DAC output offset spec with output buffer enabled

[calvin]

Hi,

Currently, we are facing an accuracy problem when using the built-in DAC on the STM32F407 (with buffer enabled). As the built-in reference voltage measured 2.0005V, we set the DOR to 0x800, expecting to get 1.25V on the DAC output. However, we actually measured 1.238V, which is 122mV away from the ideal. All tests were made with the DAC under no load.

As I've seen in datasheets and many posts, the mentioned DAC output range will be 0.2V~ (VDDA-0.2V) when the buffer is enabled. Could I suspect that the DAC output offset specification is within (IDEAL ±0.2V)? Given the 122mV error mentioned above, it's hard to estimate the output accuracy without any listed specifications to refer to.

Overall, I think the built-in buffer actually works and is usable, but we really need more information about it to leverage it better.

Thanks.

[waclawek.jan]

Thanks a lot for the numbers.

The buffer opamp appears to have a consistent offset, as probably expected; which maybe could've been compensated by calibration. However, that offset is individual, probably will have a temperature-dependent component to it, and as you've demonstrated in your second post, is also supply-voltage dependent.

JW

[Michal Dudka]

I've measured STM32F303 DAC output with and without buffer against multimeter (about 0.1% , UT71A) point by point in loaded and unloaded condition. 

Results with buffer loaded into 27k+27k (To GND and to VCC) are there:
 http://mcu.cz/images/newspost_images/40/94/buff_27k27k_2.png
And results without buffer and unloaded are there:
http://mcu.cz/images/newspost_images/40/94/wobuff_hiz_2.png 

complete info (in Czeh) is there:
http://mcu.cz/comment-n4094.html