2 Replies Latest reply on Nov 28, 2011 4:55 PM by ki.leung

    Buy One Get Three Free - Making Four VDAC8s from One

      After Parallel DAC and Dithering techniques, the next trick to create four DAC outputs using a single DAC, the SC/CT blocks, an LUT and the DMA.


      The four SC/CT blocks are configured as Track and Hold circuits whose inputs are connected to the VDAC8.  A four byte array in the RAM holds the values for the four DAC outputs. The DMA reads from this four byte array and writes to the DAC in sequence. An LUT component generates the trigger for the DMA and the Track and Hold circuits. The sequence goes like this.

      DMA updates VDAC with the first value, LUT generates the strobe for the TrackAndHold1 and the output of the DAC is held on output1.
      The LUT generates a trigger to the DMA which updates DAC with the second value. LUT generates a trigger to TrackAndHold2 and Output2 now has the 2nd DAC value.
      This repeats for all the four DAC values and the cycle continues.

      The refresh rate is chosen fast enough such that there is very little droop in the TrackAndHold outputs. Also, there is no CPU overhead as everything is taken care of by the DMA. The only overhead for the CPU is to update the RAM array with the desired DAC value. As the TrackAndHold has low output impedance, there is no need to buffer the outputs.

        • 1. Re: Buy One Get Three Free - Making Four VDAC8s from One

          Many case the VDAC outputs are used as just refernces for low impedance inputs. in such cases an elaborate output buffer might not be required. you can achieve more channels using external capacitors for the hold and analog pin hardware switches for the sampler. samplers being on pins mean more multiplexed channels.




          The switching frequency here would be limited by the RC constant of the series impedance of the pin switches and external capacitors. The Buffer on the VDAC output takes the VDACs output impedance out of the equation..

          • 2. Re: Buy One Get Three Free - Making Four VDAC8s from One

            Not sure if just using a capacitor is a good idea if the loading is low impedance, you need a larger cap. to hold the value, not to mention the "ripples"  due to charing and discharging of the capacitor. It may improve with a faster switching time, but would use a standalone DAC if a stable reference is neede.