TechTip: Basic Analog Input Signal Connections (Part 2)

Objective

To provide DAQ product users with recommended single-ended/differential wiring guidelines for analog input measurements after considering sensor grounding, DAQ system grounding and isolation, and common mode voltage.

Intended Audience
DAQ product users who must decide how to connect analog input signals to optimize the performance of their DAQ device.

Recommended Wiring Configurations
Building upon the information provided in TechTip: Basic Analog Signal Connections (Part 1), all possible grounding and input type combinations are summarized in the table below, along with MCC-recommended uses.

Recommended analog input configuration (based on a DAQ device with ±10 V common mode range)

Ground Relationship between
Sensor and DAQ
DAQ Input Configuration  MCC Recommendation
Common ground Single-ended Recommended if all inputs are common ground
Common ground Differential Acceptable if some inputs are not common ground
Common mode voltage < ±10 V Single-ended Not recommended
Common mode voltage < ±10 V Differential Recommended
Common mode voltage > ±10 V Single-ended Unacceptable without adding isolation
Common mode voltage > ±10 V Differential Unacceptable without adding isolation
Isolated grounds Single-ended Acceptable
Isolated grounds Differential Recommended

Recommended Ground Connections

On MCC AI DAQ devices, both digital grounds (GND) and analog grounds (AGND) are tied directly to the PC common ground. Unless you have a high-current return connected to one of the grounds on a device, you should connect your signals to AGND.

Ground high-current lines to GND and analog inputs to the low-current AGND line. This distinction prevents high current on the ground for your analog input signals from introducing error into the voltage measurements. Since GND and AGND have different paths to PC common ground, current on the GND line has little or no effect on the AGND line, and the accuracy of measurements of analog input signals grounded to AGND are not affected.

Common Ground / Single-Ended Inputs

MCC recommends using a single-ended configuration for the common ground scenario. However, if only some input channels/channel pairs are common ground, we recommend using the differential mode for all inputs. Figure 1 shows the recommended connections for a common ground/single-ended input system.



Figure 1. Signal source and AI DAQ device sharing common ground connected to single-ended input

Common Ground / Differential Inputs

Using a differential configuration is acceptable for common ground connections, although it requires more wiring and offers fewer channels than a single-ended configuration (see Figure 2).

Signal source with common ground

Figure 2. Signal source and AI DAQ device sharing common ground connected to differential input

Common Mode Voltage < ±10 V / Single-Ended Inputs
The phrase common mode has no meaning in a single-ended configuration. A single-ended configuration with common mode voltage is a system with offset grounds, and MCC does not recommend this configuration. However, if your application requires the increased number of channels in a single-ended configuration and the common mode voltage is small, no system damage should occur. Depending on the overall accuracy you need, you may receive acceptable results.

Common Mode Voltage < ±10 V / Differential Inputs
MCC recommends using a differential configuration to monitor systems with varying ground potentials. Make sure that the common mode voltage – which is the sum of the input signal and the ground differential – does not exceed the common mode range of the AI DAQ device (usually ±10 V).

Figure 3 shows recommended connections in this configuration.

Signal source with common mode voltage

Figure 3: Signal source and AI DAQ device sharing common mode voltage connected to differentia input

Most MCC devices cannot directly monitor signals with common mode voltages greater than ±10 V, but you can adjust for this limitation by changing the system ground configuration to reduce the overall common mode voltage, or by adding isolated signal conditioning between the source and the device (see Figure 4 and Figure 5).

Common Mode Voltage > ±10 V / Single-Ended Inputs

System with a common mode voltage

Figure 4. System with a common mode voltage > ±10 V connected to a single-ended input

Common Mode Voltage > ±10 V / Differential Inputs

System with a common mode voltage

Figure 5. System with a common mode voltage > ±10 V connected to a differential input

Isolated Grounds / Single-Ended Inputs
Using single-ended inputs is acceptable for monitoring isolated inputs, although using differential mode increases system immunity to noise. Figure 6 shows the recommended connections in this configuration.

Isolated signal source connected to single-ended input

Figure 6. Isolated signal source connected to single-ended input

Isolated Grounds / Differential Inputs
For the best performance with isolated signal sources, MCC recommends using a differential input configuration. Figure 7 shows the recommended connections for this configuration.

Isolated signal source connected to differential inpu

Figure 7. Isolated signal source connected to differential input

More Information
Please contact Measurement Computing Corporation if you have any questions or if you would like any further information.

Additional TechTips are available on our Data Acquisition Technical Articles web page.