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Digital voltmeters generally display decimal digits. The resolution implied by that is not necessarily the same as the underlying binary A/D. Since voltmeters for human display don't need to be f...
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#2: Post edited
by
Olin Lathrop
·
2020-12-12T13:43:17Z (over 3 years ago)
- Digital voltmeters generally display decimal digits. The resolution implied by that is not necessarily the same as the underlying binary A/D.
- Since voltmeters for human display don't need to be fast, but high resolution is good, these usually use delta-sigma A/Ds inside. These can have 20 or more bits, but are "slow" relative to other A/D technologies. When you want to update a visible number no more than every 500 ms, even a "slow" 20 ms or 50 ms A/D is plenty fast.
- However, what the voltmeter manufacturer promises is more than just resolution. Accuracy and linearity are important too. As a result, the internal A/D usually has significantly more resolution than is ultimately displayed, with some filtering between the raw values and the displayed value.
- So, it's not so simple as a "4½ digit" voltmeter having a 16 bit A/D internally, although that is at least theoretically possible. However, it is very unlikely that the internal A/D steps match the decimal display steps. There is a processor between the internal A/D and the decimal display that can perform lots of functions, including applying some factory-measured calibration offset and scaling, filtering, range scaling, and then ultimate conversion to decimal.
Basically, the raw internal binary A/D result is a lot more decoupled from the displayed decimal value than you are imagining in your simplistic example.
- Digital voltmeters generally display decimal digits. The resolution implied by that is not necessarily the same as the underlying binary A/D.
- Since voltmeters for human display don't need to be fast, but high resolution is good, these usually use delta-sigma A/Ds inside. These can have 20 or more bits, but are "slow" relative to other A/D technologies. When you want to update a visible number no more than every 500 ms, even a "slow" 20 ms or 50 ms A/D is plenty fast.
- However, what the voltmeter manufacturer promises is more than just resolution. Accuracy and linearity are important too. As a result, the internal A/D usually has significantly more resolution than is ultimately displayed, with some filtering between the raw values and the displayed value.
- So, it's not so simple as a "4½ digit" voltmeter having a 16 bit A/D internally, although that is at least theoretically possible. However, it is very unlikely that the internal A/D steps match the decimal display steps. There is a processor between the internal A/D and the decimal display that can perform lots of functions, including applying some factory-measured calibration offset and scaling, filtering, range scaling, and then ultimate conversion to decimal.
- Basically, the raw internal binary A/D result is a lot more decoupled from the displayed decimal value than you are imagining in your simplistic example.
- <hr>
- <blockquote>some DMM is rated for 20000 counts, so it can only be 16 bits</blockquote>
- It's not clear exactly what "20,000 counts" means. Almost certainly there is a binary A/D deep down in the meter. 20,000 is between 14 and 15 bits, so there is some conversion going on.
- Since a 15 bit A/D is unlikely, all you can really say is that there is a 16 bit <i>or more</i> A/D inside. The "20,000 counts" spec may be the resolution after all conversions, range scaling, non-linearity, and other accuracy issues are taken into account. That actual A/D could be a 20 bit delta-sigma, for example.
- Good spec sheets for binary A/Ds will show the number of <i>effective</i> bits after various sources of error are taken into account. For example, a 16 bit A/D produces 16 bits of result for each reading, but might only be good to 14½ bits under certain conditions. If you tilt your head just right, you could possibly describe that as 2<sup>14.5</sup> = 23,170, rounded to "20,000" counts.
#1: Initial revision
by
Olin Lathrop
·
2020-12-09T18:19:55Z (over 3 years ago)
Digital voltmeters generally display decimal digits. The resolution implied by that is not necessarily the same as the underlying binary A/D. Since voltmeters for human display don't need to be fast, but high resolution is good, these usually use delta-sigma A/Ds inside. These can have 20 or more bits, but are "slow" relative to other A/D technologies. When you want to update a visible number no more than every 500 ms, even a "slow" 20 ms or 50 ms A/D is plenty fast. However, what the voltmeter manufacturer promises is more than just resolution. Accuracy and linearity are important too. As a result, the internal A/D usually has significantly more resolution than is ultimately displayed, with some filtering between the raw values and the displayed value. So, it's not so simple as a "4½ digit" voltmeter having a 16 bit A/D internally, although that is at least theoretically possible. However, it is very unlikely that the internal A/D steps match the decimal display steps. There is a processor between the internal A/D and the decimal display that can perform lots of functions, including applying some factory-measured calibration offset and scaling, filtering, range scaling, and then ultimate conversion to decimal. Basically, the raw internal binary A/D result is a lot more decoupled from the displayed decimal value than you are imagining in your simplistic example.