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JRE
JRE
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In addition..., the cost of the A/D converter is exponential with additional bits. You can scale the input, to read 2V, 20V, 200V etc, but with 10 bits, 0-1023, you only ever get approx 3 digits of accuracy: 000 -- 999. So what you get on a 10 bit, general purpose multi-meter is approximately 3 digits of accuracy.

You can scale that 0.999, 1.998, 3.996, 8.992, or to any intermediate value, but as you add extra range at the top, you loose accuracy in the bottom digit.

If you drop the bottom digit, you looselose information: that bottom digit '8' may be plus or minus '4', but if you don't show it at all, it's plus or minus '9'.

Because it's generally useful, the bottom digit was often scaled to +/- 0.5. That is, a reading of 1.999 means 1.999 +/- 0.0005 -- which is 10 bits of resolution.

So 3 1/2 digits means 10 bits, and 10 bits gives 3 1/2 digits.

Better meters sometimes have 11 bits, 3.999 full scale +/- 0.0005, or 12 bits, 9.999    (Yes, if you scale to 9.999 instead of 7.999 you aren't as accurate on the last digit, but that last bit is difficult anyway, and the last digit is probably +/- 0.001 on a 12-bit bench meter.)

In addition... the cost of the A/D converter is exponential with additional bits. You can scale the input, to read 2V, 20V, 200V etc, but with 10 bits, 0-1023, you only ever get approx 3 digits of accuracy: 000 -- 999. So what you get on a 10 bit, general purpose multi-meter is approximately 3 digits of accuracy.

You can scale that 0.999, 1.998, 3.996, 8.992, or to any intermediate value, but as you add extra range at the top, you loose accuracy in the bottom digit.

If you drop the bottom digit, you loose information: that bottom digit '8' may be plus or minus '4', but if you don't show it at all, it's plus or minus '9'.

Because it's generally useful, the bottom digit was often scaled to +/- 0.5. That is, a reading of 1.999 means 1.999 +/- 0.0005 -- which is 10 bits of resolution.

So 3 1/2 digits means 10 bits, and 10 bits gives 3 1/2 digits.

Better meters sometimes have 11 bits, 3.999 full scale +/- 0.0005, or 12 bits, 9.999  (Yes, if you scale to 9.999 instead of 7.999 you aren't as accurate on the last digit, but that last bit is difficult anyway, and the last digit is probably +/- 0.001 on a 12-bit bench meter)

In addition, the cost of the A/D converter is exponential with additional bits. You can scale the input, to read 2V, 20V, 200V etc, but with 10 bits, 0-1023, you only ever get approx 3 digits of accuracy: 000 -- 999. So what you get on a 10 bit, general purpose multi-meter is approximately 3 digits of accuracy.

You can scale that 0.999, 1.998, 3.996, 8.992, or to any intermediate value, but as you add extra range at the top, you loose accuracy in the bottom digit.

If you drop the bottom digit, you lose information: that bottom digit '8' may be plus or minus '4', but if you don't show it at all, it's plus or minus '9'.

Because it's generally useful, the bottom digit was often scaled to +/- 0.5. That is, a reading of 1.999 means 1.999 +/- 0.0005 -- which is 10 bits of resolution.

So 3 1/2 digits means 10 bits, and 10 bits gives 3 1/2 digits.

Better meters sometimes have 11 bits, 3.999 full scale +/- 0.0005, or 12 bits, 9.999  (Yes, if you scale to 9.999 instead of 7.999 you aren't as accurate on the last digit, but that last bit is difficult anyway, and the last digit is probably +/- 0.001 on a 12-bit bench meter.)

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david
david
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In addition... the cost of the A/D converter is exponential with additional bits. You can scale the input, to read 2V, 20V, 200V etc, but with 10 bits, 0-1023, you only ever get approx 3 digits of accuracy: 000 -- 999. So what you get on a 10 bit, general purpose multi-meter is approximately 3 digits of accuracy.

You can scale that 0.999, 1.998, 3.996, 8.992, or to any intermediate value, but as you add extra range at the top, you loose accuracy in the bottom digit.

If you drop the bottom digit, you loose information: that bottom digit '8' may be plus or minus '4', but if you don't show it at all, it's plus or minus '9'.

Because it's generally useful, the bottom digit was often scaled to +/- 0.5. That is, a reading of 1.999 means 1.999 +/- 0.0005 -- which is 10 bits of resolution.

So 3 1/2 digits means 10 bits, and 10 bits gives 3 1/2 digits.

Better meters sometimes have 11 bits, 3.999 full scale +/- 0.0005, or 12 bits, 9.999 (Yes, if you scale to 9.999 instead of 7.999 you aren't as accurate on the last digit, but that last bit is difficult anyway, and the last digit is probably +/- 0.001 on a 12-bit bench meter)