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AM447 Folha de dados(PDF) 5 Page - List of Unclassifed Manufacturers |
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5 / 6 page High Precision Instrumentation Amplifier AM447 analog microelectronics April 2000 5/6 FUNCTIONAL DESCRIPTION The IC AM447 is an integrated high precision amplifier for low bridge output signals. Basically the AM447 is composed of 2 functional sections as shown in Figure 1: 1. A high accuracy instrumentation amplifier (IA) allows amplification with a high signal-to-noise ratio. The two offset compensation resistors ROC1 and ROC2 offer the possibility to make the input offset voltage of the instrumentation amplifier to nearly zero. But offset compensation over tem- perature is only given if the resistors ROC1 and ROC2 have the same temperature coefficients. Fur- thermore, these resistors have to be placed together very close. It is also necessary to use similar metals for the connection of the sensor bridge and the AM447 to avoid thermocouple effects. The internal gain of the IA is fixed to the value 102 = INT G . The output voltage VIAOUT (pin 10) of the IA is given by the following equation: () VG V V V IAOUT INT IN IN VREF =⋅ − + +− 2. An output stage de–couples the IA and thus improves the performance of the AM447. The gain factor G is fixed by the two external resistors RG1 and RG2. The gain factor of the output stage is defined by G R R ADJ G G =+ 1 2 1 The output signal VOUT (pin 10) can be calculated with () ( ) { rs transduce kalibrated for adjustment Offset adjustment Span VREF IN IN ADJ INT VREF IN IN OUT V V V G G V V V G V + − ⋅ = + − = − + − + 4 4 4 43 4 4 4 42 1 The AM447 is suited for two types of transducers. The IC is designed for usage with non- compensated and non-calibrated sensors using resistors R1 and R2 for offset calibration as well as for sensor systems with calibrated transducers. The remaining offset of the transducers can be cali- brated by variation of VVREF. The adjustment of the offset is then: CC O O O VREF V R R R V 2 1 2 + = The entire sensor systems realised with the different types of AM447 and only a few external com- ponents are shown in Figures 2 and 3. Offset calibration of the instrumentation amplifier The offset compensation has to be handled with care because the entire system performance de- pends on it. Please note, that this offset adjustment doesn't include the bridge offset. The offset compensation has to be done in the following order: • T =° 25 C and V CC = 5V • VVV IN IN REF +− == = 25 .V • The voltage between VIAOUT (pin 12) and VREF (pin 6) has to be adjusted to zero Volt. • VIAOUT is increased by increasing ROC1 and is decreased by increasing ROC2. |
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