MAX1459 Datasheet, PDF (19/24 Pages) Maxim Integrated Products – 2-Wire, 4-20mA Smart Signal Conditioner

2-Wire, 4–20mA
Smart Signal Conditioner
NewOFFTC COEF = CurrentOFFTC COEF -
 ₄₀₉₆
[[ _] ] _{ 2.3}
V_OFFSET(T1₎ - V_OFFSET(T2₎
V_BDRIVE(T1₎ - V_BDRIVE(T2₎


Note: CurrentOFFTC COEF is the current value
stored in the offset TC DAC. If the offset TC sign bit
(SOTC) is low, this number is negative.
3) Load this value into the offset TC DAC.
4) If NewOFFTC COEF is negative, set the offset TC
sign (SOTC) bit low; otherwise, set it high.
Offset TC compensation is now complete.
OFFSET Calibration
At this point, the sensor should still be at temperature
T2. The final offset adjustment can be made at T2 or T1
by adjusting the offset DAC (and optionally the offset
sign bit, SOFF) until the output (V_OUT(PMIN)) reads 0.5V
at zero input pressure. Use the following procedure:
1) Set offset DAC to zero (offset COEF = 0).
2) Measure the voltage at OUT.
3) If V_OUT is greater than the desired offset voltage
(0.5V in this example), set SOFF low; otherwise, set
it high.
4) Increase offset COEF until V_OUT equals the desired
offset voltage.
Offset calibration is now complete. Table 7 and Figure
12 compare an uncompensated input to a typical com-
pensated transducer output.
Sensor Selection
Silicon Piezoresistive Sensors
The MAX1459 is optimized for use with sensors
designed for current mode operation that have a TCR in
the neighborhood of 2000ppm/°C or more. Voltage-
mode excitation sensors have a characteristically low
TCR, which may necessitate the use of a temperature
sensor (internal or external). For more information on
using the MAX1459 in conditions such as TCR < TCS,
low TCS, or low TCR, refer to the MAX1459 Reference
Manual. The ideal sensor used with the MAX1459 will
not change input impedance as a function of mechani-
cal excitation (pressure). PRTs that are imbalanced
behave poorly.
Strain-Gauge Sensors
The MAX1459 was optimized for signal conditioning of
piezoresistive sensors; however, it offers powerful per-
formance for signal conditioning strain-gauge sensors
as well. Strain-gauge sensors vary greatly in perfor-
mance and compensation requirements since they are
used to measure many variables (e.g., pressure, accel-
eration, force, torque, etc.) and use a variety of materi-
als for the sensing element (e.g., constantan, manganin,
etc.) and spring elements (e.g., steel, glass, aluminum,
etc.). This makes signal conditioning extremely applica-
tion dependent. For more information on this applica-
tion, refer to the MAX1459 Reference Manual.
Ratiometric Output Configuration
Ratiometric output configuration provides an output that
is proportional to the power-supply voltage. When used
with ratiometric A/D converters, this output provides
digital pressure values independent of supply voltage.
Most automotive and some industrial applications
require ratiometric outputs.
The MAX1459 provides a high-performance ratiometric
output with a minimum number of external components
(Figure 13). These external components include the fol-
lowing:
• One power-supply bypass capacitor (C1)
• Two optional resistors, one from FSOTC to ISRC, and
another from ISRC to V_SS, depending on the sensor
type
• One optional capacitor C2 from BDRIVE to V_SS
2-Wire, 4–20mA Configuration
In the 2-wire configuration, a 4mA current is used to
power a transducer, and an incremental current of 0mA
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.
Only a few components (Figure 14) are required to
build a 4–20mA output configuration. Use a low-quies-
cent-current voltage regulator with a built-in bandgap
reference (such as the MAX875). Since the MAX1459
performs temperature and gain compensation of the
circuit, the temperature coefficient and the calibration
accuracy of the reference voltage are of secondary
importance.
The MAX1459 controls the voltage across resistor
RSENSE. With RSENSE = 50Ω, a 0.2V to 1.0V range
would be required during the calibration procedure.
Resistors RB, RC, and ROFF are used to set the voltage
across RSENSE. For overvoltage protection, place a
zener diode across VIN- and VIN+ (Figure 14). A
feedthrough capacitor across the inputs reduces
EMI/RFI. For more information on this application, refer
to the MAX1459 Reference Manual.
In 4–20mA applications, pay close attention to thermal
management. Q1 will dissipate significant power and, if
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