Datasheet
PGA309
www.ti.com
SBOS292C –DECEMBER 2003–REVISED JANUARY 2011
FUNCTIONAL DESCRIPTION
Converter (Temp ADC). In order to compensate for
second-order and higher drift nonlinearity, the span
OVERVIEW
drift can be fitted to piecewise linear curves during
The PGA309 is a programmable analog signal
calibration with the coefficients stored in an external
conditioner designed for resistive bridge sensor
nonvolatile EEPROM lookup table.
applications. It is a complete signal conditioner with
Following the fine gain adjust stage is the Output
bridge excitation, initial span and offset adjustment,
Amplifier that provides additional programmable gain.
temperature adjustment of span and offset,
Two key output amplifier connections, V
FB
and V
SJ
,
internal/external temperature measurement capability,
are brought out on the PGA309 for application
output over-scale and under-scale limiting, fault
flexibility. These connections allow for an accurate
detection, and digital calibration. The PGA309, in a
conditioned signal voltage while also providing a
calibrated sensor module, can reduce errors to the
means for PGA309 output overvoltage and large
level approaching the bridge sensor repeatability. See
capacitive loading for RFI/EMI filtering required in
Figure 24 for a block diagram of the PGA309.
many end applications.
Following is a brief overview of each major function.
OFFSET ADJUSTMENT
SENSOR ERROR ADJUSTMENT RANGE
The sensor offset adjustment is performed in two
The adjustment capability of the PGA309 is
stages. The input-referred Coarse Offset Adjust DAC
summarized in Table 1.
has approximately a ±60mV offset adjustment range
for a selected V
REF
of 5V. The fine offset and the
Table 1. PGA309 Adjustment Capability
offset drift are canceled by the 16-bit Zero DAC that
FSS (full-scale sensitivity) 1mV/V to 245mV/V
sums the signal with the output of the front-end
Span TC Over ±3300ppmFS/°C
(1)
instrumentation amplifier. Similar to the Gain DAC,
Span TC nonlinearity ≥ 10%
the input digital values of the Zero DAC are controlled
by the data in the Temperature Compensation
Zero offset ±200%FS
(2)
Lookup Table, stored in external EEPROM, driven by
Zero offset TC Over ±3000ppmFS/°C
(2)
the Temp ADC. The programming range of the Zero
Zero offset TC nonlinearity ≥ 10%
DAC is 0V to V
REF
with an output range of 0.1V to
Sensor impedance Down to 200Ω
(3)
V
SA
– 0.1V.
1. Depends on the temperature sensing scheme.
VOLTAGE REFERENCE
2. Combined coarse and fine offset adjust.
The PGA309 contains a precision low-drift voltage
3. Lower impedance possible by using a dropping
reference (selectable for 2.5V or 4.096V) that can be
resistor in series with the bridge.
used for external circuitry through the REF
IN
/REF
OUT
pin. This same reference is used for the Coarse
GAIN SCALING
Offset Adjust DAC, Zero DAC, Over/Under-Scale
The core of the PGA309 is the precision low-drift and
Limits and sensor excitation/linearization through the
no 1/f noise Front-End PGA. The overall gain of the
V
EXC
pin. When the internal reference is disabled, the
Front-End PGA + Output Amplifier can be adjusted
REF
IN
/REF
OUT
pin should be connected to an
from 2.7V/V to 1152V/V. The polarity of the inputs
external reference or to V
SA
for ratiometric-scaled
can be switched through the 2x2 input mux to
systems.
accommodate sensors with unknown polarity output.
SENSOR EXCITATION AND LINEARIZATION
The Front-End PGA provides initial coarse signal gain
using a no 1/f noise, auto-zero instrumentation
A dedicated circuit with a 7-bit + sign DAC for sensor
amplifier. The fine gain adjust is accomplished by the
voltage excitation and linearization is provided on the
16-bit attenuating Gain Digital-to-Analog Converter
PGA309. This block scales the reference voltage and
(Gain DAC). This Gain DAC is controlled by the data
sums it with a portion of the PGA309 output to
in the Temperature Compensation Lookup Table
compensate the positive or negative bow-shaped
driven by the Temperature Analog-to-Digital
nonlinearity exhibited by many sensors over their
applied pressure range. Sensors not requiring
linearization can be connected directly to the supply
(V
SA
) or to the V
EXC
pin with the Linearization DAC
(Lin DAC) set to zero.
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