Motorola AN-587: on the design of an op-amp current source


It is true that I hold a high regard for current sources due to their limitless applications in the biosciences. Current sources are at the heart of electroplating systems for electrode manufacture, this web
stimulation of tissue and imaging. This is only a small part of the reason that I find this 1973 application note so appealing. The most important message, to me, is the complete walk-through from the basic governing equations based on ideal op-amps, to non-ideal characteristics, to error propagation.

In this case, the current source becomes less important than the design process. One of the subtle issues that can be seen from the equations is that resistor matching can degrade circuit accuracy just as much as the op-amp quality. For this reason, it may be beneficial to spend an $0.05 on more accurate resistors and save $1.00 on an op-amp. The converse is also true: you can spend an extra $1.00 on a more precise op-amp to save $0.05 on passive components. The standard error from your complete circuit may end up being comparable in the two cases.

( an587-d.pdf )

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