1, foreword
Because the influence of certain little resistance causes oversight,in circuit regular meeting of test process middling is come up against bigger error exists between experimental data and theoretical value, affect test result thereby. Cupreous resistor often exists in inductor, transformer for example, the resistor of subway rail; Because its are numeric lesser, avometer of general pressing with a finger cannot be measured come out; Normally the meeting in the lab undertakes metrical with bridge, but bridge operation formalities is more irritated, cannot numerate directly again value of block of the resistor that be measured. In view of this, we used sheet piece machine, use sheet piece the advantage of machine designed this measuring instrument. This measuring instrument can numerate from LCD indication screen directly the resistance that measures, measure limits to be Ω of 2.9999k of ～ of 10 μ Ω , can have the data of the test storing at the same time, send via serial mouth next go up a machine, through going up the powerful function of a machine, the data that can measure to place undertakes analysis, processing. The measurement of this instrument precision is as high as ± 0.1% , use 4 end to measure a way, resistance does not suffer down-lead accident and the effect that contact resistor. Measure not only handy, reading is intuitionistic, and measure precision, resolution also prep above is general bridge. Can use at lab, institute, apply to working site especially.

2, test principle

As above pursues the fundamental of this machine is to use pass on the resistor that be measured foregone constant electric current, take a resistor that be measured to go up pressure fall, via amplifier enlarge changeover is 0 ～ 3V volts d.c. , send the input that C8051F005A/D changes end next, classics is odd a machine processing, show through the LCD resistor block is worth directly finally.
Because want to realize the measurement of pair of little resistance, the resolution of requirement amplifier is so high (the climax amounts to 10 μ V) , degree of linearity is good, input impedance is tall, ask drift is low, control noise and interference rejection capacity strong, if pursue,for this we were designed 2 in the differential amplifier that place of signal processing circuit shows. This amplifier puts A1, A2 to comprise the first class to differ the circuit that distribute form by carry, a3 comprises the 2nd class to differ the circuit that distribute form, r3, R4, RW forms feedback network, introduced deepness voltage series connection to lose feedback, reason has taller input impedance, and A1, A2 chooses inphase end to regard an input as end, criterion their in all modular output voltage and drift voltage are equal also, the difference that repass A3 comprises divides type circuit, can offset each other, it has friend very strong in all the model restrains ability and lesser output drift voltage; A4 is voltage follows reversely implement, its action is to make around class is kept apart. Analyse this circuit to be able to get following equation.
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