EDN Access -- 08.18.94 Twin DAC's produce true bipolar operatio

-August 18, 1994

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Design Ideas:August 18, 1994

Twin DAC's produce true bipolar operation

S Ravindranathan,
Naval Physical and Oceanographic Laboratory

The 8-bit multiplying DAC-08 is a versatile device that offers excellent performance at low cost. This DAC produces an output current that is the product of an 8-bit digital number and the input reference current. Either a unipolar voltage source for normal D/A conversion or a bipolar signal voltage for multiplying D/A conversion produces this reference current. In the latter case, the bipolar signal has to have positive dc offset. Along with the multiplicand signal, the DAC also multiplies this dc component, which then appears at the output. In attenuator applications for which the digital number acts as the attenuation-control word, the desired bipolar component in the output signal can be recovered through an output-decoupling capacitor. However, in amplitude-modulator applications where the multiplying number represents the digitized time samples of the modulating signal, this dc component, which now varies in accordance with the modulating waves, can pass through the capacitor.

The scheme in Fig 1 overcomes this drawback and delivers a true bipolar output signal. ICacts as a tracking DAC and ICas the multiplying DAC. When the dc bias at the VREF(+) input of ICis exactly equal to the quiescent bias of ICat its VREF(+) input, the voltage output at IC1's Iand Ipins precisely tracks in amplitude and phase the dc components at IC2's Iand Ipins. Differential-amplifier ICrejects these common-mode signals, leaving a true bipolar signal.

Any op amp with high CMRR, low dc offset, and gain-bandwidth product sufficient to handle the highest frequency components in the applied analog input signal at the desired gain is suitable for IC3. Adjusting Rprovides the necessary dc offset required by ICduring bipolar operation.

This circuit is much faster than any CMOS multiplying DAC.

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