MAX1478 Datasheet, PDF (13/20 Pages) Maxim Integrated Products – 1% Accurate, Digitally Trimmed,Rail-to-Rail Sensor Signal Conditioner

1% Accurate, Digitally Trimmed,
Rail-to-Rail Sensor Signal Conditioner
Table 7. Compensation Components and
Values
PARAMETER
R_ISRC
R_FTC
A_PGA
IRO
IRO Sign
IRS
OFF COEF
OFF Sign
OFFTC COEF
OFFTC Sign
FSO COEF
FSOTC COEF
DESCRIPTION
Internal (approximately 75kΩ) or user-
supplied resistor that programs the nomi-
nal sensor excitation current
Internal (approximately 75kΩ) or user-
supplied resistor that compensates
FSO TC errors
Programmable-gain amplifier gain
Input-referred offset correction DAC
value
Input-referred offset sign bit
Internal resistor selection bit
Offset-correction DAC coefficient
Offset sign bit
Offset TC compensation DAC coefficient
Offset TC sign bit
FSO trim DAC coefficient
FSO TC compensation DAC coefficient
Selecting R_FTC
When using an external resistor, use the equation
below to determine the value for R_FTC, and place the
resistor between ISRC and FSOTC. Since the 12-bit
FSOTC DAC provides considerable dynamic range, the
R_FTC value need not be exact. Generally, any resistor
value within ±50% of the calculated value is accept-
able.
R _FTC
≅
⋅ R_ISRC 500ppm/°C
TCR - | TCS |
≅
70kΩ ⋅ 500ppm/°C
= 70kΩ
2600ppm/°C - | -2100ppm/°C |
This approximation works best for bulk, micromachined,
silicon PRTs. Negative values for R_FTC indicate uncon-
ventional sensor behavior that cannot be compensated
by the MAX1478 without 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 · V_BDRIVE · ∆P
= 1.5mV/V per PSI · 2.5V · 10 PSI
= 0.0375V
where S is the sensor sensitivity at T1, V_BDRIVE 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:
A_PGA = ^OUTFSO
SensorFSO
= ^4V = +106V/V
0.0375V
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).
First, calculate the ideal IRO correction voltage using
the following formula, and select the nearest setting
from Table 1:
[ ] ^{( ) ( ) ⋅ IROideal = - O T1 V}BDRIVE ^T1
= - ₍0.012V/V₎ ⋅ 2.5V
= - 30mV
where IROideal is the exact voltage required to perfect-
ly null the sensor, O(T1) is the sensor offset voltage in
V/V at +25°C, and V_BDRIVE(T1) is the nominal sensor
excitation voltage at +25°C. In this example, 30mV
must be subtracted from the amplifier front end to null
the sensor perfectly. From Table 1, select an IRO value
of 3 to set the IRO DAC to 27mV, which is nearest the
ideal value. To subtract this value, set the IRO sign bit
to 0. The residual output-referred offset error will be
corrected later with the Offset DAC.
Determining OFFTC COEF Initial Value
Generally, OFFTC COEF can initially be set to 0 since
the offset TC error will be compensated in a later step.
However, sensors with large offset TC errors may
require an initial coarse offset TC adjustment to prevent
the PGA from saturating during the compensation pro-
cedure as temperature is increased. An initial coarse
offset TC adjustment is required for sensors with an off-
set TC greater than about 10% of the FSO. If an initial
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