Question

P5-Consider a standard strain-gage bridge with R, as the only active gage and R3-R4. Obtain an expression for R, in terms of R2. vo, and vref. Show that when R1-R2. we get v 0 -a balanced bridge- as required. Note that the equation for R. assuming that vo is measured using a high-impedance voltmeter, can be used to detect large resistance changes in R1. Now suppose that the active gage R1 is connected to the bridge using a long, twisted wire pair, with each wire having a resistance of Rc. The bridge circuit has to be modified as in Figure P5 in this case, Using the expression obtained earlier for R1, show that the equation of the modified bridge is given by re 2 2v ref 2 re Obtain an expression for the fractional error in the R1 measurement due to cable resistance Rc. Show that this error can be decreased by increasing R2 and vref-

Answer 1

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