is tied to the analog ground. For detailed information on connections to floating signal
sources and single-ended inputs, refer to your device documentation.
Connecting Ground-Referenced Signal Sources to
Analog Inputs
A grounded signal source is connected in some way to the building system ground; therefore,
the signal source is already connected to a common ground point with respect to the device
(assuming the host computer is plugged into the same power system). Nonisolated outputs of
instruments and devices that plug into the building power system fall into this category.
•Differential Inputs—Ground-referenced signal sources do not require special
components added to the SCB-100 board. For detailed information about connections to
ground-referenced signal sources and differential inputs, refer to your device
documentation.
•Single-Ended Inputs—When measuring ground-referenced signals, the external signal
supplies its own reference ground point; the device should not supply one. Configure the
device for nonreferenced, single-ended (NRSE) input mode. In this configuration, all of
the signal grounds should be tied to AI SENSE, which connects to the negative input of
the instrumentation amplifier on the device. Referencing the signal to AI GND can cause
inaccurate measurements resulting from an incorrect ground reference. For detailed
information on connections to ground-referenced signal sources and single-ended inputs,
refer to your device documentation.
Using the SCB-100 for Thermocouple Measurements
The maximum voltage level generated by thermocouples is typically a few millivolts.
Therefore, for best resolution, use an multifunction I/O device with a high gain.
Thermocouples can be measured in either differential or single-ended configurations:
•Differential Inputs—Differential configuration has better noise immunity than
single-ended configuration. You must install bias resistors when the device is set for
differential mode; the device must have a ground reference because thermocouples are
floating signal sources.
•Single-Ended Inputs—Single-ended configuration has twice as many inputs as
differential configuration. Configure the device for referenced single-ended (RSE) inputs.
Cold-junction compensation with the SCB-100 is accurate only if the temperature sensor
reading is close to the actual temperature of the screw terminals. Therefore, when reading
thermocouples, keep the SCB-100 away from drafts or other temperature gradients such as
those caused by heaters, radiators, fans, and warm equipment.
Optional Input Filtering and Broken Thermocouple Detection
To reduce noise, you can build a simple RC lowpass filter in the breadboard area.
Build broken-thermocouple-detection circuitry by connecting a high-value resistor between
the positive input and +5 V. The value of this resistor is relatively unimportant; a few
10 | ni.com | SCB-100 Installation Guide
