Measurement Computing USB-2408 Series - Manual (Page 22)

USB-2408 Series User's Guide Specifications
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Note 1: Placing a notch of the A/D digital filter at 60 Hz (setting A/D data rate = 60 S/s, 10 S/s, 5 S/s or
2.5 S/s) further improves the common mode rejection of this frequency.
Channel configurations
When any item is changed, the firmware stores channel configurations in the EEPROM of the isolated
microcontroller. An external application issues commands over the USB to make changes, and the configuration
is made non-volatile through the use of the EEPROM.
When connecting differential voltage inputs to a floating voltage source, provide a DC return path from each
voltage input to ground. To do this, connect a resistor from each input to an AGND pin. A value of
approximately 100 kΩ can be used for most applications. Leave unused input channels either floating or tied to
AGND. For each voltage/thermocouple channel configured as differential, you essentially lose one single-ended
channel.
Keep source impedances as small as possible to avoid settling time and accuracy errors.
Table 2. Channel configurations
Channel Category Specification
CxH/CxL Thermocouple 8 differential channels
CxH/CxL Voltage 16 individually-configurable channels that can be configured as either single-ended or
8 differential.
CxH/CxL Voltage
Compatible sensors
Table 3. Compatible sensor type specifications
Parameter Conditions
Thermocouple J: –210 °C to 1200 °C
K: –270 °C to 1372 °C
R: –50 °C to 1768 °C
S: –50 °C to 1768 °C
T: –270 °C to 400 °C
N: –270 °C to 1300 °C
E: –270 °C to 1000 °C
B: 0 °C to 1820 °C
Accuracy
Thermocouple measurement accuracy
Thermocouple measurement accuracy specifications include polynomial linearization error, cold-junction
compensation measurement error, and are for sample rates up to 60S/s. These specs are for one year, or 3000
operating hours, whichever comes first.
There is a CJC sensor for each terminal block of the module. The accuracy listed below assumes the screw
terminals are at the same temperature as the CJC sensor.
The accuracy errors shown do not include the inherent accuracy error of the thermocouple sensor itself. Contact
your thermocouple supplier for details on the actual thermocouple sensor accuracy limitations.
Connect thermocouples to the USB-2408 Series device such that they are floating with respect to AGND .
When configuring thermocouple sensors, keep any stray capacitance relative to AGND as small as possible to
avoid settling time and accuracy errors.
The AGND and DGND pins are isolated from earth ground. You can connect thermocouple sensors to voltages
referenced to earth ground as long as isolation between the AGND/DGND pins and earth ground is maintained.