28314 Quad
Dynamic
Charge/IEPE Conditioner
with LDTEDS

Operating in either Charge or IEPE modes, the 28314 offer a choice of 4 or 8-pole low-pass filters or 8-pole band-pass filters. Both operating modes provide accurate measurements of wide band, low-level signals guaranteed by channel gains to 8192, frequency response to 204.6 kHz, low noise, and high accuracy circuits.

• 4 channels per card, up to 64 channels per 28000 system chassis

• Dual mode: piezoelectric or IEPE

• Floating or grounded input

• Up to 200 kHz "filtered" bandwidth or 500 kHz "wide-band" bandwidth

• Two charge conversion ranges for 50,000 or 500,000 pC FS inputs

• Extremely large time constant for quasi-static charge measurements

• Programmable IEPE current to 25 mA

• TEDS compatible with proprietary Long-Distance TEDS (LDTEDS)

• Programmable amplifier: x1 to x8192 with 0.05/vernier

• 4 or 8-pole low-pass or 8-pole band-pass filters with filter bypass (wide-band)

• 2° phase matching between any channels, DC to Fc

• Overload detection

• Precise automatic calibration

• Auxiliary front panel output connection to support the use of custom output modules

Charge Mode

In charge mode, the 28314 provides two charge conversion ranges with full-scale inputs of 50,000 or 500,000 pC. Channel gains of up to 8192 provides charge sensitivity as high as 1.6 V/pC. A programmable input stage allows operation with either grounded or isolated accelerometers. Extremely large time constants allow measurement of quasi-static charge phenomena. Low noise, low-distortion and high accuracy circuits guarantee accurate high frequency measurements of even low-level signals.

Overall measurement accuracy is assured with built-in charge gain adjustment circuits, which can be used in-situ to adjust gain under run time conditions. Verification and documentation of actual charge gain can be performed using built-in shunt calibration with secondary standard shunt calibration capacitors. The calibrated value of shunt cal capacitors is stored on card EEPROM and can be recalled by host software for exact span verification or data post processing.

IEPE Mode

In IEPE mode, the 28314 accommodates long cable runs with programmable IEPE current up to 25 mA. As with charge mode, accurate measurements of wide band, low-level signals is guaranteed by channel gains to 8192, frequency response to 200 kHz, low noise, and high accuracy circuits.

Input signal visibility is a crucial aspect of IEPE sensors as the sensors bias voltage is a useful indicator of sensor, cable and connector health. The 28314 card IEPE input stage continually monitors the DC bias voltage present on the channel input prior to the AC coupling stage. Not only is this voltage level displayed for each channel but it is also compared to user programmable upper and lower threshold limits to alert the user to a sudden shift of the bias level. A system bias level report can be requested at any time, creating a file useful for pre-test gage health documentation.

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Input Stage

The 28314 input stage connects to either the charge or the IEPE front end. The input stage has low distortion, low DC drift and ultra-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.

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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. A fully buffered output having over 20 mA of drive capability may be used to drive long output cable runs.

The 28314 contains a 4 or 8-pole low-pass filter or an 8-pole band-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.

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Long Distance TEDS

The 28314 provides a mixed mode transducer interface in conformance with IEEE 1451.4 Smart Transducer Interface. The mixed mode interface supports IEPE (Integrated Electronic Piezoelectric) sensors powered by current source and TEDS (Transducer Electronic Data Sheet) capable sensors. TEDS information such as manufacturer name, serial number, calibration data, etc. are readable by the system for use in system scaling, identification, bookkeeping, troubleshooting and other functions.

TEDS sensors may be effectively applied to test models; however, there is a restriction that the cable run between the signal conditioner and the sensor be limited to 400 feet in order to be able to properly read the TEDS. For applications such as weapons test or vibration test on large structures, safety, environment, test article size and other factors often require cable runs in excess of 1000 feet that have until now precluded the use of TEDS equipped sensors.

To overcome the communications distance limitations of conventional TEDS, the 28314 is equipped with Precision Filters’ proprietary Long-Distance TEDS (LDTEDS) hardware. The proprietary LDTEDS circuitry uses an analog-to-digital converter to digitize the TEDS waveforms and utilizes a digital signal processor to process the TEDS data. LDTEDS can communicate with sensors at distances out to 1500 feet.

To learn more about LDTEDS read the white paper "LDTEDS: A Method for Long Distance Communication to Smart Transducers with TEDS".

Applications Examples

• Load, torque, dynamic force, dynamic pressure, shock, vibration and acoustic measurements

• Piezoelectric crash tests

• Ballistics shock testing

• Machine health monitoring

• Structural response tests

• Flight tests

• Wind tunnels

• Ultrasonic transducers