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EEVblog => EEVblog Specific => Topic started by: j33pn on May 09, 2019, 02:50:06 pm

Title: EEVBlog #232 Power Supply part 5
Post by: j33pn on May 09, 2019, 02:50:06 pm
Regarding the 2.048 voltage reference and 12 bit ADC, if b000000000000 is 0 current and b111111111111 is full scale current, then you would only have 2^12-1 divisions, not 2^12, so the math is all off a little and you have 500.122... uA per bit, not 500uA as stated.  Am I wrong?
Title: Re: EEVBlog #232 Power Supply part 5
Post by: thm_w on May 09, 2019, 09:08:51 pm
Yes this is a common mistake, but the error is so small usually it doesn't matter.

https://learn.sparkfun.com/tutorials/analog-to-digital-conversion/relating-adc-value-to-voltage

edit: see below
Title: Re: EEVBlog #232 Power Supply part 5
Post by: j33pn on May 10, 2019, 01:56:01 pm
You would think they would make 2.047V references.
Title: Re: EEVBlog #232 Power Supply part 5
Post by: thm_w on May 10, 2019, 08:42:48 pm
Good point, looks like I was wrong.

For AD7823 ADC for example, say we have 2.560V Vref. The datasheet states LSB is Vref/256 or 10mV. So at full scale reading we would have 2.55V. Full scale seems to be limited to VRef - 1 LSB.
So the 2.048V reference is OK, but you will be limited to reading 2.047V input FS.


https://www.analog.com/media/en/technical-documentation/data-sheets/AD7823.pdf (https://www.analog.com/media/en/technical-documentation/data-sheets/AD7823.pdf)
https://masteringelectronicsdesign.com/an-adc-and-dac-least-significant-bit-lsb/ (https://masteringelectronicsdesign.com/an-adc-and-dac-least-significant-bit-lsb/)
https://electronics.stackexchange.com/questions/364940/why-use-2-048v-and-4-096-as-a-reference (https://electronics.stackexchange.com/questions/364940/why-use-2-048v-and-4-096-as-a-reference)
https://www.st.com/content/ccc/resource/technical/document/application_note/group0/3f/4c/a4/82/bd/63/4e/92/CD00211314/files/CD00211314.pdf/jcr:content/translations/en.CD00211314.pdf (https://www.st.com/content/ccc/resource/technical/document/application_note/group0/3f/4c/a4/82/bd/63/4e/92/CD00211314/files/CD00211314.pdf/jcr:content/translations/en.CD00211314.pdf)