Gadget Freak

Possible sparks for your inspiration, when it comes to designing your own circuits. Two more Circuit Design Ideas that have been added to the site.

They involve an easy-to-build, easy-to-use, low-cost current source, and a composite instrumentation amplifier.

Programmable current source requires no power supply, courtesy of John Guy, National Semiconductor; Edited by Charles H Small and Fran Granville

Composite instrumentation amplifier challenges single-chip device for bandwidth, offset, and noise, courtesy of Marián Štofka, Slovak University of Technology, Bratislava, Slovakia; Edited by Charles H Small and Fran Granville

Programmable current source requires no power supply
Engineering labs are usually equipped with various power supplies, voltmeters, function generators, and oscilloscopes. One piece of equipment missing from many such labs, however, is a current source. This omission is unfortunate, because a current source is useful for creating I-V (current-versus-voltage) curves, charging and discharging batteries, preloading power supplies, and many other applications.

The circuit in Figure 1 is an easy-to-build, easy-to-use, low-cost current source. It comprises three sections of BCD (binary-coded-decimal) switches, a three-terminal adjustable regulator, a handful of 1%-tolerant resistors, and a National Semiconductor LM317 three-terminal adjustable regulator.
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Composite instrumentation amplifier challenges single-chip device for bandwidth, offset, and noise
Although the prevailing number set in electronics is binary, human-machine interaction uses a decimal-number set. For this reason, designs often require the use of amplifiers with gain programmable in steps in the power of 10. Currently, Analog Devices' AD8253 monolithic instrumentation amplifier is digitally programmable with voltage gains of one, 10, 100, and 1000 (Reference 1).

This IC has high bandwidth at lower gains, but you inevitably sacrifice this bandwidth when the amplifier has a gain of 1000. If your application's demands for bandwidth reach the megahertz range at a gain of 1000 and if offset and noise performance prevail over circuit complexity, then a composite amplifier may fill the bill (Figure 1).
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