Electronic Design

Cancel Multiplexed DAC's Output Error, And Add A Sign Bit

In some applications, like dataloggers, it's desirable to have many multiplexed analog outputs. As only a single output is active at once, a voltage-output digital-to-analog converter (DAC) and an analog multiplexer can do this. You can use this kind of signal to excite bridges. A bipolar output may also be needed, but you don't want to give up a bit of resolution.

To address the first problem, designers usually implement a multiplexer (Fig. 1a). But this arrangement has a serious drawback: The output accuracy could be greatly reduced due to the multiplexer's internal impedance, RON. Figure 1b shows the added error, E, of AOx, the final output.

Low-internal-impedance multiplexers (RON less than 4Ω) are commercially offered at medium prices (about $0.95). Compare this to the HCT4052's RON (under 280 Ω) that runs $0.10. Alternatively, you can use four op amps to buffer each output.

The circuit of Figure 2 employs an original, low-cost technique to take advantage of the HCT4052's low price. The multiplexer's internal resistance is inserted in the feedback loop of an op amp, and the error, E, is almost cancelled. To add a sign bit, the versatile circuit employs another multiplexer and a unity-gain differential amplifier, which is fed with the DAC's output, or its inverse. This arrangement can be easily modified to add an arbitrary number of outputs by simply changing U3.

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