MAX1457 Maxim, MAX1457 Datasheet - Page 7

• Manufacturer Part Number: MAX1457
• Description: 0.1%-Accurate Signal Conditioner for Piezoresistive Sensor Compensation
• Manufacturer: Maxim

The MAX1457 corrects pressure nonlinearity in an ana-
log fashion by providing a resistive feedback path
(resistor R
(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.
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.
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.
Figure 4. Pressure Nonlinearity Correction
_____________Applications Information
V
BR
I
LIN
BR
V
Ratiometric Output Configuration
DD
Pressure Nonlinearity Correction
in Figure 4) from a buffered main output
2-Wire, 4–20mA Configuration
_______________________________________________________________________________________
for Piezoresistive Sensor Compensation
R
LIN
111...1
16 BIT
FSO
DAC
0.1%-Accurate Signal Conditioner
LIN
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
required during the calibration procedure. If needed,
the PGA output can be divided using resistors R
R
For overvoltage protection, place a Zener diode across
V
across the inputs reduces EMI/RFI.
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):
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:
C
IN-
A
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)
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
. With R
.
and V
A
IN+
= 50Ω, a 0.2V to 1.0V range would be
(Figure 6). A feedthrough capacitor
Test System Configuration
B
and
7