Relationship between DMM resolution and "Counts"

[raptor1956]

How is a meters "Counts" determined?  That is, the value of, say, 6000 counts doesn't correlate to a 12-bit (4096) or 14-bit (16384) value so just where does the count value come from?


Brian

[MosherIV]

DMMs do not use ADCs from micros, they are too noisey or expensive or not precise enough.

DMMs have always used the dual slope integrator ADC becuase there is little need for high conversion speed, dual slope are cleaner/quieter and have better precision due to the dual slope.

The counts thing seems to be traditional in volt meter world. No reason why you cannot have any number of counters, just they like to use 2000, 4000, 6000 etc counts as features to sell

[Jay_Diddy_B]

How is a meters "Counts" determined?  That is, the value of, say, 6000 counts doesn't correlate to a 12-bit (4096) or 14-bit (16384) value so just where does the count value come from?


Brian

Brian,

A lot of DMMs don't standard off the shelf ADCs. Some use a DMM chipset and others like the Keithley 2001 use a multi-slope ADC implemented in a FPGA.

So they are not constrained by powers of 2.

A lot of the early low cost DMMs were based on the Intersil ICL7106 or ICL7107 which were 2000 counts. Actually 0 -1999 with the sign bit they are +/- 1999 counts

This was described as 3 1/2  (three and a half digits)

Then there were 4  1/2 digit meters.

Then some marketing people came up with 6000 count or 60000 counts  some times called 3 3/4 digits or 4 3/4 digits.

These are useful because you get more resolution when measuring common voltages like 24 Vdc, 48 Vdc and 230 VAC.

Be  careful, don't confuse resolution and accuracy. You can have 5 1/2 digit meters that are more accurate than some 6 1/2 digit meters.

Regards,

Jay_Diddy_B

[helius]

Additionally, the meaning of "6½ Digits" may be different for some meters. The 34410A, for instance, interprets it as "six digits from 0-9, and a less significant digit that may be either 0 or 5", i.e. a range of 0.0000000 to 0.9999995, except that this meter also has a 20% over-range, so it is actually 0.0000000 to 1.2000000.

[David Hess]

DMMs have always used the dual slope integrator ADC becuase there is little need for high conversion speed, dual slope are cleaner/quieter and have better precision due to the dual slope.

Except for the ones which use voltage to frequency conversion, charge balancing, delta-sigma, multi-slope integration, etc.  And then there is whatever the old Tektronix DMM series used before they made an agreement with Fluke to stay out of the handheld DMM market.  Dual-slope integration is nice because the tolerance of the integration resistor and capacitor cancel out but it is not the only conversion technique where that is the case.

One thing in common with these designs is that they can integrate the input over an integer number of power line cycles creating a sin(x)/x response with nulls at the power line frequency and harmonics reducing interference from power line EMI.

[Srbel]

Additionally, the meaning of "6½ Digits" may be different for some meters. The 34410A, for instance, interprets it as "six digits from 0-9, and a less significant digit that may be either 0 or 5", i.e. a range of 0.0000000 to 0.9999995, except that this meter also has a 20% over-range, so it is actually 0.0000000 to 1.2000000.

I've never understood this. 6½ Digits is 0.0000000 to 0.9999995. So 6½ Digits are 8 digits???

[David Hess]

Additionally, the meaning of "6½ Digits" may be different for some meters. The 34410A, for instance, interprets it as "six digits from 0-9, and a less significant digit that may be either 0 or 5", i.e. a range of 0.0000000 to 0.9999995, except that this meter also has a 20% over-range, so it is actually 0.0000000 to 1.2000000.

I've never understood this. 6½ Digits is 0.0000000 to 0.9999995. So 6½ Digits are 8 digits???

This actually make sense.

6-1/2 digits is 1,999,999 by 1 in one format which is 2,000,000 counts or 999,999.5 by 0.5 in another format which is also 2,000,000 counts; they have exactly the same resolution.