21-07-2016, 07:44 AM
Did the sums for the DAC yesterday. Conventional R-2R ladder design. Theory is well explained here: http://www.tek.com/blog/tutorial-digital...3-r-2r-dac As explained the DAC can be modelled as a voltage source and resistor. This is the Thevenin equivalent. For an bit bit DAC the voltage source is (255/256) * V. V is 3.3V is my case as the PLD outputs swing pretty much all the way to the rail if they're not loaded too heavily.
This is a crude design so I just want to roughly match 75R and get over 1V swing. THus avoiding the need for an opamp buffer. I also want to use convenient resistor values. For R/2R this means 100R/200R, 120R/240R or 150R/300R. Looks like 120/240 will be about right. The thevenin resistance of the DAC is 120R. Putting 200R across the output will lower than to 75R (120R//200R = 75R) The thevenin voltage will be 3.3V * (255/256) * (200/(200+120)) = 2.054V. This is just over the 2V that I need to give 1Vp-p into a 75R load. A single LSB will be 4.03mV.
Obviously I can't do 100% colour bars with that DAC - I'd need over 1.2Vp-p for that - but EBU bars are fine and they don't go over 1V.
Might give it a go on the bench today. Solder up some resistors and program in a simple ramp to test the DAC.
A crude low pass filter wouldn't come amiss. I could probably program a frequency sweep to check that but it also doesn't want too much overshoot or ringing. It will be crude, though not as crude as the 470p hung across the output of my 3 bit DAC. It only really needs to get subcarrier and low frequencies at the same ampplitude while giving half decent looking edges.
This is a crude design so I just want to roughly match 75R and get over 1V swing. THus avoiding the need for an opamp buffer. I also want to use convenient resistor values. For R/2R this means 100R/200R, 120R/240R or 150R/300R. Looks like 120/240 will be about right. The thevenin resistance of the DAC is 120R. Putting 200R across the output will lower than to 75R (120R//200R = 75R) The thevenin voltage will be 3.3V * (255/256) * (200/(200+120)) = 2.054V. This is just over the 2V that I need to give 1Vp-p into a 75R load. A single LSB will be 4.03mV.
Obviously I can't do 100% colour bars with that DAC - I'd need over 1.2Vp-p for that - but EBU bars are fine and they don't go over 1V.
Might give it a go on the bench today. Solder up some resistors and program in a simple ramp to test the DAC.
A crude low pass filter wouldn't come amiss. I could probably program a frequency sweep to check that but it also doesn't want too much overshoot or ringing. It will be crude, though not as crude as the 470p hung across the output of my 3 bit DAC. It only really needs to get subcarrier and low frequencies at the same ampplitude while giving half decent looking edges.
www.borinsky.co.uk Jeffrey Borinsky www.becg.tv
