Measurement Computing Personal Daq rev.6.0 - Manual (Page 53)

Single-Ended and Differential Connections to Analog Inputs
In Personal Daq applications, thermocouples should not be connected single-ended. Doing
so can result in noise and false readings. This is especially true when acquiring other high-
amplitude signals in conjunction with thermocouple signals that are connected single-ended.
Measurement Duration Column
⎯ Indicates the amount of time used for sampling a channel’s input
signal. The measurement durations range from very slow (610 milliseconds/sample) to very fast
(12.5 milliseconds/sample). Some related terms are defined as follows:
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 (Bit 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 least significant
bits are unreliable, resulting in 15 bit accuracy.
Speed vs. Resolution
1
Speed Designation Measurement
Duration
(Scan Period)
Maximum
Sample Rate
2
(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 12.5 ms 80 / sec 15
Notes: 1. Each channel can have independent measurement speed and resolution.
2. The sample rates shown were obtained with continuous self-calibration disabled.
The table values do not include the use of CJC measurements.
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