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LTC2431 Fiches technique(PDF) 34 Page - Linear Technology |
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LTC2431 Fiches technique(HTML) 34 Page - Linear Technology |
34 / 40 page LTC2430/LTC2431 34 24301f practice in earlier generations of load-cell interfaces, how- ever the accuracy of the LTC2430/LTC2431 changes the rationale. Achieving high gain accuracy and linearity at higher gains may prove difficult, while providing little benefit in terms of noise reduction. At a gain of 100, the gain error that could result from typical open-loop gain of 160dB is –1ppm, however, worst-case is at the minimum gain of 116dB, giving a gain error of –158ppm. Worst-case gain error at a gain of 34, is –54ppm. The use of the LTC1051A reduces the worst- case gain error to –33ppm. The advantage of gain higher than 34, then becomes dubious, as the input referred noise sees little improvement and gain accuracy is poten- tially compromised. Note that this 4-amplifier topology has advantages over the typical integrated 3-amplifier instrumentation ampli- fier in that it does not have the high noise level common in the output stage that usually dominates when an instru- mentation amplifier is used at low gain. If this amplifier is used at a gain of 10, the gain error is only 10ppm and input referred noise is reduced to 0.28 µVRMS. The buffer stages can also be configured to provide gain of up to 50 with high gain stability and linearity. Figure 40 shows an example of a single amplifier used to produce single-ended gain. This topology is best used in applications where the gain setting resistor can be made to match the temperature coefficient of the strain gauges. If the bridge is composed of precision resistors, with only one or two variable elements, the reference arm of the bridge can be made to act in conjunction with the feedback resistor to determine the gain. If the feedback resistor is incorporated into the design of the load cell, using resis- tors which match the temperature coefficient of the load- cell elements, good results can be achieved without the need for resistors with a high degree of absolute accuracy. The common mode voltage in this case, is again a function of the bridge output. Differential gain as used with a 350 Ω bridge is: A RR R V == + +Ω 995 12 1 175 . Common mode gain is half the differential gain. The maximum differential signal that can be used is 1/4 VREF, as opposed to 1/2 VREF in the 2-amplifier topology above. Remote Half Bridge Interface As opposed to full bridge applications, typical half bridge applications must contend with nonlinearity in the bridge output, as signal swing is often much greater. Applications include RTD’s, thermistors and other resistive elements that undergo significant changes over their span. For APPLICATIO S I FOR ATIO 0.1 µF 5V REF+ REF– IN+ IN– GND VCC 3 2 4 6 7 350 Ω BRIDGE 2431 F40 – + LTC1050 5V 0.1 µV R2 46.4k 20k 20k 175 Ω 1 µF 10 µF R1 4.99k AV = 9.95 = R1 + R2 R1 + 175 Ω + + 1 µF + LTC2430/ LTC2431 Figure 40. Bridge Amplification Using a Single Amplifier |
Numéro de pièce similaire - LTC2431_15 |
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Description similaire - LTC2431_15 |
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