OMB-DAQ-2408 Series User's Guide
Functional Details
16
Noise filtering, data rate and throughput rate
Although the OMB-DAQ-2408 Series A/D converter has a maximum data rate of 3,750 samples per second, the
actual throughput rate you observe for voltage and temperature data is determined by these formulas.
Maximum single-channel throughput:
Maximum multiple-channel throughput:
where
n
is the number of channels
See Table 18 and Table 19 in the "Specifications" chapter for details.
This drop-off in throughput rate is due to the noise filtering feature in the device. You can control the amount of
the noise filtering by adjusting the data rate setting. By reducing the data rate, the averaging of samples
increases, and noise drops correspondingly.
Figure 8 illustrates this inverse relationship. This graph applies to the A/D converter only – do not expect this
level of performance from a OMB-DAQ-2408 Series device itself.
Figure 8. OMB-DAQ-2408 Series A/D converter data rate vs. noise graph
If low noise is your main concern, you can operate the OMB-DAQ-2408 Series device at very low data rates
starting from 2.5 samples per second (S/s). At low rates, much of the noise is averaged out of the data, and
issues such as reference noise become less important.
At higher data rates, higher-frequency noise sources are not averaged out and begin to be troublesome. These
noise sources include the noise inherent in the A/D converter itself, which is not reducible.
Since TCs can pick up noise in your environment, select a data rate based on the primary noise frequency. For
example, to reduce the effect of 60 Hz noise, select a data rate of 60 (or a supported submultiple of 60, such as
10 or 5).
Multiple-channel throughput rates
When setting different sample rates for channels, be aware that all the channels will be sampled within the same
sample window
based on the channel with the lowest sample rate.
For example, if you set a 10 Hz data rate for channel 0, and a 50 Hz data rate for channel 1, basically, both
channels pass the same number of samples per second to the host computer. However, more averaging is
performed on channel 0 samples; therefore, channel 0 is sampled at a higher resolution.
The OMB-DAQ-2408 Series A/D converter performs averaging, and the number of averages equals
30,000/data rate.
In this example, channel 0 is sampled 3000 times over 100 ms, and all samples are averaged into one sample.
Then, channel 1 is sampled 600 times over 20 ms, and samples are likewise averaged into one sample.
The final samples are available to you at a maximum rate of about 8 Hz (8.245 Hz).