Bridge and Strain Conditioning Overview

Balanced Constant Current

Precision Filters’ proprietary Balanced Constant Current (BCC™) excitation is optimized for making dynamic and static strain measurements on single active strain gages and RTD’s. Use 2-wire BCC to make accurate dynamic measurements on a single active strain gage or 4-wire BCC for static and dynamic strain or RTD measurements.

BCC excitation provides a true balanced input for rejection of common-mode signals. Electrostatic pickup is reduced when compared to single-ended constant current excitation or a quarter-bridge configuration with remote completion resistors or unbalanced current sources. The balanced current excitation circuit operates properly even under certain common gage fault conditions such as a direct short of the gage to the test model.

Measuring Resonant Transducers

Some transducers such as piezo-resistive pressure transducers can have very high resonant frequencies that can be excited by transient events and this out-band signal can be larger than the signal of interest.

The 28000 amplifier gain is distributed as “pre-filter” and “post-filter” gain to allow the filter to remove out-band signals such as transducer resonance while still achieving the required full-scale in-band signal level. Using the 28000 “reserve” feature, the user can specify the required protection against out-band noise and the pre- and post-filter gain are automatically distributed to provide this protection. The pre-filter gain is set to achieve the required amplifier headroom for the out-band signals prior to the filter while maximizing dynamic range. After the filter removes the out-band signals, the post-filter gain is set to achieve the desired full-scale output. The 28144 card may be equipped with either a low-pass filter or a high-pass filter cascaded with a low-pass filter to form a band-pass filter. The band-pass configuration and the distributed amplifier gain provide the user ultimate flexibility to eliminate sources of either high frequency or low frequency noise.

Frequency or Time Domain Measurements

The Precision Filters’ FLAT/PULSE filter technology allows the user to set the frequency response characteristics of the signal conditioner under program control. For transient tests or tests where time-domain wave shape preservation is important, program the filter characteristic to PULSE mode. The PULSE mode provides linear phase response, which is required for time-domain wave shape reproduction and outstanding transient response with low overshoot and ringing. For frequency domain analysis, selecting the FLAT filter characteristic will result in outstanding transfer function flatness and a sharp, selective filter response. FLAT/PULSE filter technology is available on all Precision Filters’ strain and bridge conditioners.

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Bridge Conditioners with Voltage Excitation

Where applications call for high performance, high channel-density bridge conditioning, the 28108 and 28118 octal filter/amplifiers with constant voltage excitation deliver low-noise and DC stability for either AC or DC bridge-based sensor or low-level voltage measurements.

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Bridge Conditioners with Voltage Excitation Common Features:

• 8-channels/card, 128-channels/system
• Balanced programmable constant voltage excitation
• 6-wire + shield transducer interface
• Automatic bridge balance

• Transducer suppress up to 10V of DC bias
• 4-pole FLAT/PULSE low-pass filters w/filter bypass
• Test Modes: Test-bus, Short, Zero-Excitation, Excitation Monitor

28108 4-Channel Bridge Conditioner

• 255 programmable cutoff frequency settings
• 25 kHz filtered or 100 kHz “wideband” bandwidth
• Low noise, high CMRR amplification from x1/16 to x8192 with 0.05% resolution

View the 28108 Specification Sheet >

28118 4-Channel Bridge Conditioner

• 5 programmable cutoff frequency settings from 100 Hz to 30 kHz
• 30 kHz filtered or 190 kHz “wideband” bandwidth
• Programmable amplification from x1/16 to x1,024 with 0.05% resolution

View the 28118 Specification Sheet >

Dynamic Strain with Current Excitation

Measurements of dynamic strain with PFI’s balanced constant current technology are much less susceptible to EMI pickup and lead resistance desensitization that plague single-ended constant current topologies.

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28458 Dynamic Strain Conditioners Features:

• 8-channels/card, 128-channels/system
• Balanced programmable constant current excitation with 20 V compliance
• 2-wire + shield transducer interface
• 4-pole FLAT/PULSE low-pass filters w/filter bypass
• 5 programmable cutoff frequency settings
• 100 kHz filtered or 190 kHz “wideband” bandwidth

• Programmable amplification from x1/16 to x1024 with 0.05% resolution
• On-the-fly report of measured transducer excitation resistance and leakage to ground
• Sensor open and short detection
• Test Modes: Test-bus, Short, Zero-Excitation

View the 28458 Specification Sheet >