Measurement Computing USB-2408 Series - Manual (Page 16)

USB-2408 Series User's Guide Functional Details
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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 USB-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).
Figure 9. USB-2408 Series data rate vs. resolution example
Analog output (USB-2408-2AO only)
The USB-2408-2AO has two 16-bit analog outputs ( AOUT0 and AOUT1 ). Both outputs can be updated
simultaneously at a rate of 500 S/s per channel. One output can be updated at a rate of 1000 S/s. The output
range is fixed at ±10 V. The outputs default to 0 V when the host computer is shut down or suspended, or when
a reset command is issued to the device.
Digital I/O
You can connect up to eight digital I/O lines to DIO0 through DIO7 . The digital I/O terminals can detect the
state of any TTL-level input.
Digital input voltage ranges of up to 0 to +15 V are permitted, with thresholds of 0.6 V (low) and 2.2 V (high).
Each DIO channel is an open-drain, which, when used as an output, is capable of sinking up to 150 mA for
direct drive applications.