USB-2416-4AO User's Guide
Functional Details
14
Leave unused input channels either floating or tied to GND (pins 18, 36, 39, 42, 44, 46, 66, 81, 84).
Source impedances should be kept as small as possible to avoid settling time and accuracy errors.
Analog input mode
The USB-2416-4AO makes high-resolution voltage measurements in either a single-ended or fully differential
mode. You can configure the USB-2416-4AO for the following voltage input ranges.
±20 V
±10 V
±5 V
±2.5 V
±1.25 V
±0.625 V
±0.3125 V
±0.15625 V
±0.078125 V
Each analog input channel has the following measurement parameters:
signal type of volts or thermocouple type J, K, T, E, R, S, B, or N
full scale voltage from -20 to + 20 VDC
Thermocouple inputs
You can make up to 16 high-resolution differential thermocouple measurements with the USB-2416-4AO.
Built-in cold-junction sensors are provided for each of the screw-terminal blocks, and any supported TC type
can be attached to any of the 16 thermocouple channels.
Do not connect thermocouples as
single-ended
—doing so can cause false readings.
You do not need to use ground-referencing resistors because the analog front-end circuit level-shifts the
thermocouple's output into the common-mode input range of the A/D.
When configuring thermocouple sensors, keep any stray capacitance as small as possible relative to GND
(pins 18, 36, 39, 42, 44, 46, 66, 81, 84) to avoid settling time and accuracy errors.
The USB-2416-4AO also provides an open thermocouple detection (OTD) feature for each of the analog input
channels configured for thermocouple measurement. This feature is automatically enabled as part of the overall
acquisition process, and detects if an open-circuit condition exists at the thermocouple sensor.
The USB-2416-4AO provides electrostatic discharge (ESD) protection for each of the thermocouple inputs.
However, before handling thermocouple sensors, follow standard ESD practices and discharge any accumulated
ESD charge.
The USB-2416-4AO includes an ESD protection device at each of the analog input terminal block pins. If an
ESD or transient voltage is coupled into the device, the input protection device(s) clamps the voltage to protect
the thermocouple input circuitry from possible damage.
Once the thermocouple sensor is connected to the USB-2416-4AO, the configuration options have been
selected, and the recommended 45 minute warm up has elapsed, the USB-2416-4AO is ready to make high-
resolution differential voltage measurements.
Cold junction compensation (CJC) is needed to compensate for the additional thermal EMF's generated by
connecting the thermocouple leads to the USB-2416-4AO terminal blocks.
CJC is performed using a high-resolution temperature sensor connected close to the USB-2416-4AO's terminal
blocks. The USB-2416-4AO includes four separate CJC sensors—two on each side of the PCB. Software
corrects for the additional thermocouples created at the terminal.
Once the A/D and CJC data is collected, the application software uses this data to linearize to an accurate
temperature reading. The thermocouple linearization uses the latest NIST linearization coefficients for each of
the eight thermocouple types supported by the USB-2416-4AO.
