General Description
The MAX1450 evaluation kit (EV kit) demonstrates sili-
con piezoresistive sensor calibration and temperature
compensation using the MAX1450. The kit includes an
assembled and tested PC board with a Lucas
NovaSensor
®
pressure sensor calibrated at room tem-
perature. It also supports several popular pressure-sen-
sor packages. The board uses multiturn potentiometers
and configuration switches to calibrate and tempera-
ture-compensate a sensor to 1% accuracy.
Features
o
Proven PC Board Layout
o
Convenient Test Points Provided On-Board
o
Includes Calibrated (at room temperature)
Lucas NovaSensor Pressure Sensor
o
Fully Assembled and Tested
o
Supports Many Popular Sensor Packages
Quick Start
Use the quick-start procedure to evaluate initial factory
calibration accuracy or to modify the calibration set-
tings. Before recalibrating the MAX1450, you should
test the precalibrated EV board. Supply power to the
PC board and measure the output voltage as a function
of pressure and temperature. As shipped from the fac-
tory, the EV kit has been calibrated at room tempera-
ture but not compensated over temperature.
Required Equipment
· Precision regulated power supply capable of providing
+5.000V
· Multimeter with at least five significant digits
· 015psi gauge pneumatic pressure controller/calibrator
Initial Setup
The four-pin sensor connector (P2) can be used to
probe the four sensor nodes: sensor excitation (IN+),
sensor ground (IN-), sensor positive output (OUT+), and
sensor negative output (OUT-). This connector may also
serve as a means of wiring to an alternate sensor.
Connector P2 allows the user to supply power to the
board, and to measure output voltage and sensor exci-
tation voltage.
Room Temperature Bench Test
The board's output is ratiometric to the supply; there-
fore, supply voltage must be set accurately to minimize
measurement error.
Evaluates: MAX1450
MAX1450 Evaluation Kit
________________________________________________________________ Maxim Integrated Products
1
19-1611; Rev 0; 1/00
Component List
PART
MAX1450EVKIT
-40°C to +125°C
TEMP. RANGE
IC PACKAGE
20 SSOP
For free samples & the latest literature: http://www.maxim-ic.com, or phone 1-800-998-8800.
For small orders, phone 1-800-835-8769.
Ordering Information
Five-position SPST dip switch
1
SW1
Open
0
R6, R8R11
1M
±5% resistors (1206)
5
R1R5
DESIGNATION
10-pin headers
2
P3, P4
4-pin headers
2
P1, P2
6.2V, 500mW, surface-mount zener
diode
1
D1
0.01µF ceramic capacitor
1
C4
0.1µF ceramic capacitors
3
C1, C2, C3
DESCRIPTION
QTY
MAX1450 EV kit data sheet
1
None
PC board
1
None
Maxim MAX1450CAP (20-pin SSOP)
1
U1
Sensor site 2
Sensym SDX and ISO series,
IC sensors LP series, or
Lucas NovaSensor NPI series
0
S2
Sensor site 1
Lucas NovaSensor NPH-8-100GH
(TO-8, 100kPa gauge)
Other possible sensors:
Sentir AP-301 (modified TO-5) or
IC sensors models 10/20/30/40 (TO-8)
1
S1
MAX1450 data sheet
1
None
Open
0
JU1, JU2
50k
trim potentiometers
3
VR1, VR2, VR3
500k
trim potentiometer
(supplied but not mounted)
1
VR4
100k
trim potentiometer
1
VR5
NovaSensor is a registered trademark of Lucas Varity.
Evaluates: MAX1450
MAX1450 Evaluation Kit
2
_______________________________________________________________________________________
Connect the negative power-supply terminal to the ana-
log connector pin labeled VSS. Connect the positive
power-supply terminal to the analog connector pin
labeled VDD. Connect the DVM to the analog connector
pin labeled VOUT; the ground return should be connect-
ed to the VSS pin.
IMPORTANT:
To avoid problems with ground loops and
noise, connect all equipment to the same AC circuit and
use one common earth ground
. If the power supply
has a programmable current limit, set it to about 10mA.
Adjust the supply voltage to +5.000V, measuring the
voltage at test point VDD with respect to test point VSS.
There should be no connection to the sensor pressure
port. Since the sensor supplied is a gauge type, the out-
put voltage at connector P2 should read approximately
0.5V.
Room Temperature Pressure Test
Carefully remove the plastic sensor protector (if sup-
plied) and connect a silicone pressure tube to the sen-
sor pressure port. Grasp the sensor, not the PC board,
while fitting the tube in place. Perform any required
pressure controller initialization/calibration procedures,
then vent the system. At 0psig, the output voltage
should read between 0.49V and 0.51V. Perform a few
pressure cycles (015psi with supplied sensor) to mini-
mize hysteresis effects. Apply full-scale pressure and
confirm that the output reads between 4.49V and 4.51V.
Test at intermediate pressures to measure for pressure
linearity errors.
Detailed Description
of Hardware
The MAX1450 EV kit performs analog calibration and
temperature compensation of a silicon piezoresistive
sensor. The board is shipped fully assembled and tested,
with a calibrated Lucas Novasensor. Three sensor con-
nection sites are provided: S1, S2, and P1. The board
operates with a 5V nominal supply voltage. Since its
output is ratiometric (proportional) to the supply voltage,
it is critical to maintain a precise supply voltage during
test. The nominal calibrated output voltage range will
be 0.5V (at minimum pressure) to 4.5V (at maximum
pressure). Other output voltage ranges are also possible,
within the common-mode output range of the MAX1450.
Replacing the Sensor
The MAX1450 may be used to calibrate and tempera-
ture-compensate a wide variety of pressure sensor
types; however, some sensors may require additional
external circuitry. This EV kit is designed for bulk micro-
machined silicon piezoresistive pressure sensors. See
Table 1. Connector P1 will accept any generic sensor
configured as a four-wire closed Wheatstone bridge. It
is most useful with sensors that require off-board mount-
ing, such as those that have large threaded pressure
ports. This connector may also be used as a sensor
test point. Sensor sites S1 and S2 provide several over-
lapping footprints for popular metal, plastic, and ceramic
sensors, which use a slip tube as a pressure port. See
the Component List for models that are accepted at
these sites. Only one sensor may be installed at a time.
Sensor sites S1 and S2 are not provided for accommo-
dating differential pressure-sensing applications that
require two pressure sensors.
Power Requirements
The MAX1450 EV kit operates with a nominal supply
voltage of +5V and requires about 5mA of supply cur-
rent, including the sensor. The supply may vary
between +4.5V and +5.5V, and a 6.2V zener diode has
been added across the power-supply lines (VDD and
VSS) to protect the MAX1450. Connector P2 is provided
for connecting power, as well as for measuring the out-
put voltage and sensor excitation voltage.
Calibration and Temperature-
Compensation Procedure
The MAX1450 EV kit can correct four common types of
sensor errors: offset, full-span output (FSO), offset temp-
co, and FSO tempco. The user may choose which para-
meters to correct depending on the sensor behavior,
operating temperature range, and desired accuracy.
Sensor calibration corrects the offset and FSO errors at
a single temperature. Temperature compensation,
which is optional, minimizes offset and FSO error drifts
with changes in temperature, and will require an envi-
ronmental chamber. To correct offset and FSO errors
over a range of temperatures, use the offset tempco
(VR1, OFTC) and FSO tempco (VR4, FSOTC) poten-
tiometers.
Since the PC board is not conformal coated, the envi-
ronmental chamber must not allow condensation to take
place. If condensation occurs, bake the PC board at
+125°C (with no power applied) for a minimum of one
hour. The circuit may behave erratically if moisture con-
denses on the PC board due to weak ionic paths affect-
ing high-impedance nodes on the board. The electron-
ics should be conformal coated in any application
where moisture condensation may occur.
Put the board in an environmental chamber and test the
board over any temperatures between -40°C and
+125°C. First perform one or two full excursions of tem-
perature and pressure to minimize hysteresis errors.
Most of the error after compensation is due to sensor
drift and nonrepeatable behavior. To understand the
source of these errors, monitor the low-level sensor out-
put using the P1 sensor connector to separate sensor
errors from MAX1450 errors. To avoid attenuating the
sensor output signal, take this measurement using a
multimeter with an input impedance >10M
.
Required Equipment
· Precision regulated power supply capable of providing
+5.000V
· Multimeter with at least five significant digits
· 015psi gauge pneumatic pressure controller/cali-
brator
· Dry air or nitrogen
· Noncondensing environmental chamber capable of
handling -40°C to +125°C
Initial Setup
Trim potentiometer VR4 (FSOTC) is required only when
compensating over temperature. Installing VR4 may
perturb previous calibration setpoints. Begin with the
switches and potentiometers as shown in Table 2.
Connect the sensor to a pressure source and test for
leaks. Connect the EV kit to a +5V power source. Since
output voltage is ratiometric to the power supply, an
accurate power-supply setting is required. If the power
supply has a programmable current limit, set it to about
10mA. Current consumption should not exceed 5mA.
Calibration Procedure
This procedure describes how to calibrate at a single
temperature. The following example is designed to cali-
brate a sensor with a nominal output voltage of 0.5V at
P
MIN
and an output voltage of 4.5V at P
MAX
; thus, the
ideal FSO will be 4V.
Choosing PGA Gain Setting
1) Set the temperature to T1 and allow sufficient soak
time.
2) Confirm that the supply voltage is correct.
3) Set the pressure to P
MIN
.
4) Adjust VR3 (FSO trim) until V
BDRIVE
(Vbdr, BDRV) is
approximately 2.0V.
5) Measure the differential sensor output (INP - INM).
6) Set the pressure to P
MAX
; remeasure (INP - INM).
7) Calculate the sensor FSO.
8) Divide the ideal FSO (4V) by the sensor FSO to cal-
culate the ideal PGA gain required.
9) Choose the PGA gain setting that is closest to the
calculated ideal gain.
10) Program the PGA gain chosen using the three pro-
gramming switches (SW1-1, SW1-2, SW1-3).
Example:
1) Set the temperature to T1 and allow it to soak.
2) Confirm the supply voltage.
3) Set the pressure to P
MIN
.
4) Adjust VR3 until V
BDRIVE
= 2.42V.
5) Measure (INP - INM) at P
MIN
to be -0.011V.
6) Measure (INP - INM) at P
MAX
to be 0.056V.
7) Calculate sensor FSO to be 0.067V.
8) Calculate the ideal gain to be 4 / 0.067 = 59.7V/V.
9) Determine the closest available gain setting to
be 65 (PGA value is 1).
10) Set the PGA to 001 (binary), close SW1-1 (LSB),
open SW1-2, and open SW1-3.
Evaluates: MAX1450
MAX1450 Evaluation Kit
_______________________________________________________________________________________
3
Table 1. Sensor Requirements for EV Kit
PARAMETER
VALUE
DESCRIPTION
Bridge Resistance
5k
(typ)
Sensor input impedance at +25°C
Resistance Tempco
TCR >
|
TCS
|
The sensor input impedance tempco must exceed the absolute value of the sensor pressure-
sensitivity tempco.
Pressure Sensitivity
~10 to 30
mV/V/FSO
Differential output FSO, per volt of sensor excitation
Sensitivity Tempco
TCS < 0
The sensitivity tempco must be negative.
Sensor Offset
|
V
OFFSET
|
< 100mV
The sensor offset voltage (absolute value) at minimum gain must be less than about 100mV.
At higher gains, the offset must be proportionately smaller.
Offset Tempco
|OTC| <
|
TCS
|
The absolute value of the sensor offset tempco must be less than the absolute value of its sensitivity
tempco, when both are expressed in terms of the sensor's FSO.
Evaluates: MAX1450
Determining and Setting the Ideal
Sensor Excitation Voltage at T1
1) Set the pressure to P
MIN
.
2) Measure the output voltage (V
OUT
) at P
MIN
.
3) If V
OUT
> 0.5V, open SW1-4 (SOF); if V
OUT
< 0.5V,
close SW1-4.
4) Adjust VR2 (OFST trim) until V
OUT
is 0.5V.
5) Set the pressure to P
MAX
, and measure V
OUT
at
P
MAX
.
6) Calculate the uncorrected FSO as follows: V
OUT
at
P
MAX
- V
OUT
at P
MIN
.
7) Calculate the FSO error as follows: uncorrected
FSO / ideal FSO.
8) Set the pressure to P
MIN
, and measure V
BDRIVE
(uncorrected V
BDRIVE
).
9) Determine the ideal V
BDRIVE
as follows: uncorrected
V
BDRIVE
/ FSO error.
10) Using VR3 (FSO trim), set V
ISRC
(pin 17) to ideal
V
BDRIVE
.
11) Using VR5 (R
ISRC
trim), set V
BDRIVE
to ideal
V
BDRIVE
.
Example:
1) Set the pressure to P
MIN
.
2) Measure V
OUT
at P
MIN
to be 0.987V.
3) Open SW1-4 (the voltage at OFFSET, pin 9, is
subtracted from the output).
4) Adjust VR2 until V
OUT
at P
MIN
= 0.503V.
5) Set the pressure to P
MAX
, then measure V
OUT
at P
MAX
to be 4.742V.
6) Calculate the uncorrected FSO as follows: 4.742
- 0.503 = 4.239V.
7) Calculate the FSO error as follows: 4.239 / 4 =
1.0597 (approximately 6% too high).
8) Measure V
BDRIVE
as 2.42V (uncorrected
V
BDRIVE
).
9) Calculate ideal V
BDRIVE
as follows: 2.42 /
1.0597 = 2.284V.
10) Adjust VR3 until V
ISRC
= 2.284V.
11) Adjust VR5 until V
BDRIVE
= 2.284V.
Setting Offset Voltage at T1
1) Set VR2 fully CCW (V
OFFSET
, pin 9 = 0).
2) Set the pressure to P
MIN
.
3) Measure the output voltage V
OUT
at P
MIN
.
4) If V
OUT
> 0.5V, open SW1-4; if V
OUT
< 0.5V, close
SW1-4.
5) Adjust VR2 until V
OUT
is 0.5V.
Example:
1) Set VR2 fully CCW.
2) Set the pressure to P
MIN
.
3) Measure V
OUT
P
MIN
to be 0.387V.
4) Close SW1-4 (the voltage at OFFSET, pin 9, is
added to the output).
5) Adjust VR2 until V
OUT
at P
MIN
= 0.496V.
MAX1450 Evaluation Kit
4
_______________________________________________________________________________________
PARAMETER
FUNCTION
INITIAL SETTING
SW1-1
PGA LSB
Off (open)
SW1-2
PGA NSB
Off (open)
SW1-3
PGA MSB
Off (open)
SW1-4
Offset Sign
On (closed)
SW1-5
Offset TC Sign
On (closed)
VR1
Offset TC Adjust
Fully CCW
DESCRIPTION
Minimum gain
Minimum gain
Minimum gain
Sign bit is set to positive.
Sign bit is set to positive.
No offset TC correction is performed. V
OFFSET
, pin 9 = 0.
VR2
Offset Adjust
Fully CCW
VR3
FSO Adjust
Approximately
midscale
VR4
FSO TC Adjust
Do not install VR4
unless compensating
over temperature.
VR5
R
ISRC
Adjust
Approximately
midscale
No offset correction is added. V
OFFTC
, pin 8 = 0.
Sets initial V
ISRC
to ~2.5V.
No FSO TC adjustment
Sets initial R
ISRC
to ~ 50k
.
Table 2. Initial Settings
Offset and FSO calibration (at T1) is now complete.
Remeasure the EV kit's output voltage (V
OUT
) at both
P
MIN
and P
MAX
. If required, make small readjustments
to FSO and to offset using their respective adjusting
potentiometers. If large errors are observed (~ 0.5% or
greater), recalibrate from step 1.
Temperature-Compensation Procedure
Accuracy may be improved by measuring offset and
FSO errors at a second temperature T2 (T2 > T1). The
FSOTC and OFFTC potentiometers will be used to com-
pensate for the drift in offset and FSO.
Compensating FSO TC Error at T2
1) Increase the temperature to T2 and allow sufficient
soak time.
2) Confirm that the supply voltage is correct.
3) Set the pressure to P
MIN
.
4) Measure
V
BDRIVE
(uncorrected V
BDRIVE
at T2).
5) Compare this uncorrected V
BDRIVE
with the ideal
V
BDRIVE
at T1 (see Determining and Setting the
Ideal Sensor Excitation Voltage at T1). If the uncor-
rected V
BDRIVE
is less than or equal to the ideal
V
BDRIVE
at T1, the sensor cannot be compensated
and you should abort the procedure. If it is greater
than the ideal V
BDRIVE
, the sensor can be compen-
sated and you may proceed.
6) Measure
V
OUT
at P
MIN
.
7) Set the pressure to P
MAX
, and measure V
OUT
at
P
MAX
.
8) Calculate the uncorrected FSO as follows: V
OUT
at
P
MAX
- V
OUT
at P
MIN
.
9) Calculate the FSO error as follows: uncorrected
FSO / ideal FSO.
10) Set the pressure to P
MIN
, and remeasure V
BDRIVE
(uncorrected V
BDRIVE
).
11) Determine the ideal V
BDRIVE
as follows: uncorrected
V
BDRIVE
/ FSO error.
12) Using VR4 (FSO TC trim), set V
BDRIVE
to ideal
V
BDRIVE
.
Example:
1) Increase the temperature to T2 and allow suffi-
cient soak time.
2) Confirm that the supply voltage is correct.
3) Set the pressure to P
MIN
.
4) Measure the uncorrected V
BDRIVE
at T2 as
2.961V.
5) Since it is greater than 2.284V, we can proceed.
6) Measure
V
OUT
at P
MIN
to be 0.6V.
7) Set the pressure to P
MAX
, and measure V
OUT
at
P
MAX
to be 4.72V.
8) Calculate the uncorrected FSO as follows: 4.72
- 0.6 = 4.12V.
9) Calculate FSO error as follows: 4.12 / 4 = 1.03
(approximately 3% too high).
10) Set the pressure to P
MIN
, and remeasure
V
BDRIVE
(uncorrected V
BDRIVE
).
11) Determine the ideal V
BDRIVE
as follows: 2.961 /
1.03 = 2.875V.
12) Adjust VR4 (FSO TC trim) until V
BDRIVE
=
2.875V.
Compensating Offset TC Error at T2
1) Set the pressure to P
MIN
.
2) Measure
V
OUT
at P
MIN
.
3) Calculate the offset TC error as follows: V
OUT
at
P
MIN
- 0.5.
4) Calculate the delta V
BDRIVE
as follows: ideal
V
BDRIVE
at T2 - ideal V
BDRIVE
at T1.
5) Calculate the offset TC correction factor as follows:
offset TC error / (1.15
·
delta V
BDRIVE
).
6) If offset TC error is positive, set SW1-5 to open
(negative offset TC correction); if it is negative,
close SW1-5.
7) Calculate V
OFFTC
as follows: offset TC correction
factor
·
V
BDRIVE
at T2.
8) Measure the voltage at OFFTC (pin 8), and adjust
VR1 to the value V
OFFTC
.
Example:
1) Set the pressure to P
MIN
.
2) Measure
V
OUT
at P
MIN
to be 0.75V.
3) Calculate the offset TC error as follows: 0.75 -
0.5 = +0.25V.
4) Calculate delta V
BDRIVE
as follows: 2.875 -
2.284 = +0.591V.
5) Calculate offset TC correction factor as follows:
0.25 / (1.15
·
0.591) = 0.368 (+36.7% of
V
BDRIVE
).
6) Since offset TC correction factor is positive,
open SW1-5.
7) Calculate
V
OFFTC
as 0.368
·
2.875 = 1.058V.
8) Adjust VR1 until V
OFFTC
= 1.058V.
Evaluates: MAX1450
MAX1450 Evaluation Kit
_______________________________________________________________________________________
5
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