Measurement Computing USB-2408 Series - Manual (Page 14)

USB-2408 Series User's Guide Functional Details
14
An example of a 12-element list is shown in the table below.
Sample channel gain queue list
Element Channel Range
0 CH0 BIP10V
1 CH1 BIP5V
2 CH0 BIP2Pt5VOLTS
3 CH4 BIP2Pt5VOLTS
4 CH8 BIP2Pt5VOLTS
5 CH0 BIP5V
6 CH1 BIP1Pt25VOLTS
7 CH7 BIP5V
8 CH0 BIP1Pt25VOLTS
9 CH15 BIP10V
10 CH9 BIP1Pt25VOLTS
11 CH10 BIP2Pt5VOLTS
Carefully match the gain to the expected voltage range on the associated channel or an over range condition
may occur. Although this condition does not damage the device, it does produce a useless full-scale reading,
and can introduce a long recovery time due to saturation of the input channel.
For more information about analog signal connections
For more information about analog input connections, refer to the Guide to Signal Connections (this document
is available on our web site at
www.mccdaq.com/signals/signals.pdf ).
TC input mode
You can make up to eight high-resolution DIFF TC measurements with a USB-2408 Series device. Built-in
cold-junction compensation sensors are provided for each of the screw-terminal blocks and any supported TC
type can be attached to any of the 8 TC channels.
Do not connect TCs as SE – doing so can cause false readings.
You do not need to use ground-referencing resistors for TC inputs because the analog front-end circuit level-
shifts the TC output into the common-mode input range of the A/D.
USB-2408 Series devices also provide an open TC detection feature for each of the analog input channels
configured for TC measurement. This feature is enabled or disabled by software, and when enabled, it detects if
an open-circuit condition exists at the TC sensor.
USB-2408 Series devices provide electrostatic discharge (ESD) protection for each of the TC inputs. However,
before handling TC sensors, follow standard ESD practices and discharge any accumulated ESD charge.
Once the TC sensor is connected to the device, the configuration options have been selected, and the
recommended 45 minute warm up has elapsed, the device is ready to make high-resolution DIFF temperature
measurements.
Built-in cold junction compensation (CJC) automatically compensates for the additional thermal EMFs
generated by connecting the TC leads to the USB-2408 Series device terminal blocks.
CJC is performed using a high-resolution temperature sensor connected close to the device terminal blocks. The
device includes two separate CJC sensors – one on each side of the PCB. Software corrects for the additional
TCs created at the terminals.
Once the A/D and CJC data is collected, the application software uses this data to linearize to an accurate
temperature reading. The TC linearization uses the latest NIST linearization coefficients for each of the eight
TC types supported by USB-2408 Series devices.
When configuring TC sensors, keep any stray capacitance as small as possible relative to AGND to avoid
settling time and accuracy errors. For TC channels do not provide a return path to ground as this is done
internally.