Richard Clark wrote in
:

On Thu, 30 Aug 2007 10:11:22 GMT, Owen Duffy wrote:

they are both 4 digit multimeters which
doesn't mean a lot. They were used to measure 200mV with 1mV
resolution,

Hi Owen,

The convention for decades has been to describe them as 3½ Digits, or
2000 count, not 4 digit unless they could represent 9999. Adding
digits does not generally add precision, resolution, monotonicity, or
accuracy. However, as it costs money to add a digit, the underlying
circuitry could usually support "some" of these attributes. Better
instruments perform rounding after the last digit.

Hi Richard,

It is interesting in marketing hype that reference is made to 2 digit and
3 digit instruments, which implies a log based metric (10*log
(MaxReading)) when you assume a 'full count', and the same hype refers to
the upper digit if it can only have values of 0 or 1 as half a digit,
whereas it probably has a weight of log(0.5) or 0.3... so in utility
terms, a 2 1/2 digit instrument is really a 2.3 digit instrument.

In my case, I was making the measurements straddling 200mV, so I needed a
bit of headroom for outliers, say 1dB or 225mV fsd, so it was effectively
2.35 digit instrument if you followed that argument.

Nevertheless, the error introduced by the resolution issue and instrument
accuracy does not explain the experimental results... something else is
happening, and one needs to look beyond the instrument itself to form a
realistic view of measurement uncertainty when measuring narrowband
noise.

Owen