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06-03-2014

Two-quadrant multiplying DAC utilizes octal CMOS buffer

74HC244

Ajoy Raman

EDN

Utilizing the large operating voltage range of an octal CMOS buffer, this Design Idea presents a simple 8-bit two-quadrant multiplying Digital-to-Analog-Converter (DAC) built with the buffer/line-driver IC 74HC244.

Two-quadrant multiplying DAC utilizes octal CMOS buffer
Figure 1.  

In Figure 1, an 8-bit digital word is fed through resistors R1-R8 to the eight inputs of the CMOS buffer U1. The outputs of U1 are combined through a 1:2:4:8...128 weighted resistor network formed by resistors R9-R23. The DAC reference voltage Vref is fed to U1 VCC. The outputs of U1 thus track Vref. Resistors R1-R8 are required to prevent the output voltages of U1 from being influenced by the voltage-levels of the digital inputs.

An 8-bit voltage-output unipolar DAC with Vref=VCC is formed at pin 3 of U2. C1 optionally low-pass filters this output, which is buffered, amplified by a factor two, and offset by Vref, to provide a bipolar DAC output at pin 6 of the op-amp, an LF357.

Two-quadrant multiplying DAC utilizes octal CMOS buffer
Figure 2.
 

Figure 2 shows the DAC output for a 16-samples/cycle, 3750 Hz synthesized sine wave for values of Vref from 2 V to 7 V. The measured settling time of this DAC is 200 ns with C1 = 200 pF.

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  • The output stage of the Converter is not standard. What is the sense of external Assembly resistors? This thing is quite useful. Especially when you need from mirocontrollers translate numeric value in analog.

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