17
Signametrics
2.12 Accuracy Notes
Important
All accuracy specifications for DCV, Resistance, DCI, ACV, and ACI apply for the time periods shown
in the respective specification tables. To meet these specifications, the System Calibration function must be
performed once a day. System Calibration is a simple software operation that takes a few seconds. Do it by
executing the
DMMCalibrate()
command, or selecting S-Cal in the control panel.
All three products are capable of continuous measurement as well as data transfer rates of up to 1,000 readings per
second (rps). To achieve the 6-1/2 digit resolution, the DMM should be operated at 5 rps or slower. The maximum
reading rate for 5-1/2 digits is 30 rps.
Accuracy vs. Reading Rates
All of the above specifications apply to reading rates of 2 rps or lower. For higher
reading rates, increase the noise floor for DCV, Resistance, and DCI by the square root of the increase in reading
rate from 2 rps. For example, the noise floor for the 3
.
3 VDC range is 8
µ
V at 5 rps. At 20 readings per second, or
10x the reading rate, the noise increases by the square root of 10, or 3.16 times. The noise, then, at 20 readings per
second is ± 25
µ
V.
The noise characteristics for the AC functions increases by the same number as the DC functions. For example, the
noise floor for the 3.3 VAC, 20 rps, will have digit rattle of 8.7 mV vs. 2.75 mV at 2 rps.
Reading Rates vs. Noise Rejection
The best AC (50 Hz, 60 Hz or 400 Hz) power line rejection is obtained at
reading rates that are whole number divisions greater than 1 of the line frequency, as shown in the following table.
For best AC line rejection you should use the reading rates checked. It is important to follow this table. Always use
the lowest checked rate that is practical for the application.
Reading Rate (rps)
Power Line frequency
50 Hz
60 Hz
400 Hz
0.1
√
√
√
0.2
√
√
√
0.5
√
√
√
1
√
√
√
2
√
√
√
5
√
√
√
10
√
√
√
15
√
20
√
√
25
√
√
30
√
40
√
50
√
√
60
√
80
√
100
√
200
√
400
√
Reading Rates vs. Digits of Resolution
For reading rates of 10 readings per second (rps) and slower, the DMM
has 6 ½ digits of resolution. For reading rates from 10 rps to 30 rps, the DMM has 5 ½ digits of resolution.
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