How to Connect a DAQ Device to an Analog Signal Source
Correct analog input wiring is essential for obtaining accurate and stable measurements from a data acquisition system. An incorrect connection can introduce noise, ground loops, offset errors, unstable readings or input voltages outside the permitted range of the DAQ device.
Before connecting an analog signal source, determine:
- Whether the signal source is grounded or floating
- Which terminal configuration the DAQ device supports
- The expected signal voltage and common-mode voltage
- The maximum input range of the DAQ device
- The required measurement accuracy and noise performance
- Whether the source and DAQ system share the same ground
Analog Input Terminal Configurations
Many multifunction NI DAQ devices support one or more of the following analog input configurations:
Measures the voltage difference between two analog input terminals and provides improved rejection of common-mode noise.
Measures each analog input relative to the DAQ device analog ground and provides the largest number of channels.
Measures each channel relative to a shared AI SENSE reference instead of directly using the DAQ analog ground.
Differential Input
A differential input measures the voltage between a positive input and a negative input:
Measured voltage = AI+ − AI−
Differential connections generally provide the best noise immunity because interference appearing equally on both conductors can be rejected by the DAQ input amplifier.
Differential mode is commonly preferred when:
- The signal level is low.
- The signal cable is relatively long.
- The environment contains electrical noise.
- The source and DAQ device are at different ground potentials.
- Measurement accuracy is more important than maximizing channel count.
Referenced Single-Ended Input
In RSE mode, the positive signal conductor connects to an analog input channel, and the signal return connects directly to the DAQ device analog ground.
Signal positive → AI channel
Signal return → AI GND
RSE mode provides more measurement channels and requires less wiring than differential mode. It is commonly suitable for floating signal sources, short connections and relatively high-level signals in low-noise environments.
RSE is normally not recommended for a grounded signal source because directly connecting the source ground to DAQ analog ground can create a ground loop.
Non-Referenced Single-Ended Input
In NRSE mode, each signal is measured relative to a shared remote reference connected to the DAQ device AI SENSE terminal.
Signal positive → AI channel
Signal reference or return → AI SENSE
NRSE can reduce errors caused by differences between the signal-source ground and the DAQ analog ground. It is often suitable for multiple grounded signal sources that share a common reference.
Grounded and Floating Signal Sources
Selecting the correct DAQ input configuration requires understanding the type of analog signal source.
Ground-Referenced Signal Source
A grounded signal source is connected to building ground, protective earth or another system ground. Typical examples include:
- Benchtop instruments connected through a three-wire AC power cord
- Signal generators with an earth-referenced output
- Non-isolated power supplies
- Desktop computers and plug-in instruments
- Industrial equipment connected to protective earth
Because the source and DAQ device may be connected to ground at different physical locations, a small voltage can exist between their grounds. Directly connecting these grounds can create a ground loop.
Floating Signal Source
A floating signal source is not intentionally connected to building ground or protective earth. Typical examples include:
- Batteries
- Battery-powered sensors
- Термопары
- Isolated signal conditioners
- Isolation amplifiers
- Transformer-coupled outputs
- Portable instruments operating from internal batteries
A floating source does not automatically have a defined relationship to the DAQ input ground. Without a suitable bias-current return path, its common-mode voltage can drift outside the input range of the DAQ amplifier.
Connecting a Grounded Signal Source
Recommended Method: Differential Input
Differential mode is generally the preferred connection for measuring a grounded signal source.
- Connect the source positive output to the positive DAQ input terminal.
- Connect the source return to the corresponding negative DAQ input terminal.
- Configure the channel for Differential input in NI MAX, DAQ Assistant or the NI-DAQmx API.
- Keep both signal conductors together as a twisted pair.
- Confirm the signal remains within the differential and common-mode input limits.
On E Series and M Series multifunction DAQ devices, differential channels normally use paired AI terminals. The exact channel pairing varies by device, so consult the device pinout or user manual.
Alternative Method: NRSE Input
If the DAQ device supports NRSE, a grounded source can also be connected using:
- Source positive output to the selected AI channel
- Source reference or return to AI SENSE
- DAQ channel configured for NRSE input
NRSE uses a shared reference conductor and can provide more channels than differential mode. However, differential mode normally offers better rejection of noise coupled equally into both signal wires.
Why RSE Is Not Recommended for a Grounded Source
In RSE mode, the source ground is connected directly to the DAQ analog ground. If the two systems already connect to earth at different locations, this creates a second ground path.
Current flowing through that path can introduce:
- 50 Hz or 60 Hz power-line interference
- Measurement offset
- Unstable readings
- Common-mode errors
- Noise that changes when other equipment switches on
Grounded Source Connections by DAQ Family
E Series and M Series DAQ Devices
Use Differential input: source positive to AI+, and source return to the corresponding AI− terminal.
Alternative:Use NRSE when supported: source positive to an AI channel, and source return to AI SENSE.
Generally avoid:RSE connection between a grounded source and AI GND because it may create a ground loop.
DAQPad-6015 and DAQPad-6016
Connect the grounded source using a Differential input configuration.
Alternative:Use NRSE when it is supported by the device and suitable for the measurement system.
USB-6008 and USB-6009
The USB-6008 and USB-6009 support Differential and RSE analog input configurations but do not support NRSE.
Connect the source to a Differential input pair and configure the channel for Differential measurement.
Although RSE is available, it can create a ground loop when used with an earth-referenced source. Differential mode is normally the safer measurement configuration.
Connecting a Floating Signal Source
Floating Source in Differential Mode
A floating source can be connected to a differential input, but the DAQ input amplifier requires a return path for its input bias current.
Without this return path, the input voltage can drift until the amplifier saturates or exceeds its common-mode range. The result may be unstable, clipped or incorrect readings.
- Connect source positive to AI+.
- Connect source negative to AI−.
- Connect one bias resistor from AI+ to AI GND.
- Connect an equal-value bias resistor from AI− to AI GND.
- Configure the channel for Differential input.
Using equal resistor values helps maintain a balanced input impedance and preserves common-mode rejection.
Selecting Bias Resistors
Bias resistors provide a DC return path for the DAQ amplifier input bias current and prevent the floating source from drifting outside the permitted common-mode range.
Resistors in the approximate range of 10 kΩ to 100 kΩ are commonly used when appropriate for the source and DAQ input.
- Lower resistance provides a stronger reference to ground.
- Higher resistance reduces loading on the signal source.
- Both resistors should have the same nominal value.
- The resistor tolerance may affect input balance.
- The source impedance and DAQ input impedance must be considered.
Floating Source in RSE Mode
A floating source can often be connected in RSE mode:
- Connect the source positive output to the selected AI channel.
- Connect the source negative output or return to AI GND.
- Configure the DAQ channel for RSE input.
This connection establishes the DAQ analog ground as the measurement reference. It is simple and provides more channels, but it normally offers less noise rejection than a differential connection.
Floating Source Connections by DAQ Family
E Series and M Series DAQ Devices
Use Differential input with equal bias resistors from both input conductors to AI GND when the source requires a bias-current return path.
For simple, short connections:RSE may be used by connecting the source positive to AI and the source return to AI GND.
DAQPad-6015 and DAQPad-6016
Use Differential input with the appropriate bias-current return path. Confirm the exact terminal names and supported configurations in the device documentation.
USB-6008 and USB-6009
Connect the source across a supported differential channel pair. Provide the required bias-current return path according to the device documentation.
RSE connection:Connect the source positive to an AI channel and the source negative or return to AI GND.
Because USB DAQ devices and connected computers can have different grounding arrangements depending on the USB port, computer power supply and surrounding equipment, do not assume that the system is earth-referenced. Verify the actual grounding and provide a defined signal reference.
How to Select the Correct Connection
Use Differential input.
Use NRSE when supported and when a shared source reference is appropriate.
Generally avoid this combination because it can create a ground loop.
Use Differential input with an appropriate bias-current return path.
Use RSE when the signal level and environment make a single-ended connection acceptable.
Reduce Noise in Analog Measurements
Use Twisted-Pair Wiring
Route each signal and its return together as a twisted pair. Twisting reduces the loop area and helps both conductors pick up similar interference, which a differential input can reject.
Use Shielded Cable When Necessary
A shield can reduce capacitive coupling from nearby electrical equipment. Shield termination depends on the measurement system and frequency environment, but connecting the shield at one end is commonly used to avoid creating another ground loop.
Keep Signal Cables Away from Noise Sources
Avoid routing analog signal cables next to:
- AC power cables
- Motors and variable-frequency drives
- Relays and contactors
- Transformers
- Импульсные источники питания
- High-current conductors
- High-speed digital cables
Use the Smallest Suitable Input Range
Select the smallest DAQ input range that safely contains the expected signal. A suitable range can improve measurement resolution, but the signal and common-mode voltage must never exceed the device limits.
Confirm Source Impedance
High source impedance can increase settling time, measurement error and sensitivity to leakage currents. Confirm that the signal source impedance is compatible with the DAQ input specifications and selected sampling rate.
Recommended Connection Procedure
- Identify the exact DAQ device and terminal block.
- Download the correct pinout and user manual.
- Determine whether the signal source is grounded or floating.
- Measure or confirm the maximum signal and common-mode voltage.
- Select Differential, RSE or NRSE according to device support.
- Turn off the source and DAQ system before changing wiring.
- Connect the signal using the documented terminal names.
- Add bias resistors when required for a floating differential source.
- Configure the same terminal mode in NI MAX or NI-DAQmx.
- Use a low, safe source voltage for the initial test.
- Run an NI MAX test panel and verify the reading.
- Check for offset, noise, clipping and unstable measurements.
Frequently Asked Questions
What is the best input mode for a grounded signal source?
Differential input is generally recommended because it provides improved common-mode noise rejection and avoids directly connecting two ground references.
Why should I avoid RSE with a grounded signal source?
RSE connects the source ground directly to DAQ analog ground. If both systems are already grounded elsewhere, this creates a ground loop that can introduce noise and offset.
What is the best input mode for a floating signal source?
Differential mode usually provides the best noise performance, but the input amplifier may require bias resistors to provide a DC return path. RSE can be used for simpler short-distance measurements.
Why does a floating differential measurement drift or saturate?
The input bias current may have no return path, allowing the common-mode voltage to drift outside the DAQ amplifier range. Equal bias resistors from the inputs to AI GND can provide the required path when recommended by the device manual.
Do the USB-6008 and USB-6009 support NRSE?
No. These devices support Differential and RSE analog input configurations but do not provide NRSE input.
Can I use any two AI terminals as a differential pair?
No. Many multiplexed DAQ devices use specific positive and negative channel pairings. Consult the device pinout to identify the correct differential terminal pairs.
Are 10 kΩ to 100 kΩ bias resistors suitable for every source?
No. This is a commonly used range, but the correct resistance depends on source impedance, acceptable loading, DAQ input characteristics and measurement accuracy. Follow the model-specific documentation.
