Electronic – Altering reference voltage of DAC

dac

I'm making a circuit which requires an adjustable voltage gain (0 – 1).

I thought of using a multiplying DAC (R-2R type) with input voltage connected to its Vref pin. The input code to the DAC shall be adjusted using a micro-controller and once set, the input code loaded into the DAC shall remain unchanged. I expect the transfer function Vo/Vref to remain constant.

Can someone cite the disadvantages of this circuit? I plan to test it in the next couple of days.

Best Answer

Works really well. Two small drawbacks:

  1. The gain settings are on a linear scale, so minimum attenuation for an 8-bit MDAC is -48dB. For some applications this may not be enough. In an audio application I augmented it with a 20dB FET-switched gain stage and managed to get pretty close to 1dB per step from 0 to 60, except the bottom 4 steps (0 off, 1 -68dB, 2 -62dB etc).
  2. At high frequencies (well above the audio range) , the internal network is quite complex and so frequency response will not be equally flat at different attenuation settings. I should add that an 8-bit MDAC has the advantage over a 12-bit one; not only is the latter more complex, but in order to meet higher accuracy requirements, it tends to use larger FETs with lower ON-resistance but higher parasitic capacitance so its high frequency performance may be inferior.
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