28454A
Quad Conditioner with Balanced Current Excitation
Dynamic Strain Conditioner
 28454A Quad Conditioner with
Balanced Current Excitation accommodates single arm static or
dynamic strain gages, RTDs or other resistive transducers that require
constant current to excite them. A unique balanced excitation circuit
delivers accurate current yet has excellent immunity to noise pickup.
Dynamic strain measurements may be made using only a 2-wire interface to
the transducer. Gain is programmable to x8192.
The 28454A is equipped with Precision Filters' proprietary
balanced differential constant current excitation. This topology is especially
useful for reducing pickup from interfering sources as compared with traditional
unbalanced current sources. The gage health monitor circuits indicate open or
shorted gages and the input overload detector reports overloads by out-of-band
signals which could cause in-band distortion.
Balanced Constant Current Excitation
Balanced constant current excitation provides a true balanced
input for rejection of common-mode signals. Using a proprietary constant current
source, the 28454A is able to deliver accurate excitation to the gage.
Programmable excitation provides 0 to ±25.5 mA of constant current with an
“excitation off” mode to detect self-induced signals. Gage open/short
detection is also provided.
For dynamic strain conditioning applications, the 28454A can
provide accurate measurements with only two wires by AC coupling the input. For
best AC or DC measurements (required for RTD type transducers), the 28454A provides a 4-wire Kelvin connection for remote sense.
Suppression of the gage DC operating point is performed
automatically using the zero suppress feature of the 28454A. Zero suppress allows
the use of more gain to emphasize small gage fluctuations. Zero suppress also
provides the user with an accurate means to balance a full bridge.
Excitation current flows through an accurate sense resistor in
the current loop allowing for direct monitoring of the excitation current. The
current source output may be modulated to allow AC current injection in the
loop. The frequency and amplitude of the AC current is user controlled. This
allows the user to simulate changes in gage resistance in the loop and provides
direct AC input stimulation to the signal conditioner for end-to-end system
calibration.
28454A input and output connectors
Input Stage
The 28454A input stage provides 80 dB of common-mode rejection
and may be either AC or DC coupled. AC coupling is useful for dynamic
applications where the DC bias on the transducer that can limit dynamic range
can be coupled out of the signal. With the input DC coupled, low drift and ultra
low noise (<-163 dBV/Ö Hz) is provided by the
28454A
input stage. The input stage may be shorted
to verify signal conditioner channel noise and DC offsets.
Test Bus
A 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, both drive and sink current levels
may be monitored separately making it possible to detect excitation current
leakage conditions in the external current loop.
A switch located at the output
of each channel allows for multiplexed connection to the mainframe output
monitor bus. The output monitor bus is available by a connector located on the BIF card. A second scope monitor connector is available at the front
panel of the BIF card. The monitor function is used by the
Test System or is available to the user for viewing channel outputs.
A variety of test modes are supported on the card including:
Amp short, Excitation off, Voltage Substitution, Excitation Drive Current
Monitor, Excitation Sink Current Monitor, AC Current Inject.
Amplifier
Programmable pre- and post-filter gain provides gain of 8192.
Gain is distributed both before and after the filter to provide protection from
large out of band signals or transients that could cause clipping in the filter,
distorting the data. The Gain Wizard 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. If over-voltage conditions
occur, overload detectors alert the user to the condition.
A fully buffered output having over 20 mA of drive capability
may be used to drive long output cables 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.
Custom Filters
Other filter characteristics and cutoff frequencies are
available including Butterworth and Bessel Filter types. Please consult
Precision Filters with custom filter requirements.
Click to enlarge
Precision 28454A Features
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4
channels per card, up to 64 channels in one 28000 system chassis
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Balanced differential constant current
source with 20 V compliance
-
Programmable
AC or DC coupled input
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Up
to 200 kHz "filtered" bandwidth or 700 kHz
"wide-band" bandwidth
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2
or 4-wire input plus shield per channel
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Manual
or automatic zero suppress
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Sensor
open and short detection
-
Excitation
current monitor
-
Voltage
substitution test signals
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AC
current test mode for verifying transducer, cabling and frequency response
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Programmable
amplifier: x1/16 to x8192 with 0.05% vernier
-
4 or 8-pole anti-alias filters with filter bypass (wide-band)
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2°
phase matching between any channels
-
Overload
detection
-
Precise
digital calibration
-
Auxiliary front panel output connection to support
the use of custom output modules
Channel Block Diagram
Click to enlarge
28454A Transducer Interface
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Product Information
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Applications Example
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Up to 64 channels in a single 28000 system chassis or mix
with other 28000 signal conditioning cards
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Rotating machinery
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High temperature strain gage conditioner
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Static or dynamic strain gage conditioner
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Full bridge conditioner
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RTD conditioner
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Hot wire anemometry
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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
28208 Octal Thermocouple Conditioner
28302B Dual
Vibration Amplifier
27304 Quad Dual Mode Charge Amplifier
28314 Quad Dynamic Charge/IEPE Conditioner
with LDTEDS
28316B 16-Channel TEDS/IEPE Accelerometer
Conditioner
28454A Quad Conditioner Specifications
28524 Quad Frequency to Voltage Converter
Modules
28608B Octal Programmable Filter Amplifier
27708A Octal
AC/DC Amplifier
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