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High CMRR Instrumentation Amplifier (Schematic and Layout) design for biomedical applications

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Instrumentation amplifiers are intended to be used whenever acquisition of a useful signal is difficult. IA’s must have extremely high input impedances because source impedances may be high and/or unbalanced. bias and offset currents are low and relatively stable so that the source impedance need not be constant. Balanced differential inputs are provided so that the signal source may be referenced to any reasonable level independent of the IA output load reference. Common mode rejection, a measure of input balance, is very high so that noise pickup and ground drops, characteristic of remote sensor applications, are minimized.Care is taken to provide high, well characterized stability of critical parameters under varying conditions, such as changing temperatures and supply voltages. Finally, all components that are critical to the performance of the IA are internal to the device. The precision of an IA is provided at the expense of flexibility. By committing to the one specific task of
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Speed Up Composite Amplifier Circuit Diagram

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This is a Speed Up Composite Amplifier Circuit Diagram. The term “composite op amp” can mean a variety of things. In the most general sense of the word, any additional circuitry at either the input An ultra-low-noise, low-distortion op ampthe AD797is combined with the ADS 11 op amp, which offers a high bandwidth and a 100-mA output drive capability. 



Speed Up Composite Amplifier Circuit Diagram

The composite-amplifier circuit serves quite well when driving high resolution ADC`s and ATE systems. The fast AD811 operates at twice the gain of the AD797 so that the slower amplifier need only slew one-half of the total output swing. Using the component values shown, the circuit is capable of better than -90 dB THD with a 5-V, 500-kHz output signal. If a 100-kHz sine-wave input is used, the circuit will drive a 600- load to a level of 7 V rms with less than -109 dB THD, as well as a 10-kQ load at less than -117 dB THD.

The device can be modified to supply ari overall gain of 5 by changing both the R/R-m ratio and R^R2 ratio to 4:1. This raises the gains of AD811 and the total circuit while maintaining the AD797 at unity gain. If only the R/Rm ratio is changed, the circuit might become unstable. In contrast, if only the RJR2 ratio is varied, the AD797 will then operate at gain. Subsequently, the circuit will have a lower overall bandwidth. Ri should be equal to the parallel combination of Rm and Rf.

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