MAX1478 Datasheet, PDF (12/20 Pages) Maxim Integrated Products – 1% Accurate, Digitally Trimmed,Rail-to-Rail Sensor Signal Conditioner

1% Accurate, Digitally Trimmed,
Rail-to-Rail Sensor Signal Conditioner
Writing to the Configuration and DAC Registers
When writing to the configuration register or directly to
the internal 12-bit DACs, the data field (D0–D11) con-
tains the data to be written to the respective register.
Note that all four DACs and the configuration register
can be updated without toggling the CS line. Every
register write command must be followed by a LOAD
REGISTER command.
__________Applications Information
At power-up, the following occurs:
Power-Up
1) The DAC and configuration registers are reset to
zero.
2) CS transitions from low to high after power-up (an
internal pull-up resistor ensures that this happens if
CS is left unconnected), and the EEPROM contents
are read and processed.
3) The DAC and configuration registers are updated
either once or approximately 400 times per second
(as determined by the state of WE).
4) The MAX1478 begins accepting commands in a ser-
ial format on DIO immediately after receiving the INIT
SEQUENCE.
The MAX1478 is shipped with all memory locations in
the internal EEPROM uninitialized. Therefore, the
MAX1478 must be programmed for proper operation.
Compensation Procedure
The following compensation procedure was used to
obtain the results shown in Figure 9 and Table 8. It
assumes a pressure transducer with a +5V supply and
an output voltage that is ratiometric to the supply volt-
age. The desired offset voltage (V_OUT at P_MIN) is 0.5V,
and the desired FSO voltage (V_OUT(PMAX) - V_OUT(PMIN))
is 4V; thus, the full-scale output voltage (V_OUT at P_MAX)
will be 4.5V (see Figure 1). The procedure requires a
minimum of two test pressures (e.g., zero and full
scale) at two arbitrary test temperatures, T₁ and T₂.
Ideally, T₁ and T₂ are the two points where we wish to
perform best linear fit compensation. The following out-
lines a typical compensation procedure:
1) Perform Coefficient Initialization
2) Perform FSO Calibration
3) Perform FSOTC Compensation
4) Perform Offset TC Compensation
5) Perform Offset Calibration
Coefficient Initialization
Select the resistor values and the PGA gain to prevent
overload of the PGA and bridge current source. These
values depend on sensor behavior and require some
sensor characterization data, which may be available
from the sensor manufacturer. If not, the data can be
generated by performing a two-temperature, two-pres-
sure sensor evaluation. The required sensor information
is shown in Table 6 and can be used to obtain the val-
ues for the parameters listed in Table 7.
Table 6. Sensor Information for Typical
PRT
PARAMETER
SENSOR
DESCRIPTION
TYPICAL
VALUES
Rb(T)
TCR
S(T)
TCS
O(T)
Bridge Impedance
Bridge Impedance
Tempco
Sensitivity
Sensitivity Tempco
Offset
5kΩ at +25°C
2600ppm/°C
+1.5mV/V per
PSI at +25°C
-2100ppm/°C
+12mV/V at
+25°C
OTC
S(p)
P_MIN
P_MAX
Offset Tempco
Sensitivity Linearity Error
as % FSO, BSLF
(Best Straight-Line Fit)
Minimum Input Pressure
Maximum Input Pressure
-1000ppm/°C
of FSO
0.1% FSO,
BSLF
0 psi
10 psi
Selecting R_ISRC
When using an external resistor, use the equation
below to determine the value of R_ISRC, and place the
resistor between ISRC and V_SS. Since the 12-bit FSO
DAC provides considerable dynamic range, the R_ISRC
value need not be exact. Generally, any resistor value
within ±50% of the calculated value is acceptable. If
both the internal resistors R_ISRC and R_FTC are used, set
the IRS bit at EEPROM address bit 7 high. Otherwise,
set IRS low and connect external resistors as shown in
Figure 10.
⋅ R_ISRC ≈ 14 Rb(T1)
≈ 14 ⋅ 5kΩ = 70kΩ
where Rb(T) is the sensor input impedance at tempera-
ture T1 (+25°C in this example).
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