Author Topic: EEVBlog #232 Power Supply part 5  (Read 541 times)

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j33pn

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EEVBlog #232 Power Supply part 5
« 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?

thm_w

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Re: EEVBlog #232 Power Supply part 5
« Reply #1 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.

edit: see below
« Last Edit: May 10, 2019, 08:43:50 pm by thm_w »

j33pn

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Re: EEVBlog #232 Power Supply part 5
« Reply #2 on: May 10, 2019, 01:56:01 pm »
You would think they would make 2.047V references.

thm_w

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Re: EEVBlog #232 Power Supply part 5
« Reply #3 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.