PRECISION 28000
Signal Conditioning
System

Precision 28316C
Isolated IEPE Accelerometer Conditioner with Long Distance TEDS™

Condition grounded IEPE accelerometers without introducing ground loops

The 28316C provides 16 channels of precision signal conditioning for IEPE (Integrated Electronic Piezoelectric) accelerometers or remote charge converters (RCC’s). Programmable channel gain and 4-pole low-pass filters provide clean, amplified “data acquisition ready” output signals. Each channel input can be isolated to allow connection to grounded accelerometers without introducing ground loops. In addition, the 28316C may be used as a 16-channel Precision AC Filter/Amplifier with balanced differential input.

For facilitating gain setup on large channel count systems, the automatic gain setup mode can simultaneously range 256 channels on stationary signals to provide desired output signal level.

The 28316C provides a mixed mode transducer interface in conformance with IEEE 1451.4 Smart Transducer Interface. The mixed mode interface supports IEPE sensors powered by a 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.

Conventional TEDS readers in signal conditioners have been limited to a maximum cable run of 400 feet to the TEDS sensor. For applications such as weapons test, vibration test on large structures, safety and environmental testing, test article size and other factors often require cable runs in excess of 1000 feet that have precluded the use of TEDS equipped sensors.

The 28316C is equipped with Precision Filters’ proprietary Long-Distance TEDS (LDTEDS)™. The proprietary LDTEDS circuitry digitizes the TEDS waveforms and utilizes digital signal processing to read the TEDS data. Precision Filters’ LDTEDS can communicate with sensors at distances up to 1500 feet.

The test input and output monitor architecture of the 28316C makes the system ready to work with the 28000 test subsystem hardware to support automated Go/No-Go and Factory Acceptance tests.

28316C Features

16 channels per card; 256 channels per chassis
IEPE conditioner or AC Filter/Amplifier with balanced differential input
Isolated channel input allows conditioning of grounded IEPE accelerometers without introducing ground loops
Precision Filters’ LDTEDS™ (Long-Distance Transducer Electronic Datasheet) compliant
Gain x1 to x512 with 0.1% resolution
Auto-gain setup for best ADC dynamic range
Programmable 4-pole low-pass filters
Current settings of 0, 2, 4, or 8 mA (programmable per channel)
Sensor bias detector; in-range or out-of-range
AC test current checks sensor, cable and signal conditioner health
Overload detection
Output monitor, selects any one of 512 outputs
Test input for each channel

28316C Isolation Mode

Many signal conditioners for IEPE sensors utilize a grounded input whereby the sensor “low” is connected to channel ground at the signal conditioner input. This is usually the most appropriate connection since the low input is often the shield of the coaxial cable. Occasionally however, the IEPE low connection is grounded at the sensor. This can be a troublesome situation as it creates a ground loop if the low side is also connected to ground at the signal conditioner. Simply disconnecting the low connection at the signal conditioner is not an option since IEPE current must be returned directly to the IEPE current source.

To solve this difficult problem, the 28316C has a programmable input stage which can be grounded or isolated from the channel ground. In isolated mode, the low connection can be grounded at the distant sensor location and no ground loops will be created. Proprietary IEPE circuitry assures that all IEPE current will be returned through the floating low connection and not through the chassis earth ground connection.

Sensor and Cable Health

The operating bias point of an IEPE sensor may change before failing, so measuring the bias point is a means of monitoring the sensor health. The 28316C provides a readout of the bias voltage, as well as an ‘out of range’ indicator with user specified limits.

The second check is a measure of the output impedance of the sensor buffer, which may shift with a damaged sensor. A small AC calibration current is superimposed on the DC IEPE supply. This AC current reacts with the output impedance of the sensor to stimulate a sensor based AC test signal. This signal can be used as a measure of the device’s output impedance as well as to provide a stimulus for an end-to-end check of the sensor, cable, signal conditioning and data acquisition system.

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Auto Gain Ranging Mode

The auto-gain ranging mode provides a means of setting up all 256 channels in four gain ranging periods, where the gain ranging period, T, is selected from 1/8 S to 128 S depending on the signal type (periodic or random) and the response of the structure (narrow-band or wide-band). When the auto-ranging is initiated, all channels are set

to a gain of x16 and the output level detector determines if the channel output exceeded the target level at any time during the period T1.

If the target level was exceeded during period T1, the gain is set to x4. If the target level was not exceeded, the gain is set to x64. The level detectors are reset and the level detectors operate for another period (period T2).

If the target level was exceeded during period T2, the gain is decreased. If the target level was not exceeded, the gain is decreased. The level detectors are reset and the level detectors operate for another period (period T3).

At the end of four periods (4T) all 256 channels have been set to the operating gain. See the Auto-Ranging Gain Decision Tree diagram below. The output level detector is set to allow for crest factor and creep.

TEDS Support

TEDS (Transducer Electronic Data Sheet) reader/writer option is available as Option T on the 28316C card. Option T provides the ability to read stored data on TEDS-compliant devices. Precision Filters' proprietary Long Distance TEDS (LDTEDS) support provides the capability to communicate TEDS information across signal lines up to 1500 feet long.

The separate TEDS reader/writer program 28000-TEDS-READER is required to support Option T. The reader/writer program is compatible with IEEE 1451.4 V0.9 transducers.

TEDS data is displayed as a spreadsheet that can be displayed and saved as a configuration file. 28316C system information such as channel number and a time stamp of the time that TEDS information for each channel was last read are provided.

Manufacturer’s information read from the sensor including manufacturer name, model number, version number and version letter of the sensor together with sensor serial number and cal date are displayed in the table. Accelerometer Template Information in accordance with the IEEE standard is displayed as well as application dependent information. Sensor Sensitivity is used to set channel gains for EU scaling of 28316C outputs.