Ground Loop Noise in the NI USB-5132/5133
Some measurement setups can induce ground loop noise in measurements taken with the NI 5132/5133. This noise can appear as an undesired DC offset or as a stray low-frequency signal. This section describes methods you can use to mitigate ground-loop-related noise.
Ground-Loop-Related DC Offset
When a ground loop exists, a DC offset can appear in a measurement due to power supply return currents, as shown in the following figure.
The USB, BNC, and ground lug connectors all share the same ground inside the NI 5132/5133, thus power supply return currents to the host computer can flow out of all three connections. Ideally, all of the supply current returns through the USB cable (the current labeled Ireturn2) and none returns through the BNC cable (the current labeled Ireturn1). In practice, this does not happen because RUSB is not a perfect short and RGND is finite when a ground loop exists. As shown in the figure, the unwanted voltage drop along the BNC cable, Verror, is in series with the measured voltage and represents a DC offset error. By Ohms law, Verror is as follows:
Verror = Ireturn1 × RBNC
Also, given the three ground resistances depicted in the figure, you can predict the magnitude of Ireturn1 with respect to the total supply current, Isupply, using the current division formula as follows:
Ireturn1 = (RUSB/(RGND+RUSB+RBNC)) × Isupply.
Substitute the second equation into the first:
Verror = ((RUSB × RBNC)/(RGND+RUSB+RBNC)) × Isupply
As the previous formula shows, you can reduce Verror by increasing RGND, reducing RUSB, or reducing RBNC.
How to Reduce Ground-Loop-Related Offset Error
To reduce Verror, use the following methods:
- Increase RGND by disconnecting the ground between the measurement source and the computer ground (in other words, break the ground loop). If this is not possible, increase RGND by some other method. Note that increasing RGND to only 1 Ohm usually eliminates any measurable DC offset.
- Reduce RUSB by using a higher quality or shorter USB cable. Alternatively, install a heavy ground wire between the NI 5132/5133 ground lug and the host computer ground (thus effectively reducing the value of RUSB).
- Reduce RBNC by using a shorter BNC cable. Alternatively, install a heavy ground wire between the NI 5132/5133 ground lug and the signal source ground (thus effectively reducing the value of RBNC).
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Note You can also perform any combination of these methods to further reduce Verror. |
Ground-Loop-Related AC Noise
An AC noise source existing in a ground loop can appear in NI 5132/5133 measurements, as shown in the following figure.
A loop is formed by the BNC cable, the NI 5132/5133, the USB cable, the computer, the ground interconnection between computer and signal source, and the signal source. An AC noise source, VAC, is depicted in series with the loop and causes the generation of an error voltage Verror in series with the NI 5132/5133 measurement input. Because the loop forms a voltage divider, you can write an equation for Verror in terms of VAC and the three resistances around the loop as follows:
Verror = (RBNC/(RGND+RUSB+RBNC)) × VAC
As the previous formula shows, you can reduce Verror by increasing RGND, reducing RBNC, or reducing VAC.
How to Reduce Ground-Loop-Related AC Noise
To reduce Verror, use the following methods:
- Increase RGND by disconnecting the ground between the measurement source and the computer ground (break the ground loop). If breaking the ground loop is not possible, a modest increase in the value of RGND can help. For example, if the total resistance around a ground loop is initially 0.1 Ohms, increasing the value of RGND to 1 Ohm decreases ground loop noise by 20 dB.
- Reduce RBNC by using a shorter BNC cable. Alternatively, install a heavy ground wire between the NI 5132/5133 ground lug and the signal source ground, thus effectively reducing the value of RBNC.
- Reduce VAC. You can accomplish this goal in several ways. Ideally, the host computer and measurement source should share a ground interconnection that carries little current. Both should share the same power outlet or power strip, when possible. Alternatively, you can connect a heavy wire between computer ground and measurement source ground to shunt VAC. The resistance of this wire would need to be much lower than RGND. If the noise is induced in the loop magnetically, reduce the area of the loop or separate any cables in the loop from cables carrying large AC currents.
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Note You can also perform any combination of these methods to further reduce Verror. |