Datasheet
PGA308
www.ti.com
SBOS440B –JULY 2008–REVISED DECEMBER 2010
FUNCTIONAL DESCRIPTION
OVERVIEW VOLTAGE REFERENCE
The PGA308 is an ideal building block for resistive The PGA308 V
REF
pin provides input from a
bridge sensor conditioning and general data reference voltage. The reference voltage is used by
acquisition. Digitally-programmable coarse offset, fine the Coarse Offset Adjust and Zero DACs. The fault
offset, and gain may be controlled in real time or monitor circuitry trip points, as well as the over- and
permanently programmed into the PGA308. under-scale limits, can be selected to be referenced
to either V
S
or V
REF
. This flexibility accommodates
absolute or ratiometric mode designs.
SENSOR ERROR ADJUSTMENT RANGE
The PGA308 is designed to readily accommodate the
FAULT MONITOR CIRCUIT SENSOR FAULT
following sensors:
DETECTION
Span
25°C
: 0.08mV/V to 296mV/V
To detect sensor burnout and/or short, a set of four
Initial Offset: 20mV/V
comparators (external fault comparators) are
Span and offset are based on a bridge sensor
connected to the inputs of the front-end PGA. There
excitation voltage of +5V, a PGA308 output voltage
are two fault-detect modes of operation for these
span of 4V (+0.5V to +4.5V), V
REF
of +5V, and a
comparators.
V
OUT
/V
IN
gain up to 9600. For proper PGA308 setup,
consider noise, small-signal bandwidth, V
OUT
/V
IN
Common-Mode Fault
gain, and required system error.
If either of the inputs are taken outside of the
common-mode range of the amplifier [greater than
AMPLIFICATION SIGNAL PATH
(V
S
– 1.2V), or less than 100mV], then the
corresponding comparator sets a sensor fault flag
The core of the PGA308 is a precision, low drift, and
that can be programmed to drive the PGA308 V
OUT
to
low noise front-end programmable gain amplifier
within 100mV (I
OUT
< 4mA) of either V
S
(or V
CLAMP
if
(PGA). This front-end PGA has gain capabilities from
V
CLAMP
is used) or ground. This level is well above
x4 to x1600. The output amplifier has a gain range
the set over-scale limit level or well below the set
from ×2 to ×6. A fine gain adjust in front of the output
under-scale limit level. The state of the fault condition
amplifier offers a selectable ×0.33 to ×1.0 attenuation
can be read in digital form in the ALRM register. If the
factor. This architecture yields a V
OUT
/V
IN
gain range
over-scale/under-scale limiting is disabled, the
for the PGA309 of ×2.67 to ×9600. Many applications
PGA308 output voltage is also driven within 100mV
use overall gains of ×1600 or less. The selection of
(I
OUT
< 4mA) of either V
S
(or V
CLAMP
if V
CLAMP
is used)
gains in the front-end PGA and output amplifier,
or ground, depending on the selected fault polarity
although capable of up to ×9600 overall gain, are
(high or low).
intended to allow for gain distribution throughout the
PGA308; this design enables optimum span and
Bridge Fault
offset scaling from input to output. The polarity of the
inputs can be switched through the input mux to
To assist in identifying mis-wiring, or open- or
accommodate sensors with unknown polarity output.
short-circuit conditions, the PGA308 provides bridge
Higher gains reduce bandwidth and require more
fault monitoring. For bridge fault detection, either V
S
analog filtering and/or system analog-to-digital
or V
REF
(whichever is used for bridge excitation) can
converter (ADC) averaging to reject noise.
be chosen as V
FLT
. If either of the inputs are taken to
less than the larger of either 100mV or 0.35V
FLT
, then
COARSE AND FINE OFFSET ADJUSTMENT
a fault is signaled. Also, if either of the inputs is taken
to greater than the smaller of (V
S
– 1.2V) or 0.65V
FLT
,
The sensor offset adjustment is done in two stages.
then a fault is signaled. This fault detection allows for
The input-referred Coarse Offset Adjust DAC has a
operation with bridge differential voltages of up to
±100mV offset adjustment range for a selected V
REF
30% of the bridge excitation voltage. The
of +5V. Any residual input sensor offset is corrected
corresponding comparator sets a sensor fault flag
and any desired V
OUT
offset pedestal for zero-applied
that can be programmed to drive the PGA308 V
OUT
to
sensor strain input is set by a Fine Offset Adjust
within 100mV (I
OUT
< 4mA) of either V
S
(or V
CLAMP
if
through the 16-bit Zero DAC that adds to the signal
V
CLAMP
is used) or ground. This level is well above
from the output of the front-end PGA.
the set over-scale limit level or well below the set
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