- From: Alberto LUSIANI <Alberto.Lusiani_at_pi.infn.it>
- Date: 01 Apr 2003 20:00:15 +0200
Hi everybody, I have a basic question about DAC conventions. If I would project a DAC based on resistive partitioning, the most straightforward outcome would be that, if I have say 8 bits and a range from 0V to 1V, the number "0" would output 0V, while the maximum number, 255, would output 255/256 * 1V, i.e. slightly less than 1V. By increasing the gain by 256/255 I could have 0V with 0 and exactly 1V with 255, but then each step would be 1/255 rather than 1/256 of 1V, furthermore 128 (the "zero" of a signed 8-bit number) would not output exactly 0.5V but something close to it. So both choiches have drawbacks. - having 255 output match the upper limit of the range means 128 (like any other number) does not match exactly 1/2 the range which could be useful to get bipolar output - having more straightforward bipolar output, with 128 matching exactly the 1/2 the range, means 255 won't reach the upper range limit For the boards I am using (and in general, for what I can see) Comedi seems to assume that the largest DAC input (4095 or 65535 typically) does match exactly the upper range (10V typically). Is this the case, with actual boards like NI PCI-6036E? This means that for this board, which has bipolar output from -10V to 10V one cannot set the DAC to produce (nominally) exactly zero. Perhaps the DAC step is so small that nobody cares. But I am interested in understanding the matter. Greetings, -- Alberto
Received on 2003-04-01Z17:00:15