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Date: 03-03-14

Challenge the Unconventional—Make Unipolar DACs Bipolar

By Yuriy Kurtsevoy, Senior Member of Technical Staff, Maxim Integrated

The trend in analog ICs is toward single-supply digital-to-analog converters (DACs). A DAC with only a positive 5V supply is convenient, but it limits the available applications to those not requiring high voltage, high current, or bipolar (±) outputs. In this article we illustrate how an external operational amplifier can transform a unipolar DAC to provide bipolar operation.

Take the Unconventional Route
The expression “up the down staircase” comes from a movie, a play, and a book by the same name.1 It is a comedy based on a school in New York City. The title recalls a rule that penalized students for going up the staircase reserved for coming down. It is always a great temptation for a youngster to run up the stairs, or escalator here, as the stairs are moving down. Some might say that the child is “thinking outside the box” or breaking the rules, and perhaps he is. Clearly, he is challenging the expected or mandated flow, the common thinking. He also shows how with some daring, a goal can be attained by an unconventional route. There is a lesson here for us engineers.

Sometimes when we design analog circuits, the design “elements” just do not want to fit together. The solution seems unusually elusive. An example situation is when we need a bipolar output from a unipolar DAC. The industry’s trend today is toward smaller, lower power, and higher performance devices, which is excellent when that resolves a solution problem. However, this same low-voltage unipolar DAC cannot function in high-performance, high-voltage, high-current, or bipolar applications directly. Any additional circuits must not degrade the DAC’s performance. In such a case, it is time to go up that escalator, try something different. We will show you how to make a bipolar output from a unipolar DAC by adding a high-voltage op amp.

Full article is available at http://www.eeherald.com/images/maxim-dac.pdf

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