2006-10-20 Measuring FFO offsets

The electronics man of the Telis project came up with a script to measure any offsets in the DAC and ADC that sets input and reads measurements on the FFO (Flux Flow Oscillator).

The ADC that's used has many possibilities. It has four channels and for each of those, has a register to which an offset can be written.

measuring ffo offsets.jpg

The script measures the offsets in four steps (noted in red on the schema):

  1. The FFO bias channel is set to an internal dummy resistance. ADC channel III (ch III on the schema) should not measure any current (i.e. be 0). It probably reads somewhat above or below zero, so the scripts note the first offset and writes it to the offset register in the ADC so future measurements take this into account.
  2. The first DAC (the upper one in the picture) is set to 0 mV. This time, ADC channel I shouldn't measure any current. Again, there's probably an offset but the script only notes this internally; this DAC doesn't have registers to save an offset.
  3. For the second (lower) DAC we don't have any way of measuring an offset. However, for ADC channel II the precision is not as important as ADC channels I and III. We simply assume that the offset is the same as the first DAC.
  4. When the second DAC its offset is assumed, we measure the offset on ADC channel II and set it to the appropriate register on the ADC.

Note that the ADC has possibilities to measure the offsets itself, but in this case that can't be used: there are amplifiers just outside of the ADC that are not drawn in the schema above. These can have an offset as well and this is taken into account by the above-mentioned scripts.