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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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Constant Bandwidth Amplifier Circuit Diagram

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This is a simple Constant bandwidth amplifier circuit diagram. For devices such as operational amplifiers that are designed to have a simple one-pole frequency will also be equal to the unity-gain bandwidth of the amplifier the bandwidth within which the amplifier gain is at least 1. The traditional restnct1on of constant gain-bandwidth products for a voltage amplifier can be overcome by employing feedback around a current amplifier. 



 Two current mirrors, constructed from transistors in a CA3096AE array, effectively turn the LF351 op amp into a current amplifier. Feedback is then applied by using R2 and R1, turning the whole circuit into a feedback voltage amplifier with a non-inverting gain of G of 1 + R112R2. Using the values shown, a constant bandwidth of 3.5 MHz is obtained for all voltage gains up to and beyond 100 at 10 V pk-pk output, equivalent to a gain-bandwidth product of 350 MHz from an op amp with an advertised unity gain-bandwidth of 10 MHz. 

An inverting gain configuration is also possible (see Fig. 2) where G ~ R1/ZR2. Slewing rates are significantly improved by this approach; even a 741 can manage 100 V p,s under these conditions since its output is a virtual earth. However, because the new configurations use current feedback to achieve bandwidth independence, an output buffer should be added for circuits where a significant output current is required.

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