Auto-zeroing-scale


Posted on Feb 26, 2013

Electronic scales have come into wide use and the HA-510X, as a very low noise device, can improve such designs. This circuit uses a staingauge sensing element as part of a resistive Wienbridge. An auto-zero circuit is also incorporated into this design by including a sample-and-hold network. The bridge signal drives the inverting input of a differentially configured HA-5102. The noninverting input is driven by the other half of the HA-5102 used as a buffer for the holding capacitor, CH.


Auto-zeroing-scale
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This second amplifier and its capacitor CH form the sampling circuit used for automatic output zeroing. The 20-KO resistor between the holding capacitor CHand the input terminal, reduces the drain from the bias currents. A second resistor RG is used in the feedback loop to balance the effect of R8. If R7 is approximately equal to the resistance of the strain gauge, the input signal from the bridge can be roughly nulled with R6. With very close matching of the ratio R4/Rl i:o R3/R2, the output offset can be nulled by closing Sl. This will charge CH and provide a 0-V difference to the inputs of the second amplifier, which results in a 0-V output. In this manner, the output of the strain gauge can be indirectly zeroed. RIO and potentiometer Rll provide an additional mechanism for fine tuning VouT. but they can also increase offset voltage away from the zero point. Cl and C2 reduce the circuit"s susceptibility to noise and transients.




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