calibration

Calibration

• To adjust the gain, DAC code and CV offset, you'll need a good multimeter; ideally, 40000 counts or better (often described as 4 3/4 digits or better).

• The calibration procedure menu is entered by keeping the left encoder pushed down during start up (power-on).

1. default values:

• The first screen asks whether to use default values or not. If this is the first time you calibrate the module, or if you wish to re-calibrate from scratch, leave the selection at Yes and click (press) the right encoder to proceed. Otherwise, if you just want to adjust or fine-tune your existing calibration settings, twist the right encoder ant-clockwise so that No appears and then click the right encoder.

2. display offset:

• The next screen is used to adjust the display position. Some OLED screens are offset by two pixels, some are not (SH1106 vs SSD1306, typically). You should see a complete rectangle around the perimeter of the screen. If not, twist the right encoder until you do, then click the right encoder to proceed.

3. DAC channels A-D:

• Note that at each step, you click the right encoder to proceed, or click the left encoder to go back one step.

• If you have built you module with trimpots for the output voltages, first follow these steps (skip them and proceed to the Pass 2 steps if you have built without trimpots but have used 0.1% resistors instead).

Pass 1: trimpots

• The next screen will have a heading of DAC A -3 volts. Connect your multimeter to the channel A output (typically using alligator clip connectors on a patch cable inserted in the channel A jack, or some other equivalent arrangement). If you push the the right encoder again, you'll see DAC A -2 volts and so on. Pushing the left encoder will take you back one step; you should also see the CV output for that channel move up and down by approx. 1 volt. Ignore what else is being said on the display for the time being: at this point, we'd like to adjust the gain, so that 1 volt steps are seen at the outputs.

• Calibrate one output at a time, using the respective trimpot on the back (see photo below). Go back and forth in the menu until the steps are close to being 1v apart. Don't worry if the distance isn't exactly 1.0000v and don't worry about offset (eg 2.0034v, 3.0042v,...), we'll take care of that later; at this stage we're interested in getting it pretty much right. Repeat for each of the four channels. It is rather tedious, which is why building with 0.1% resistor and no trimpots is to be preferred. When done, proceed to the Pass 2 steps below.

• the channels map to the panel as follows: A (top left), B (top right), C (bottom right), D (bottom left).

Pass 2: DAC code

• Now repeat the procedure, but this time use the encoder to adjust the voltages, rather than the trimpots. For modules without trimpots, start at this point.

• The next screen (or the first calibration screen, if you have already gone through this once to adjust the trimpots) will have a heading of DAC A -3 volts. Connect your multimeter to the channel A output (typically using alligator clip connectors on a patch cable inserted in the channel A jack, or some other equivalent arrangement), and twist the right encoder until you see exactly -3.0000 volts on the A output. The display on your multimeter (make sure it is set to high resolution mode) might jump around between -2.9999 and 3.0000, or between -3.0000 and -3.0001. That's fine, just get it as close to -3.0000V as possible. Click the right encoder to proceed.

• The next screen will be similar, but the heading will have changed to DAC A -2 volts. Once again, twist the right encoder until the output of channel A is as close to -2.0000V as possible. Click the right encoder to proceed.

• Continue these steps, adjusting the output of channel A in 1 volt increments all the way up to +6.0000V. If your multimeter has only 40,000 counts of precision, then it will drop back to 1 millivolt resolution above 4 volts. Just try to get as close as you can to exactly 5.0000V and 6.0000V. Generally, just a few notches of encoder rotation past the point where the display flicks over from, say, 4.999 to 5.000 is the correct point. If your meter has greater resolution (such as 5 1/2 digits), then you will be able to accurately measure down to the nearest 0.1mV and get the adjust exactly right. Note the progress bar at the bottom of the screen shows how many steps have been completed.

• The procedure is then repeated for channel B, then for channel C, and finally for channel D.

3. CV inputs:

• After channel D adjustments have been completed, the next step is to adjust the CV input trimpot. Ensure nothing is connected to the module, and adjust the CV input trimpot until the pair of numbers displayed are equal (they might jump around by +/- 1 - that's fine, just get them as close to equal as possible).

• The next step adjusts the CV input offsets for each channel. With no inputs connected, twist the right encoder so that the value shown is as close to 0 as possible. There may be some jitter and it might jump around between -1 and 0, or between 0 and 1 - that's fine - just get it as close to zero as possible. Repeat this for all four CV inputs.

• Next, you need to adjust the CV input scaling at 1 volt. To do this, you need a reference voltage of exactly 1 volt. Luckily, we have already calibrated the module outputs by this stage, so we can just use those. Exactly 1.0000V should now be output on channels A to D (check this with your meter, and choose the channel with the voltage nearest 1.0000V). Connect this channel output to the CV1 input using a patch cable, and long-press the right encoder to record the offset value.

• Repeat this for the 3V reference point (at this point +3.0000V will be output on channels A to D).

• Finally, save the results, and you're finished!