Electronic – Accuracy vs Precision

They're using a ADS1247 for the ADC in that app-note.

This is a 24 bit ADC, so the maximum output count is \$2^{24}\$ or 16,777,216. Considering their values are ~8,380,411, which is very close to \$\frac{2^{24}}{2}\$ (which is 8,388,608, so the offset is 8,197).

Based on that fact, I would assume that they're simply doing a improper twos complement conversion, which would effectively do exactly what you describe in the question - shift a range of +/- 8M to 0-16M.

The ADC in question (ADS1247) does not seem capable of producing a non-signed output, so I have no idea how they're getting those values, but they wouldn't make sense unless they're converting their signed readings into unsigned numbers somehow.

Basically, I suspect your thinking is entirely correct. What they're showing looks exactly like what you would see if one was improperly converting a signed value to an unsigned value.

Unfortunately, they don't detail the digital end of their test-setup in any way, so there is no way to know for sure.

1. Generally if an accuracy is given as 1.5%, it is assumed that this means ±1.5% unless otherwise stated. Thus you should not substitute one for the other.
2. Precision can be better than accuracy. In your example, however, since the readings are near the edge of the circle and spread around, the precision is not much better than the accuracy. If the readings were closer together then the precision would be better than the accuracy.
3. Precision could be expressed using ± meaning that the readings will cluster around the average reading within that percentage.
4. Again, if you express precision without the ± sign, then the ± will generally be assumed. It is better to use the ± sign and avoid any ambiguity.