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.

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Precision 28454A Features

4 channels per card, up to 64 channels in one 28000 system chassis
Balanced differential constant current source with 20 V compliance
Programmable AC or DC coupled input
Up to 200 kHz "filtered" bandwidth or 700 kHz "wide-band" bandwidth
2 or 4-wire input plus shield per channel
Manual or automatic zero suppress
Sensor open and short detection
Excitation current monitor
Voltage substitution test signals
AC current test mode for verifying transducer, cabling and frequency response
Programmable amplifier: x1/16 to x8192 with 0.05% vernier
4 or 8-pole anti-alias filters with filter bypass (wide-band)
2° phase matching between any channels
Overload detection
Precise digital calibration
Auxiliary front panel output connection to support the use of custom output modules