As part of our GPSDO work Richard Mc Corkle and I tested multiple DAC's using hardware I developed and Richard wrote the code. We limited to affordable and solderable. The LTC1655 was the clear winner because of linearity and temperature, see attached. We tested dithering 20 bits and stacking two for coarse fine, storing the test data from using the LTC2400 ADC. Limited our choices to dither and bare use. We did this 5 years ago. Contact with Richard has sadly stopped. I am very concerned, however we continue to use his contributions on several projects with very good results. We use the LTC 1655 on Rb's because its resolution and range as is, is perfect. Five years later I know no better alternative Bert Kehren In a message dated 7/16/2017 5:09:08 A.M. Eastern Daylight Time, [email protected] writes:
On Sat, 8 Jul 2017 11:42:44 -0700 Tim Lister <[email protected]> wrote: > Forgive the ignorance, but why is there a large disparity between ADC > and DAC capabilities ? > For example, Linear Technology sell a 24 bit ADC for ~$7 but an 18 bit > DAC is $30-50... Much simplified, it boils down to it being easier to measure voltage differences by averaging than keeping a voltage constant. E.g. in those >20bit ADC's you will usually find a delta-sigma ADC, usually 3rd to 5th order with a 1.5 to 5 bit ADC/DAC inside. The ADC and DAC can be laser trimmed to be in the order of 0.1% of their ideal values. With a few additional tricks you can get the most of the remaining non-linearity out. These tricks also help to remove errors due to DC-offsets in the signal path. But the biggest improvement comes from averaging over many "samples" to get the white noise out. If you look at the usual sample rates at which those ADC reach their "full" performance, it is around 1-30 (output) samples per second. On the other hand, on a DAC you need to keep the output voltage stable. You can do the same delta-sigma approach as with the ADC with much the same result, but you have one big problem: it is not easy to build an analog low pass filter that has a corner frequency down at 10Hz. This means, you have to work at a much higher frequency to have a low pass filter that can be realized (let's say 1kHz if you are building a discrete filter, higher if it's integrated). But that means that you have several orders of magnitude more (white) noise. Additionally, a lot of people expect to do a couple of 1000 samples per second at least, to have a usefull DAC. But that contradicts the need to have a narrow band low pass filter to get the noise out. Attila Kinali -- It is upon moral qualities that a society is ultimately founded. All the prosperity and technological sophistication in the world is of no use without that foundation. -- Miss Matheson, The Diamond Age, Neil Stephenson _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
InitialTestResults.pdf
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