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National Instruments 777966-21 24-Channel Strain/Bridge Input Module

The National Instruments 777966-21 (or SCXI-1521) is a 24-channel strain/bridge input module. It is ideal for applications such as static and fatigue testing using 350 Ω quarter-bridge strain gauges. Its high channel density supports multi channel test environments that make it suitable for large scale structural analysis. The module features 24 differential input channels that provide a voltage gain of 42 and is DC-coupled. Each of the inputs is protected against overvoltage with ± 11 V protection both when powered on and off. Nonlinearity in measurements is capped at a maximum of 0.02 %. The gain error after calibration is minimal at 0.1 % of reading further supporting the precision required in strain and bridge measurements.

The 777966-21 module is capable of scanning each channel at different accuracies depending on the speed. It offers options such as ± 0.012 % accuracy at a 3 µs interval or a higher ± 0.0015 % accuracy at a 20 µs interval. The module also features noise rejection and a low noise level of 2.5 µVpp in the 0.1 to 10 Hz range. The built-in 2-pole Butterworth filter with a 12 dB/octave rolloff at 10 Hz eliminates high frequency noise which is essential for clear signal capture in noisy environments. The NI 777966-21 module also includes built-in excitation settings ranging from 0.0 to 5.0 V. It delivers up to 21 mA of current to support a wide range of bridge configurations. Load regulation is maintained at 0.08 %. This ensures stable excitation under varying load conditions while temperature drift is minimized at ± 10 ppm/°C.

This strain/bridge input module offers ± 0.45 % of excitation voltage with a resolution of 4,096 counts for offset null compensation. This aids in accurate compensation for any offset errors during measurements. The built-in bridge completion resistors (350 Ω and 120 Ω options) are stable and ensure reliable long term performance with a maximum drift of 0.02 % per year. The 777966-21 module is also designed for stability under different environmental conditions with a recommended warm up time of 30 minutes to ensure consistent measurements. Gain drift is limited to ± 11 ppm/°C while offset drift is controlled at ± 3 µV/°C max. This maintains accuracy over a wide range of operating temperatures.