Noise
Noise—unwanted signals present on the output channels—can affect devices connected to the output channels.
Noise can be characterized as normal-mode or common-mode noise. Regardless of its characterization, noise is meaningful only when it is specified with an associated bandwidth.
Normal-Mode Noise
Normal-mode noise is present between the output HI terminal and the output common LO terminal, appearing either in series (Constant Voltage mode) or parallel (Constant Current mode) with the output of the device. Normal-mode noise can be expressed as voltage noise or current noise, depending on the control mode of the output channel.
AC to DC rectification causes ripple, a type of periodic normal-mode noise.
Common-Mode Noise
Common-mode noise is present between the output common LO terminal and the chassis or earth ground. In this sense, the equivalent circuit is a current noise source connected across these two terminals. When you connect an impedance between the output common/ground and chassis or earth ground, a noise current can flow in the impedance, resulting in an unexpected offset or other undesirable error.
Output Capacitance Considerations
To help reduce noise and ripple when the device is operating in a high-current range, NI recommends setting the niDCPower Output Capacitance property or the NIDCPOWER_ATTR_OUTPUT_CAPACITANCE attribute to HIGH for devices that support this feature. Remember that a larger capacitance results in a slower output response. Refer to Load Considerations for more information about capacitive loads.
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Note The only valid output capacitance setting on all channels for the NI PXI-4110 is HIGH. For more information about reducing noise in high-current ranges with the NI PXI-4130, refer to Output Capacitance Selection. |
Measurement Noise Rejection
In many environments, line noise (for example, 50 Hz or 60 Hz) or other unwanted periodic signals may be present in a system and can degrade measurement quality. You can program your device to reject periodic signals and their harmonics by configuring the niDCPower Samples to Average property or the NIDCPOWER_ATTR_SAMPLES_TO_AVERAGE attribute according to the following table.
| Number of Samples to Average | Frequencies Rejected for 3 kHz Sample Rate |
|---|---|
| 1 | 3 kHz |
| 50 | 60 Hz |
| 60 | 50 Hz |
| 300 | 50 Hz and 60 Hz |
For frequencies not listed in this table, use the following formula:
N = 3000/Fr
where N is the number of samples to average and Fr is the frequency rejected.
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Note To improve noise reduction while keeping frequency rejection, set the number of samples to average to N, 2N, 3N, and so on. The maximum allowed samples to average is 511. |