Measurement Computing USB-231-OEM, User Manual, Page: 13 / 26

USB-231-OEM User's Guide Functional Details
13
Multichannel scanning
The USB-231-OEM can scan multiple channels at high rates and digitize the signals accurately. However, you
should consider several issues when designing your measurement system to ensure the high accuracy of your
measurements:

Use low impedance sources –
To ensure fast settling times, your signal sources should have an
impedance of <1 kΩ. Large source impedances incre ase the settling time of the DAQ device and decrease
the accuracy at fast scanning rates.
 Use short high-quality cabling –
Using short high-quality cables can minimize several effects that
degrade accuracy including crosstalk, transmission line effects, and noise. The capacitance of the cable also
can increase the settling time.
To ensure the specified EMC performance, operate this product only with shielded cables and accessories.
The length of any wire or cable connected to the connector must be no longer than 0.5 m (20 in.).

Avoid scanning faster than necessary –
Design your system to scan at slower speeds to give the DAQ
device more time to settle to a more accurate level when switching between channels.
Analog output
Figure 5 shows the USB-231-OEM analog output circuitry.
Figure 5. USB-231-OEM analog output circuitry
The main analog output circuitry blocks are as follows:
 Protection –
The protection circuit prevents damage of the buffers in case of a short circuit or an
overvoltage condition.
 Buffer –
The buffer amplifies the analog signal to the ±10 V range and ensures the driving capability for
the external load.
 DAC 0 and DAC 1 –
The digital-to-analog converters (DAC) convert the digital signals into low-level
analog signals.
 AO FIFO –
The AO FIFO (first-in-first out) ensures that data is transferred to the DACs in a timely manner
without being affected by USB latencies.
 DAC Control –
The DAC control sets the DAC data rate and the startup condition, which could be
triggered by the
TRIG
signal.
Analog output signals are referenced to AGND. Connect the loads between AOUT0 or AOUT1 and AGND as
shown in Figure 6 on page 14.