I'm trying to familiarise myself with the DAC of the ADuC7128 (I have the eval. board Rev. 01). What I've done is to test the output of the DAC, trying both the single-ended mode and the differential mode. From what I understand (from reading the datasheet), the output range of the two modes should be different (Vref/2 +/- Vref/2 for single-ended and AVdd/2 +/- Vref/2 for differential mode.)
Since the DAC uses the internal 2.5 volts as reference, I'd expect the output of single-ended mode to range from 0V to 2.5V, and the differential mode to vary from 0.35 to 2.85 (AVdd = 3.2V, and Vref = 2.5). But what I obtained instead was that the voltage for both modes were within the range of 0.2v to 1.54v. Am I doing something wrong? I measured the output from the VDAC pin.
I followed the instructions in the datasheet, which is to enable the internal 2.5V (REFCON = 0x01), and then selected the functions by setting the values of the DACCON ( 0x036 for differential mode and 0x026 for single-ended mode), and then enabling the DAC through DACEN (set to 0x01).
From the datasheet, the output of the DAC can be controlled through the DACDAT register, so I've placed some values inside the register to test the output. What I obtain for 0x01FF (full scale) was 1.54V, 0x0000 (mid-scale) 0.88V, and 200mV.
I'd appreciate any inputs from any members of the forum,
Have you recognized the specification for the voltage swing on page 4 of the data sheet:
(0.33 × VREF ± 0.2 × VREF) × 1.33
I suggest its best to test all modes and generate a small test program which steps through the whole output-range, so you see the physical outrange on a scope i.e. very easily.