Datasheet

PGA309
www.ti.com SBOS292C –DECEMBER 2003–REVISED JANUARY 2011
ADC FOR TEMPERATURE SENSING algorithm for accurate DAC adjustments between
stored temperature indexes. This approach allows for
The temperature sense circuitry drives the
a piecewise linear temperature compensation of up to
compensation for the sensor span and offset drift.
17 temperature indexes and associated temperature
Either internal or external temperature sensing is
coefficients.
possible. The temperature can be sensed in one of
the following ways: If either Checksum1, Checksum2, or both are
incorrect, the output of the PGA309 is set to
• Bridge impedance change (excitation current
high-impedance.
sense, in the positive or negative part of the
bridge), for sensors with large temperature
FAULT MONITOR
coefficient of resistance (TCR > 0.1%/°C).
• On-chip PGA309 temperature, when the chip is
To detect sensor burnout or a short, a set of four
located sufficiently close to the sensor.
comparators are connected to the inputs of the
Front-End PGA. If any of the inputs are taken to
• External diode, thermistor, or RTD placed on the
within 100mV of ground or V
EXC
, or violate the input
sensor membrane. An internal 7mA current source
CMR of the Front-End PGA, then the corresponding
may be enabled to excite these types of
comparator sets a sensor fault flag that causes the
temperature sensors.
PGA309 V
OUT
to be driven within 100mV of either V
SA
The temperature signal is digitized by the onboard
or ground, depending upon the alarm configuration
Temp ADC. The output of the Temp ADC is used by
setting (Register 5—PGA Configuration and
the control digital circuit to read the data from the
Over/Under-Scale Limit). This will be well above the
Lookup Table in an external EEPROM, and set the
set Over-Scale Limit level or well below the set
output of the Gain DAC and the Zero DAC to the
Under-Scale Limit level. The state of the fault
calibrated values as temperature changes.
condition can be read in digital form in Register
8—Alarm Status Register. If the Over/Under-Scale
An additional function provided through the Temp
Limit is disabled, the PGA309 output voltage will still
ADC is the ability to read the V
OUT
pin back through
be driven within 100mV of either V
SA
or ground,
the Temp ADC input mux. This provides flexibility for
depending upon the alarm configuration setting.
a digital output through either One-Wire or Two-Wire
interface, as well as the possibility for an external
There are five other fault detect comparators that
microcontroller to perform real-time custom
help detect subtle PGA309 front-end violations that
calibration of the PGA309.
could otherwise result in linear voltages at V
OUT
that
would be interpreted as valid states. These are
EXTERNAL EEPROM AND TEMPERATURE
especially useful during factory calibration and setup,
COEFFICIENTS
and are configured through Register 5—PGA
Configuration and Over/Under-Scale Limit. The
The PGA309 uses an industry-standard Two-Wire
respective status of each can also be read back
external EEPROM (typically, a SOT23-5 package). A
through Register 8—Alarm Status Register.
1k-bit (minimum) EEPROM is needed when using all
17 temperature coefficients. Larger EEPROMs may
OVER-SCALE AND UNDER-SCALE LIMITSE
be used to provide space for a serial number, lot
code, or other data.
The over-scale and under-scale limit circuitry
combined with the fault monitor circuitry provides a
The first part of the external EEPROM contains the
means for system diagnostics. A typical
configuration data for the PGA309, with settings for:
sensor-conditioned output may be scaled for 10% to
• Register 3—Reference Control and Linearization
90% of the system ADC range for the sensor normal
• Register 4—PGA Coarse Offset and Gain/Output
operating range. If the conditioned pressure sensor is
Amplifier Gain
below 4%, it is considered under-pressure; if over
• Register 5—PGA Configuration and
96%, it is considered over-pressure.
Over/Under-Scale Limit
The PGA309 over/under-scale limit circuit can be
• Register 6—Temp ADC Control
programmed individually for under-scale and
This section of the EEPROM contains its own
over-scale values that clip or limit the PGA309 output.
individual checksum (Checksum1).
From a system diagnostic view, 10% to 90% of ADC
range is normal operation, less than 4% is
The second part of the external EEPROM contains
under-pressure, and greater than 96% is
up to 17 temperature index values and corresponding
over-pressure. If the fault detect circuitry is used, a
temperature coefficients for the Zero DAC and Gain
detected fault will cause the PGA309 output to be
DAC adjustments with measured temperature, and
driven to positive or negative saturation.
also contains its own checksum (Checksum2). The
PGA309 lookup logic contains a linear interpolation
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