ADC reading discrepancy; Avcc vs. 2.56V ----**Resolved**

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Using an STK500 & Atmega8. When taking ADC readings using a 10K pot between Vcc and Gnd, wiper connected to ADC input, I find as much as a 20mV difference (at ~1.25V input) in readings(that is, difference between voltage readings as measured via ADC) when using Avcc vs. the internal 2.56V reference. The difference exists in either 8 or 10 bit mode. Readings are stable..and the STK has 100nF cap at Aref. Vcc, and thus Avcc, is at 4.95V. Now..., I might be able to accept that difference in 8 bit mode, i.e., 4.95/255 ~ 0.02V resolution, but in 10 bit mode 4.95/1024 ~ 0.005V resolution. Operation is at room temp. Any thoughts? Perhaps I should add that I'm operating in single conversion mode, and that I always take multiple readings to verify stability.
TIA
John

Last Edited: Wed. Apr 16, 2008 - 05:17 PM
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I see you have measured AVcc down to 10mV. Have you done the same with the internal reference voltage? (Hint: See the datasheets on the specs--you will find something like +/- 8%.)

As we are seeing new models introduced--Xmega, Mega8HVA (but not in our grubby hands yet)--Atmel is adding a much more accurate 1.100V onboard reference.

Lee

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I've never met a pig I didn't like, as long as you have some salt and pepper.

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Lee,

Aha! I had looked at the thermal variation, but not absolute max variation. Turns out that measured Avref (with internal reference selected) is 2.65V..., which pretty much reduces any differences to within precision differences/noise realm. Curiously, the data sheet information (graphs, tables) all show the bandgap as ~1.3V. Presumably the buffer has a gain of 2.., though such is never mentioned.., or I've never seen it. At any rate Fig. 166, pg. 273, Bandgap vs. Vcc, shows a nominal 1.3V at room temp, and Table 16, pg. 42, Internal Voltage Reference Characteristics shows Typical Vbg as 1.3V. Assuming a gain of 2 in the buffer, and using Table 16 max/min, that means a low of 2.3V and a high of 2.8V for internal Vref. Heck.., I'm lucky I was even close......Many thanks for the tip.

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I'm a bit puzzled with the voltage references too, in a the Tiny26.

The datasheet's text calls it a 2.56V nominal reference. But the tables say the nominal value is 2.7V. Why didn't they called it 2.7V nominal reference?

Now the 1.18V reference. The text and the tables agree that it's 1.18V nominal. However, why does the graph at the end section of the datasheet shows the variation between 1.218V and 1.234V? 1.18V is not even on the graph's scale.

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