Digital Electronics: Principles, Devices and Applications

490 Digital Electronics12.7 D/A Converter ApplicationsIn addition to being an integral part of some of the architectures of popular varieties of A/Dconverters, D/A converters are extensively used in a variety of other application circuits. Some commonapplications include multipliers, digitally controlled dividers, programmable integrators, low-frequencyfunction generators and digitally controlled filters.12.7.1 D/A Converter as a MultiplierThe basic D/A converter operated in the current steering mode with the output opamp wired as acurrent-to-voltage converter works as a multiplier where the output voltage is the product of theanalogue input applied at the V ref terminal and the digital word input. CMOS D/A converters aremuch better suited to multiplying applications as the multiplying capabilities of other types of D/Aconverter are restricted to a limited range of input voltage. One such application circuit where themultiplying capability of the D/A converter is used is the digitally controlled audio signal attenuator.Figure 12.19 shows the circuit diagram. The audio signal is applied to the V ref input and theattenuation code is applied to the digital input. The analogue output is the attenuated version of theinput.As audio attenuators, conventional D/A converters provide a limited range of attenuation which is256:1 or 48 dB for an eight-bit converter and 4096:1 or 72 dB for a 12-bit converter. Logarithmic D/Aconverters, which give a logarithmic relationship between the digital fraction and the output signalmatching the response of the human ear, are particularly suitable for this application. These are codedto give attenuation in equal decimal steps.12.7.2 D/A converter as a DividerIf the feedback resistance is used as the input resistor and the D/A converter is connected as a feedbackelement, the circuit acts as a divider or a programmable gain element. Figure 12.20 shows the circuitconfiguration. The output is given by V o =−V in /D For smaller values of digital fraction D theoutput increases, and the designer should ensure that the amplifier does not saturate under theseconditions.+VV refV DD R FBOUT-1DGNDD/AConverterAGND-+VOUTV INFigure 12.19 Digitally controlled audio signal attenuator.DigitalInput