# [volt-nuts] Basic question concerning voltage references

David davidwhess at gmail.com
Mon Feb 20 21:39:58 EST 2017

```On Mon, 20 Feb 2017 19:29:55 +0000, you wrote:

>...
>
>The A/D in a 6½ digit instrument by definition must have at least
>22 bits (2^21 = 2097152 + sign bit) and if we scale things so the
>least significant bit is 5 microvolts, we have a full scale voltage
>of 2^21 * 5e-6V = 10.48576V.
>
>QED:  Uncertainty on your 6½ digit meter on a 10V signal will always
>be at least ± 5 microvolts.
>
>If you do the same math for a true 8½ digit DVM, you need a 29 bit
>A/D converter (2^28 = 268435456 + sign) which means you can use 40
>nV as stepsize yielding 10.73V range, and thus in teory get ± 40nV
>measurements.

I just have a minor quibble with the above which does not alter your
point.

There is no requirement for an ADC output to be a power of 2.  Slope
integration and voltage-to-frequency designs commonly use or used BCD
counters to produce a power of 10 output.

I remember when supressing negative zero was a new feature.
```