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PRECISION 28000
Signal Conditioning
System
 
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 28104A Quad-Channel Resistive Bridge Conditioner
28104A Quad Bridge Conditioner offers four
channels of conditioning to support a wide variety of transducers including
those that require constant voltage excitation in a bridge configuration such as
strain gages and pressure transducers. The 28104A is easily configured to operate
as a precise voltage filter/amplifier on low-level AC or DC voltage inputs.
The 28104A will meet your most demanding 1-, 2-, and 4-arm bridge
applications. Up to 64 channels may reside in a single 28016B-M3 chassis.
Bridge Input
The 28104A provides constant voltage excitation and conditioning for 1-, 2-,
and 4-arm resistive bridges. The 28104A features automatic calibration of gain
and offset for the entire channel, including the amplifier, filter, and
excitation supply. The 2- to 10-wire input connection provides 6 wires for the
bridge, 2 wires for shunt calibration, 1 wire for the shield, and 1 wire for
single-arm bridges.
The 28104A card with bridge configuration card and
bridge resistor module.
Precision bridge configuration resistors are contained on a plug-on module
located on each 28104A card. The resistors may be programmed to complete the
bridge for quarter, half, or full bridge configurations. ID registers are
provided to read back bridge configuration and resistance.
Bridge Auto Balance
Automatic balance of the bridge is accomplished by inserting a voltage
ratiometric with the excitation supply to the amplifier input stage. This
balance method provides outstanding stability without loading the bridge. A wide
range of unbalanced conditions may be accommodated.
Shut Calibration
Two methods of shunt calibration are available. A voltage insertion technique
provides 4096 steps of bipolar shunt calibration that may be used to simulate a
bridge unbalance. The 28104A also supports the use of precision resistors for
bipolar single step shunt calibration. For either technique, the 28104A utilizes
dedicated shunt cal lines to the bridge that allow the user to perform the shunt
cal on non-current carrying leads. Also, since the bridge balance technique is
non-invasive, shunt calibration results are unaffected by bridge balance
circuitry.
Balanced Voltage Excitation
The 28104A features a programmable constant voltage excitation supply that
can source up to 20 volts to the bridge. Balanced constant voltage excitation
offers a number of advantages over single-ended excitation. It enables a true
balanced instrumentation amplifier input for outstanding rejection of high
frequency common-mode signals. Single-ended voltage excitation to balanced
bridges produces a relatively large common-mode voltage at half the excitation
supply. The instrumentation amplifier must reject this signal. Balanced voltage
excitation applied to balanced bridges results in lower common mode input
voltages to the amplifier input stage.
The excitation supply has automatic gain and offset correction that may be
run on the unit in place at any time. Dedicated remote sense lines allow the
excitation supply regulator to deliver an accurate voltage to the bridge.
Input Stage
The 28104A input stage has 120 dB of common-mode rejection, low DC drift and
ultra-low noise. The input can be equipped with programmable AC/DC coupling. AC
coupling is useful in applications where DC levels that limit dynamic range can
be coupled out of the signal.
A programmable switch at the input stage is provided to
connect the amplifier to the 28000 system test bus. The test bus is used to
inject signals for performance verification. In addition, the excitation supply
may be connected to the input stage for direct verification of the excitation
supply level.
Amplifier and Filter
Programmable pre- and post-filter amplifiers provide an overall gain of 8192.
Gain is distributed both before and after the filter to provide protection from
large out-of-band energy or transients that could cause clipping before the
filter, distorting the data. The Gain Wizard in the GUI allows the user to set a
gain reserve and then apportions the gain between the input and output. This
provides input gain for best noise performance yet conforms to the limitations
of the user’s worst case estimate of out-band or transient signals. Overload
detectors alert the user to over-voltage conditions.
The post-filter gain has resolution of better than 0.05% to
enable precise system scaling of 28104A outputs to match the full-scale input of
the external recording device, given the transducer sensitivity and fullscale
input in measurement units. A fully buffered output having over 20 mA of drive
capability may be used to drive long output cable runs.
4 or 8-Pole Low Pass Filters
The 28104A contains a 4- or 8-pole low-pass filter with
cutoffs programmable from 1 Hz to 204.6 kHz and programmable "flat" or "pulse"
mode.
The "flat" mode provides pass-band characteristics nearly
identical to a Butterworth filter while providing a much sharper roll-off. This
mode is a good choice for applications such as spectral analysis.
The "pulse" mode has time domain response similar to the
Bessel filter yet provides superior amplitude response characteristics. The
"pulse" mode is ideal for time domain applications including transient (shock)
measurements and time domain waveform analysis.
Click to enlarge
Download the
28104A Specifications Sheet
Precision 28104A Features
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4 channels per card, 64 channels
per 28016B-M3 chassis
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Balanced programmable constant
voltage source with remote sense
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Up to 20 V excitation delivered to
the bridge
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±10 Vcm operation
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Up to 200 kHz "filter" bandwidth or
700 kHz "wide-band" bandwidth
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2 to 10 wire plus shield transducer
input interface
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Automatic bridge balance
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4096 step bipolar shunt cal or single
step bipolar resistive shunt cal; shunt any bridge arm
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Switch selectable bridge configuration
(1-arm, 2-arm, or 4-arm) with read
back
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Programmable AC/DC input coupling
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Programmable amplifier: x1/16 to x8192
with 0.05% vernier settings
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4 or 8-pole low-pass filters with filter bypass (wide-band)
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2° phase matching between any channels, DC to Fc
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Overload detection
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Precise automatic calibration
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Auxiliary front panel output connection to support
the use of custom output modules
28104A Channel Block Diagram
Click to enlarge
Sample Bridge Configuration Diagram
Click to enlarge
System Block Diagram
Click to enlarge
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Product Information
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Applications
Example
Up to 64 channels of quality resistive bridge conditioning in a single
28000 system
chassis or mix with other 28000 signal conditioning cards.
Strain gage conditioner
Load cell conditioner
Pressure transducer conditioner
Low level
AC or DC amplifier
(<1 mV to 10 V inputs)
Anti-aliasing filter/amplifier
Reference
The following Specifications
are available in Precision Filters'
On-line Library
for downloading:
28104A Quad Bridge Conditioner Specifications
28108 Octal Wideband Bridge Conditioner
28144 Quad-Channel Wideband Transducer
Conditioner with Voltage and Current Excitation
28208 Octal Thermocouple Conditioner
28302B Dual
Vibration Amplifier
27304 Quad Dual Mode Charge Amplifier
28314 Quad Dynamic Charge/IEPE Conditioner
with LDTEDS
28316C Isolated IEPE Accelerometer
Conditioner with LDTEDS
28454A Quad Conditioner Specifications
28524 Quad Frequency to Voltage Converter
Modules
28608B Octal Programmable Filter Amplifier
28708 Octal
AC/DC Amplifier
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