MAX1478 Datasheet, PDF (5/20 Pages) Maxim Integrated Products – 1% Accurate, Digitally Trimmed,Rail-to-Rail Sensor Signal Conditioner

1% Accurate, Digitally Trimmed,
Rail-to-Rail Sensor Signal Conditioner
FSO TC Compensation
Silicon piezoresistive transducers (PRTs) exhibit a large
positive input resistance tempco (TCR) so that, while
under constant current excitation, the bridge voltage
(V_BDRIVE) increases with temperature. This depen-
dence of V_BDRIVE on the sensor temperature can be
used to compensate the sensor temperature errors.
PRTs also have a large negative full-span output sensi-
tivity tempco (TCS) so that, with constant voltage exci-
tation, FSO will decrease with temperature, causing a
full-span output temperature coefficient (FSO TC) error.
However, if the bridge voltage can be made to increase
with temperature at the same rate that TCS decreases
with temperature, the FSO will remain constant.
FSO TC compensation is accomplished by resistor
R_FTC and the FSOTC DAC, which modulate the excita-
tion reference current at ISRC as a function of tempera-
ture (Figure 3). FSO DAC sets V_ISRC and remains
constant with temperature, while the voltage at FSOTC
varies with temperature. FSOTC is the buffered output
of the FSOTC DAC. The reference DAC voltage is
V_BDRIVE, which is temperature dependent. The FSOTC
DAC alters the tempco of the current source. When the
tempco of the bridge voltage is equal in magnitude and
opposite in polarity to the TCS, the FSO TC errors are
compensated and FSO will be constant with tempera-
ture.
Offset TC Compensation
Compensating offset TC errors involves first measuring
the uncompensated offset TC error, then determining
the percentage of the temperature-dependent voltage
V_BDRIVE that must be added to the output summing
junction to correct the error. Use the Offset TC DAC to
adjust the amount of BDRIVE voltage that is added to
the output summing junction (Figure 2).
Analog Signal Path
The fully differential analog signal path consists of four
stages:
• Front-end summing junction for coarse offset correction
• 3-bit PGA with eight selectable gains ranging from
41 through 230
• Three-input-channel summing junction
• Differential to single-ended output buffer (Figure 2)
Coarse Offset Correction
The sensor output is first fed into a differential summing
junction (INM (negative input) and INP (positive input))
with a CMRR >90dB, an input impedance of approxi-
mately 1MΩ, and a common-mode input voltage range
from V_SS to V_DD. At this summing junction, a coarse off-
set-correction voltage is added, and the resultant volt-
age is fed into the PGA. The 3-bit (plus sign)
input-referred Offset DAC (IRO DAC) generates the
coarse offset-correction voltage. The DAC voltage ref-
erence is 1.25% of V_DD; thus, a V_DD of 5V results in a
front-end offset-correction voltage ranging from -63mV
to +63mV, in 9mV steps (Table 1). To add an offset to
the input signal, set the IRO sign bit high; to subtract an
offset from the input signal, set the IRO sign bit low.
The IRO DAC bits (C2, C1, C0, and IRO sign bit) are
programmed in the configuration register (see Internal
EEPROM section).
4.5
FULL-SPAN OUTPUT (FSO)
0.5
OFFSET
P_MIN
FULL SCALE (FS)
P_MAX
PRESSURE
1.25% V_DD
IRO
DAC
INP
INM
BDRIVE
OFFTC
DAC
SOTC
A2 A1 A0
A = 2.3
±
Σ
PGA
Σ
OUT
A=1
A = 2.3
V_DD
OFFSET
DAC
±
SOFF
Figure 1. Typical Pressure-Sensor Output
Figure 2. Signal-Path Block Diagram
_______________________________________________________________________________________ 5