MAX1457 Datasheet, PDF (7/12 Pages) Maxim Integrated Products – 0.1%-Accurate Signal Conditioner for Piezoresistive Sensor Compensation

0.1%-Accurate Signal Conditioner
for Piezoresistive Sensor Compensation
Pressure Nonlinearity Correction
The MAX1457 corrects pressure nonlinearity in an ana-
log fashion by providing a resistive feedback path
(resistor R_LIN in Figure 4) from a buffered main output
(LINOUT pin) to the current source (ISRC pin). The
feedback coefficient is then set by writing a 16-bit word
to the FSO LIN DAC.
For many silicon sensors, this type of nonlinearity cor-
rection may reduce sensor nonlinearity by an order of
magnitude.
_____________Applications Information
Ratiometric Output Configuration
Ratiometric output configuration provides an output that
is proportional to the power-supply voltage. When used
with ratiometric ADCs, this output provides digital pres-
sure values independent of supply voltage.
The MAX1457 has been designed to provide a high-
performance ratiometric output with a minimum number
of external components (Figure 5). These external com-
ponents typically include an external EEPROM (93C66),
decoupling capacitors, and resistors.
2-Wire, 4–20mA Configuration
In this configuration, a 4mA current is used to power a
transducer, and an incremental current of 0 to 16mA
proportional to the measured pressure is transmitted
over the same pair of wires. Current output enables
long-distance transmission without a loss of accuracy
due to cable resistance.
V_DD
R_LIN
I_BR
FSO
LIN
DAC
111...1
16 BIT
V_BR
PGA
V_OUT
Only a few components (Figure 6) are required to build
a 4–20mA output configuration. A low-quiescent-cur-
rent voltage regulator with a built-in bandgap reference
(such as the REF02) should be used. Since the
MAX1457 performs temperature and gain compensa-
tion of the circuit, the temperature stability and calibra-
tion accuracy of the reference voltage is of secondary
importance.
The external transistor forms the controllable current
loop. The MAX1457 controls the voltage across resistor
R_A. With R_A = 50Ω, a 0.2V to 1.0V range would be
required during the calibration procedure. If needed,
the PGA output can be divided using resistors R_B and
R_C.
For overvoltage protection, place a Zener diode across
V_IN- and V_IN+ (Figure 6). A feedthrough capacitor
across the inputs reduces EMI/RFI.
Test System Configuration
The MAX1457 is designed to support an automated
production pressure-temperature test system with inte-
grated calibration and temperature compensation.
Figure 7 shows the implementation concept for a low-
cost test system capable of testing up to five transduc-
er modules connected in parallel. Three-state outputs
on the MAX1457 allow for parallel connection of trans-
ducers.
The test system shown in Figure 7 includes a dedicated
test bus consisting of six wires (the capacitive loading
of each transducer module should not exceed the
EEPROM fan-out specifications):
• Two power-supply lines
• One analog output voltage line from the transducers
to a system digital voltmeter
• Three MicroWire/SPI interface lines: EDI (data-in),
EDO (data-out), and ECLK (clock)
For simultaneous testing of more than five transducer
modules, use buffers to prevent overloading the data bus.
A digital multiplexer controls the two chip-select signals
for each transducer:
• Module Select (MCS) places the selected module
into an active state, enabling operation and compen-
sation
• EEPROM Select (ECS) enables writing to the trans-
ducer’s EEPROM
Figure 4. Pressure Nonlinearity Correction
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