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STM32 Dac strange behaviour...

Question asked by webjorn.gullik on Jan 2, 2014
Latest reply on Jan 7, 2014 by webjorn.gullik
I have been trying some waveform generation using DAC + DMA. This works quite nicely, and I am now generating I + Q waveforms for a small SDR radio project. Just to see what cand be done, I have pushed up the rates. As rates goes up, output slewing reduces the amplitude. This is expected, but I am still able to generate a reasonable waveform at 200 khz.
However, going to this rate, output level only spans 2 V pp, with a measured VDD of 2.956 V . DAC 2 is centered at half VDD, and reaches within 0.5 V from either VDD or GND.
But DAC 1 is centered on 2 V, and it does reach VDD, but the negative part only reaches 1 V, so it seems like a DC component is introduced.
So, I enabled the output buffers. Now I get some more amplitude, but the signal looks like a triangular, so slewing rate is *worse* when enabling buffers. The unbuffered waveform is a fairly nice sine, (which is from the sinetable), and slewing without buffers just smooths the signal to within 3-5% of an ideal sine, BUT with a DC offset on one DAC. Does anyone have an idea what is happening here?