Measurement Computing USB-2408 Series - Manual (Page 13)

USB-2408 Series User's Guide Functional Details
13
Analog input mode
You can configure the analog inputs for SE or DIFF mode. The input voltage range is software selectable for
±10 V, ±5 V, ±2.5 V, ±1.25 V, ±0.625 V, ±0.312 V, ±0.156 V, or ±0.078 V.
With SE mode, connect up to 16 inputs to screw terminals CH0 to CH15 . SE mode requires two wires:
 Connect one wire to the signal you want to measure ( CHx ).
 Connect one wire to the analog ground reference ( AGND ).
Refer to Figure 3 on page 11 for the location of the SE inputs.
With DIFF mode, connect up to eight differential inputs to screw terminals CH0H/CH0L to CH7H/CH7L .
DIFF mode requires two wires plus a ground reference:
 Connect one wire to the high/positive signal (
CHxH ).
 Connect one wire to the low/negative signal (
CHxL ).
 Connect one wire to the analog ground reference (
AGND ).
Refer to Figure 4 on page 11 for the location of the DIFF inputs.
When connecting DIFF voltage inputs to a "floating" voltage source, make sure there is a DC return path from
each voltage input to ground. You make this path by connecting a resistor from each low channel input to an
AGND pin. A value of approximately 100 kΩ can be used for most applications. This does not apply to
channels configured for TC input, as they have their own internal reference.
Leave unused input channels either floating or tied to an AGND terminal.
Source impedances should be kept as small as possible to avoid settling time and accuracy errors.
Figure 6 shows a DIFF voltage connection using a ground path resistor.
Figure 6. DIFF voltage connection with ground path resistor example
Gain queue
Use the USB-2408 Series channel - gain queue feature to configure a list of channels, modes, and gains for each
scan. The settings are stored in a channel – gain queue list that is written to local memory on the device.
The channel - gain queue list contains one or more channel numbers and range settings. You can configure up to
64 elements. The channels can be listed in any order, and can include duplicate channels for sampling at
different ranges.