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.