MAX1450EAP Fast Delivery |IC PHOENIX CO.,LIMITED

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MAX1450EAP
Low-Cost, 1%-Accurate Signal Conditioner for Piezoresistive Sensors
General Description
The MAX1450 sensor signal conditioner is optimized for
piezoresistive sensor calibration and temperature com-
pensation. It includes an adjustable current source for
sensor excitation and a 3-bit programmable-gain amplifi-
er (PGA). Achieving a total typical error factor within
1% of the sensor’s inherent repeatability errors, the
MAX1450 compensates offset, full-span output (FSO), off-
set tempco,FSO tempco, and FSO nonlinearity of silicon
piezoresistive sensors via external trimmable resistors,
potentiometers, or digital-to-analog converters (DACs).
The MAX1450 is capable of compensating sensors that
display close error distributions with a single tempera-
ture point, making it ideal for low-cost, medium-accuracy
applications. Although optimized for use with popular
piezoresistive sensors, it may also be used with other
resistive sensor types such as strain gauges.
Customization
Maxim can customize the MAX1450 for unique require-
ments including improved power specifications. With a
dedicated cell library consisting of more than 90 sen-
sor-specific functional blocks, Maxim can quickly pro-
vide customized MAX1450 solutions. Contact the
factory for additional information.
Applications
Piezoresistive Pressure and Acceleration
Transducers and Transmitters
Manifold Absolute Pressure (MAP) Sensors
Automotive Systems
Hydraulic Systems
Industrial Pressure Sensors
Features1% Sensor Signal ConditioningCorrects Sensor Errors Using Coefficients Stored
in External Trimmable Resistors, Potentiometers,
or DACsCompensates Offset, Offset TC, FSO, FSO TC,
and FSO LinearityRail-to-Rail®Analog OutputProgrammable Current Source for Sensor
ExcitationFast Signal-Path Settling Time (< 1ms)Accepts Sensor Outputs from 10mV/V to 30mV/V Fully Analog Signal Path
MAX1450
Low-Cost, 1%-Accurate Signal Conditioner
for Piezoresistive Sensors
Pin Configuration
19-1365; Rev 0; 5/98
Functional Diagram
Ordering Information
Rail-to-Rail is a registered trademark of Nippon Motorola, Ltd.
MAX1450
Low-Cost, 1%-Accurate Signal Conditioner
for Piezoresistive Sensors
ABSOLUTE MAXIMUM RATINGS
ELECTRICAL CHARACTERISTICS
(VDD= +5V, VSS= 0, TA= TMINto TMAX, unless otherwise noted. Typical values are at TA= +25°C.)
Stresses beyond those listed under “Absolute Maximum Ratings” may cause permanent damage to the device. These are stress ratings only, and functional
operation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure to
absolute maximum rating conditions for extended periods may affect device reliability.
Supply Voltage, VDDto VSS......................................-0.3V to +6V
All Other Pins...................................(VSS- 0.3V) to (VDD+ 0.3V)
Short-Circuit Duration, OUT, BBUF, BDRIVE.............Continuous
Continuous Power Dissipation (TA= +70°C)
SSOP (derate 8.00mW/°C above +70°C)....................640mW
Operating Temperature Range
MAX1450CAP.....................................................0°C to +70°C
MAX1450EAP..................................................-40°C to +85°C
Storage Temperature Range.............................-65°C to +165°C
Lead Temperature (soldering, 10sec).............................+300°C
MAX1450
Low-Cost, 1%-Accurate Signal Conditioner
for Piezoresistive Sensors
ELECTRICAL CHARACTERISTICS (continued)
(VDD= +5V, VSS= 0, TA= TMINto TMAX, unless otherwise noted. Typical values are at TA= +25°C.)
Note 1:Contact factory for high-volume applications requiring less than 1.5mA.
Note 2:All electronics temperature errors are compensated together with the sensor errors.
Note 3:The sensor and the MAX1450 must always be at the same temperature during calibration and use.
Note 4:This is the maximum allowable sensor offset at minimum gain (39V/V).
Note 5:This is the sensor’s sensitivity normalized to its drive voltage, assuming a desired full-span output (FSO) of 4V and a bridge
voltage of 2.5V. Operating at lower bridge excitation voltages can accommodate higher sensitivities.
______________Detailed Description
Analog Signal Path
The MAX1450’s signal path is fully differential and com-
bines the following three stages: a 3-bit PGA with
selectable gains of 39, 65, 91, 117, 143, 169, 195, and
221; a summing junction; and a differential to single-
ended output buffer (Figure 1).
Programmable-Gain Amplifier
The analog signal is first fed into a programmable-gain
instrumentation amplifier with a CMRR of 90dB and a
common-mode input range from VSSto VDD. Pins A0,
A1, and A2 set the PGA gain anywhere from 39V/V to
221V/V (in steps of 26).
MAX1450
Low-Cost, 1%-Accurate Signal Conditioner
for Piezoresistive Sensors
Pin Description
Summing Junction
The second stage in the analog signal path consists of
a summing junction for offset, offset temperature com-
pensation, and the PGA output. The offset voltage
(VOFFSET) and offset temperature-compensation volt-
age (VOFFTC) add or subtract from the PGA output
depending on their respective sign bits, offset sign
(SOFF), and offset TC sign (SOTC). VOFFSETand
VOFFTCcan range in magnitude from VSSto VDD.
Output Buffer
The final stage in the analog signal path consists
of a unity-gain buffer. This buffer is capable of swinging
to within 250mV of VSSand VDDwhile sourcing/sinking
up to 1.0mA, or within 50mV of the power supplies with
no load.
Bridge Drive
Figure 2 shows the functional diagram of the on-chip
current source. The voltage at FSOTRIM, in conjunction
with RISRC, sets the nominal current, IISRCwhich sets
the FSO (refer to Figure 3 for sensor terminology.) IISRC
is additionally modulated by components from the
external resistor RSTCand the optional resistor RLIN.
RSTCis used to feed back a portion of the buffered
bridge-excitation voltage (VBBUF), which compensates
FSO TC errors by modulating the bridge-excitation cur-
rent over temperature. To correct FSO linearity errors,
feed back a portion of the output voltage to the current-
source reference node via the optional RLIN resistor.
Applications Information
Compensation Procedure
The following compensation procedure assumes a pres-
sure transducer with a +5V supply and an output voltage
that is ratiometric to the supply voltage (see Ratiometric
Output Configurationsection). The desired offset voltage
(VOUTat PMIN) is 0.5V, and the desired FSO voltage
(VOUT(PMAX) - VOUT(PMIN)) is 4V; thus the FS output volt-
age (VOUTat PMAX) will be 4.5V. The procedure requires
a minimum of two test pressures (e.g., zero and full scale)
and two temperatures. A typical compensation procedure
is as follows:
1) Perform Coefficient Initialization
2) Perform FSO Calibration
3) Perform FSO TC Compensation
4) Perform OFFSET TC Compensation
5) Perform OFFSET Calibration
6) Perform Linearity Calibration (Optional)
Coefficient Initialization
Select the resistor values and the PGA gain to prevent
gross overload of the PGA and bridge current source.
These values depend on sensor behavior and require
some sensor characterization data. This data may be
available from the sensor manufacturer. If not, it can be
generated by performing a two-temperature, two-pres-
MAX1450
Low-Cost, 1%-Accurate Signal Conditioner
for Piezoresistive Sensors
Figure 2. Bridge Drive Circuit
MAX1450
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 RISRC
RISRCprograms the nominal sensor excitation current
and is placed between ISRC and VSS. 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 RSTC
RSTCcompensates 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 RSTCindicate 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, VBDRIVEis the
sensor excitation voltage (initially 2.5V), and ΔP is the
maximum pressure differential.
Low-Cost, 1%-Accurate Signal Conditioner
for Piezoresistive Sensors
Table 1. Sensor Information
Table 2.CompensationComponents/Values

Partno	Mfg	Dc	Qty	Available	Descript
MAX1450EAP	MAX	N/a	13	avai	Low-Cost, 1%-Accurate Signal Conditioner for Piezoresistive Sensors