Measurement-computing Personal Daq rev.6.0 Manuale Utente Scaricare il pdf (Pagina 100)

5-4 Signal Management 878695 Personal Daq User’s Manual

Signal Acquisition

Measurement Duration, Sample Rate, and Resolution

Measurement Duration (per channel) ⎯ the amount of time used for sampling a channel’s input signal.

For Personal Daq, the measurement durations range from very slow (610 milliseconds/sample) to very fast

(12.5 milliseconds/sample).

Sample rate – Samples per second. The sample rate is the number of samples that take place per second.

With a slow measurement duration of 610 milliseconds, there will only be 1.6 samples per second. With a

very fast measurement duration of 12.5 milliseconds, there will be 80 samples per second.

Resolution (Bits RMS) – The number of reliable data bits that exist for a signal’s measurement. The

greater the resolution, the more detailed the reading, for example, with increased resolution a reading of

5.12 V could become 5.11896 V. Personal Daq actually provides for 24 bits of data information; however,

the accuracy of the least significant bits becomes less as the measurement duration speeds up. At a

measurement duration of 610 milliseconds, the last two bits are considered unreliable, resulting in a

resolution of 22 bits. At a very fast measurement duration (12.5 milliseconds), the nine most least

significant bits are unreliable, resulting in 15 bit accuracy.

Speed vs. Resolution

Speed Designation Measurement

Duration

(per channel)

Maximum

Sample Rate

(Samples/sec)

Resolution

(Bits RMS)

(-4 V to +4 V range)

Very Slow, 50 / 60 Hz rejection 610 ms 1.6 / sec 22

Slow, 50 Hz rejection 370 ms 2.7 / sec 22

Slow, 60 Hz rejection 310 ms 3.2 / sec 22

Medium, 50 Hz rejection 130 ms 7.7 / sec 21

Medium, 60 Hz rejection 110 ms 9.2 / sec 21

Medium 40 ms 25 / sec 19

Fast 20 ms 48 / sec 17

Very Fast 12.5ms 80 / sec 15

Notes:

1. Each channel can have independent measurement duration and resolution.

2. The sample rates shown were obtained with a 10-channel scan and with

continuous self-calibration disabled.

3. Duration does not include the use of CJC measurements.

When you select the measurement duration you also determine the sample rate and resolution for the

applicable channel. For Personal Daq’s analog input applications, sample rates range from 1.6 samples/sec

up to 80 samples/sec and corresponding resolution ranges from 22 to 15 bits.

Under Sampling and Aliasing

When you select the measurement duration you also determine the sample rate and resolution for the

applicable channel. As shown in the previous table, sample rates range from 1.6 samples/sec up to

80 samples/sec and corresponding resolution ranges from 22 to 15 bit (for Personal Daq’s analog input

applications).

When measuring highly variable input signals (as opposed to relatively steady input signals), the variable

signals will require more samples/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.

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