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Continuous_output_sampled_bridge

2016-05-17 10:31  
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Continuous_output_sampled_bridge
Continuous_output_sampled_bridge
Figure 4-16 shows a bridge signal conditioner with sampled continuous output.Figure 4-17 shows the waveforms. Traces A, B, C, and D show the Q2 collector, LT1021-5 output, A1 output, and S1 input, respectively. The output is made continuous by the addition of a sample-hold stage. Q2 is off when the sample command is low. Under these conditions, only A2 and S1 receive power, resulting in a current drain of less than 60 μA. When the sample command is pulsed high, the Q2 collector (trace A, Fig. 4-17) goes high, providing power to all other circuit elements. During the sampling phase, supply current approaches 20 mA, but a 10-Hz sampling rate cuts effective drain below 250 μA. Slower sampling rates will further reduce drain, but the C1 droop rate (about 1 mV/100 ms) sets the accuracy constant. The 10-Hz rate provides adequate bandwidth for most transducers. For 3-mV/V-slope-factor transducers, the gain trim shown (1 M) allows proper calibration. It might be necessary to rescale the gain trim for other transducer types. The A2 output is accurate enough for 12-bit systems. Although the output is continuous, information is collected at a 10-Hz rate, and the Nyquist limit applies when interpreting the results. LINEAR TECHNOLOGY, APPLICATION NOTE 43, P. 21.