Scan Rate vs. Resolution2
Resolution
Scan Rate3,4(Scans (ENOB5
Speed Designation Scan Time1(s) per Second - SPS) RMS)
Very Slow, 50 / 60 Hz rejection 3600s>=ST >= 6.5s 0.0002778=< SR <= 22.5
0.153
Slow, 50 Hz rejection 6.5s> ST >= 1s 0.153 < SR <=1 21
Slow, 60 Hz rejection 1s> ST >=0.6s 1 < SR <= 1.67 20.9
Medium, 60 Hz rejection 0.6s> ST >=0.3s 1.67 < SR <= 3.33 20.4
Medium, 50 Hz rejection 0.3s> ST >=0.2s 3.33 < SR <= 5 20.3
Medium 0.2s> ST >=0.05s 5 < SR <= 20 19.2
Fast 0.05s> ST >=0.02s 20 < SR <= 50 18.7
Very Fast 0.02s> ST >=0.001s 50< SR <= 1000 17
Table 5-1. Scan Rate vs. Resolution (ENOB, RMS)
Notes:
1. Scan Rate = 1/(Scan Time);
2. Each channel can have independent measurement duration and resolution.
3. The scan rates and resolutions shown were obtained with a 16-channel scan and with continuous
self-calibration.
4. When measuring TC input, the CJC measurements will be treated as a channel.
5. ENOB, Effective Number of Bits
5.3 Under Sampling and Aliasing
When you select the Scan Time you also determine the scan rate and resolution
for the applicable channel. As shown in the previous table, scan rates range from
0.0002778 scans/sec up to 1000 scans/sec and corresponding resolution ranges
from 22.5 to 17 bit (for the DAQ’s analog input applications).
When measuring highly variable input signals (as opposed to relatively steady
input signals), the variable signals will require more scans/sec to obtain a
realistic signal representation. When too few samples are taken, the term under
sampling is often used. Under sampling tends to lower the signal’s graphed
amplitude. This is also known as aliasing. In such cases, the inadequate number
of samples results in a “flattening” of the signal. Gross under sampling can even
result in a relatively flat line, even though the actual signal is a waveform.
For best results the sample rate (scan rate) should be at least 10 times the
measurement frequency. If possible it is recommended that you set the scan rate
to 20 times the frequency of the input signal.
Signal Management
5
5-2