Measurement Computing USB-1408FS, User Manual

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USB-1408FS User's Guide Functional Details
21
+2.5VREF terminal
The +2.5VREF connection (pin 16) is an output terminal that supplies 2.5 volts. The images below show the
+2.5VREF pin configured as the voltage source for channel 0.
Single-ended measurement Differential measurement
SYNC terminal
The SYNC connection (pin 19) is a bidirectional I/O signal. You can use it for two purposes:

Configure as an external clock input to externally source the A/D conversions. The SYNC terminal
supports TTL-level input signals of up to 48 kHz.

Configure as an output to synchronize with a second USB-1408FS and acquire data from 16 channels. For
information on synchronizing multiple units, refer to page 24.
Ground terminals
The four analog ground (
AGND
) connections provide a common ground for all USB-1408FS input channels.
Four ground (
GND
) connections provide a common ground for the
DIO
,
TRIG_IN
,
CTR
,
SYNC
and
PC +5V
connections. Refer to the " Main connector and pin out " diagrams on page 16 for the location of these pins.
Accuracy
The overall accuracy of any instrument is limited by the error components within the system. Quite often,
resolution is incorrectly used to quantify the performance of a measurement product. While "14-bits" or "1 part
in 16383" does indicate what can be resolved, it provides little insight into the quality of an absolute
measurement. Accuracy specifications describe the actual results that can be realized with a measurement
device.
There are three types of errors which affect the accuracy of a measurement system:

offset

gain

nonlinearity
The primary error sources in the USB-1408FS are offset and gain. Nonlinearity is small in the USB-1408FS,
and is not significant as an error source with respect to offset and gain.
Figure 11 shows an example of an ideal, error-free, USB-1408FS transfer function. The typical calibrated
accuracy of the USB-1408FS is range-dependent, as explained in the "
Specifications " chapter on page 26. We
use a ±10 V range here as an example of what you can expect when performing a measurement in this range.