A weighing platform uses a full-bridge load cell of sensitivity

PHYSICS · Multi-part · Fermi University Physics · Electronics, Instrumentation & Communication · Circuit foundations and passive networks · Sensors, detectors and experimental instrumentation

Electronics for Measurement, Ch Sensors, transducers and the measurement front end

Problem

2\,\mathrm{mV/V}

, excited at

V_{\rm ex} = 10\,\mathrm{V}

dc, with a full-scale rated load of

500\,\mathrm{kg}

. The load cell and its instrumentation amplifier are mounted

5\,\mathrm{m}

apart, and a ground loop between the load cell's chassis and the amplifier's ground (of the kind discussed for grounding and shielding) injects a common-mode interference of

15\,\mathrm{mV}

onto the bridge's output leads. The system must keep the error from this interference below

0.05\%

of full scale, referred to the bridge's output. The instrumentation amplifier available is specified (like the INA103) with a minimum common-mode rejection ratio of

72\,\mathrm{dB}

at a gain of 1, rising to

100\,\mathrm{dB}

minimum at any gain of 100 or more. Downstream, a 12-bit analogue-to-digital converter has an input range of

2.00\,\mathrm{V}

but, once the noise and the anti-alias filter behind it are accounted for, resolves only to an effective

10.8

bits. ((a)) Calculate the minimum common-mode rejection ratio, in dB, needed to keep the interference error within the stated limit. ((b)) Choose an instrumentation-amplifier gain that makes the full-scale bridge signal fill the converter's input range, and state whether the amplifier meets the requirement of part (a) at that gain, with its margin in dB. ((c)) Using the converter's effective resolution, calculate the smallest change in load, in kilograms, that the chain can resolve.

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