
R
ISRC
Resistor that programs the nominal sensor
excitation current
R
STC
Resistor that compensates FSO TC errors
A
PGA
Programmable-gain amplifier gain
OFFTC
Offset TC correction voltage, including its
respective sign bit
R
LIN
Resistor that corrects FSO linearity errors
(optional)
V
PRESSURE
P
MAX
P
MIN
4.5
0.5
FULL-SCALE (FS)
FULL-SPAN OUTPUT (FSO)
OFFSET
Figure 3. Typical Pressure-Sensor Output