Specifications

Signametrics 124
6.10 Calibration
Each SMX2040 DMM uses its own SM40CAL.DAT calibration file to ensure the accuracy of its functions and
ranges. The SM40CAL.DAT file is a text file that contains the DMM identification number, calibration date, and
calibration constants for all DMM ranges. For most functions, the calibration constants are scale factor and offset
terms that solve the "y = mx + b" equation for each range An input "x" is corrected using a scale factor term "m"
and an offset term "b"; this gives the desired DMM reading, "y". Keep in mind that for ranges and functions that
are unavailable for a particular product in the SMX2040 family, the calibration record contains a place-holder. An
example SM40CAL.DAT is shown:
card_id 10123 type 2044 calibration_date 06/15/1999
ad ; A/D compensation
72.0 20.0
vdc ; VDC 330mV, 3.3V, 33V, 330V ranges, offset and gain parameters
-386.0 0.99961
-37.0 0.999991
-83.0 0.999795
-8.8 1.00015
vac ; VAC 1st line - DC offset. Than offset, gain and freq each range 330mV to 250V
5.303
0.84 1.015461 23
0.0043 1.0256 23
0.0 1.02205 0
0.0 1.031386 0
idc ; IDC 3.3mA to 2.5A ranges, offset and gain
-1450.0 1.00103
-176.0 1.00602
-1450.0 1.00482
-176.0 1.00001
iac ; IAC 3.3mA to 2.5A ranges, offset and gain
1.6 1.02402
0.0 1.03357
1.69 1.00513
0.0 1.0142
2w-ohm ; Ohms 33, 330, 3.3k,33k,330k,3.3M,33M,330Meg ranges, offset and gain
1.27e+4 1.002259
1256.0 1.002307
110.0 1.002665
0.0 1.006304
0.0 1.003066
0.0 1.001848
0.0 0.995664
0.0 1.00030
The first column under any function, e.g.,"vdc", is the offset term "b", expressed as a value proportional to
analog-to-digital (a/d) counts. The second column is the scale factor term "m". Within each function, the "b" and
"m" terms are listed with the lowest range at the beginning. For example, under "2
w-ohm" above, " 1.27e+4
1.002259 " represents the offset term for the 33 Ω range, and " 1.002259 " is the scale factor for this range. This
record must be for the SMX2044 since the SMX2040 does not have the 33 Ohms range, and therefore these values
will be set to 0.0 and 1.0.
For the ACV function, the first line in the calibration record is the DC offset value. The rest of the lines contain the
RMS offset, gain correction factor, and a third column that represents a digital code from 0 to 31 that controls the
high frequency performance of each AC function. A large value, e.g., 31, implies high attenuation.
The SM40CAL.DAT file is created by performing external calibration. The general calibration algorithm consists
of applying a zero value to the DMM followed by a value of 2/3
rd
of the top of each range. Calibration of your
SMX2040 is best performed using calibration software available from Signametrics.
When using multiple DMMs in a single chassis, the SM40CAL.DAT file must have a calibration record for each
DMM. You can combine the unique calibration records of each DMM into one SM40CAL.DAT file using any
ASCII text editor.
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