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28108 Octal-Channel Wideband Bridge Conditioner






 
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28108 Octal-Channel Wideband Bridge Conditioner
Bridge Conditioner with Constant Voltage Excitation
The
28108 Octal Wideband Bridge Conditioner provides eight 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 28108 is easily configurable to operate as a
precise voltage filter/amplifier on low-level voltage inputs.
The 28108 provides constant voltage excitation and
conditioning for 4-arm resistive bridges. The 28108 features automatic
calibration of gain and offset for the entire channel, including the amplifier
and filter.
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.
Up to sixteen 28108 cards may reside in the 28000 system to
provide up to 128 channels per chassis. In addition, the 28108 may be mixed with
other conditioners in the 28000 family to meet your unique signal conditioning
requirements.
Precision 28108 features
- 8 channels per card, 128 channels per chassis
- Balanced programmable constant voltage source with remote sense
- Up to 20 V excitation delivered to the bridge
- ±10 Vcm operation
- Up to 25.5 kHz "filtered" bandwidth or 100 kHz "wide-band" bandwidth
- 2- to 6-wire plus shield transducer input interface
- Automatic bridge balance/transducer suppress
- Programmable amplifier: x1/4 to x8192 with 0.05% vernier
- 4-pole low-pass filters with filter programmable pulse/flat
characteristics
- 2° phase matching between any channels
- Overload detection
- Precise automatic calibration
- Auxiliary front panel output connection to support the use of custom
output modules (for unit without option 4)
Precision 28108 Description
Balanced Voltage Excitation
The 28108 features a programmable constant voltage excitation
supply that can source up to 20 volts to the bridge. Dedicated remote sense
lines allow the excitation supply regulator to deliver an accurate voltage 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.
Input Stage
The 28108 input stage has 100 dB of common-mode rejection, low
DC drift and low noise. 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.
28000/28108 System Block Diagram

Click diagram to enlarge
Amplifier
Programmable pre- and post-filter amplifiers provide an
overall gain of 8,192. 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 the 28108 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 5 mA of drive
capability may be used to drive long output cable runs.

Click diagram to enlarge
4-Pole Low Pass Flat or Pulse Filter
The 28108 contains a 4-pole low-pass filter
with cutoffs programmable from 100 Hz to 25.5 kHz in "flat" mode or 50 Hz to
12.75 kHz in 50 Hz steps in "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.


Custom Filters
Other filter characteristics and cutoff frequencies are
available including Butterworth and Bessel Filter types. Please consult
Precision Filters for custom filter requirements.
28108 Operations
The 28000 system with the 28108 card is controlled by the
supplied Graphical User Interface (GUI) operating on a Windows compatible PC
controller or over the network using the built-in 28000-LCS interactive command
line interpreter.


28108 Group Control Panel
click to enlarge
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Product Information
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Applications Example
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Strain gage conditioner
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Load cell conditioner
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Pressure transducer conditioner
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Piezoresistive transducer conditioner
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General purpose bridge conditioner
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Potentiometer conditioner
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Low level AC
or DC amplifier (<1 mV to 10 V inputs)
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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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