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DAC and op amp provide variable-control voltage

-December 06, 2001

Early DACs contained standard R-2R ladder networks, and produced a negative output voltage. These early DACs, such as the MAX7837/7847 and the MAX523, require both positive and negative supply rails to accommodate their negative output. With the transition to single-supply ICs, however, many modern DACs operate with a single supply rail and an inverted R-2R ladder network. The inverted R-2R network produces a positive output voltage. Despite the popularity of single-supply ICs, some applications still require a negative control voltage. Figure 1 shows a circuit that satisfies this requirement. The circuit contains a modern, inverted R-2R ladder DAC and one op amp. In comparison with older DACs containing standard R-2R ladders, this approach offers lower supply voltages, higher speed, and smaller packages. The DAC, IC3, operating with a 2.5V reference voltage from IC1 and driven by microcontroller IC2, produces an output swing from 0 to 2.5V. Op amp IC4 inverts and amplifies this output to produce a 0 to –5V output. For test purposes, the software routine in Listing 1 commands the microcontroller to generate a 0 to –5V triangle-wave output.

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