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MAX1459
Otherwise, set IRS high and connect external resistors
as shown in Figure 13:
where Rb(T) is the sensor input impedance at tempera-
ture T1 (+25°C in this example).
Selecting RFTC
When using an external resistor, use the equation
below to determine the value for RFTC, and place the
resistor between ISRC and FSOTC. Since the 12-bit
FSOTC DAC provides considerable dynamic range, the
RFTC value need not be exact. Generally, any resistor
value within ±50% of the calculated value is accept-
able:
This approximation works best for bulk, micromachined,
silicon PRTs. Negative values for RFTC indicate uncon-
ventional sensor behavior that can be compensated by
the MAX1459 with additional external circuitry.
Selecting the PGA Gain Setting
To select the PGA gain setting, first calculate
SensorFSO, the sensor full-span output voltage at T1:
SensorFSO = S x VBDRIVE x
P
= +1.5mV/V per PSI x 2.5V x 10 PSI
= 0.0375V
where S is the sensor sensitivity at T1, VBDRIVE is the
sensor excitation voltage (initially 2.5V), and
P is the
maximum pressure differential.
Then calculate the ideal gain using the following formula,
and select the nearest gain setting from Table 2:
where OUTFSO is the desired calibrated transducer
full-span output voltage, and SensorFSO is the sensor
full-span output voltage at T1.
In this example, a PGA value of 2 (gain of +95V/V) is
the best selection.
Determining Input-Referred OFFSET
The input-referred offset (IRO) register is used to null
any front-end sensor offset errors prior to amplification
by the PGA. This reduces the possibility of saturating
the PGA and maximizes the useful dynamic range of
the PGA (particularly at the higher gain values).
A
OUTFSO
SensorFSO
4V
0.0375V
+106V/V
PGA ≈
=
R
500ppm/ C
60k
500ppm/ C
60k
FTC
ISRC
×°
×°
°°
TCR - TCS
2600ppm/ C - -2100ppm/ C
||
R
Rb(T
kk
ISRC ≈×
≈×
≈
)
12
5
60
2-Wire, 4–20mA
Smart Signal Conditioner
16
______________________________________________________________________________________
Internal (approximately 100k
) or user-
supplied resistor that compensates
FSOTC errors
RFTC
FSOTC compensation DAC
FSOTC COEF
FSO trim DAC
FSO COEF
Offset TC sign bit
OFFTC Sign
Offset TC compensation DAC coefficient
OFFTC COEF
Offset sign bit
OFF Sign
Offset correction DAC coefficient
OFF COEF
Internal resistor selection bit
IRS
Input-referred offset sign bit
PARAMETER
IRO Sign
DESCRIPTION
Input-referred offset correction DAC
value
Internal (approximately 100k
) or user-
supplied resistor that programs the nomi-
nal sensor excitation current
IRO
RISRC
Programmable-gain amplifier gain
APGA
Table 9. Compensation Components and
Values
Maximum Input Pressure
PMAX
10 psi
Minimum Input Pressure
PMIN
Sensitivity Linearity Error
as % FSO, BSLF
(best straight-line fit)
0 psi
S(p)
0.1% FSO,
BSLF
Offset Tempco
OTC
Offset
-1000ppm/°C
of FSO
O(T)
+12mV/V at
+25°C
Sensitivity Tempco
TCS
Sensitivity
-2100ppm/°C
S(T)
+1.5mV/V per
PSI at +25°C
PARAMETER
Bridge Impedance
Tempco
SENSOR
DESCRIPTION
TCR
Bridge Impedance
2600ppm/°C
Rb(T)
TYPICAL
VALUES
5k
at +25°C
Table 8. Sensor Information for Typical PRT