Section 4: Measure considerations
Model DMM7510 7½ Digit Graphical Sampling Multimeter Reference Manual
4-10
DMM7510-901-01 Rev. B / May 2015
Radio frequency interference
Radio Frequency Interference (RFI) is a general term used to describe electromagnetic interference
over a wide range of frequencies across the spectrum. RFI creates problems at low signal levels, but
it can also affect measurements at high levels if the fields are of sufficient magnitude.
RFI can be caused by steady-state sources, such as radio or TV signals, or some types of electronic
equipment, such as microprocessors and high speed digital circuits. It can also result from impulse
sources, as in the case of arcing in high-voltage environments. The effect on the measurement can
be considerable if enough of the unwanted signal is present.
You can minimize RFI in several ways:
•
Keep the Model DMM7510 voltage source and signal leads away from RFI sources.
•
Shield instrument, signal leads, sources, and other measuring instruments.
•
In extreme cases, a specially-constructed screen room may be required to sufficiently attenuate
the RFI signal.
In some situations, the Model DMM7510 digital filter may help to reduce RFI effects. In some cases,
additional external filtering may also be required. However, filtering may have detrimental effects,
such as increased settling time on the signal.
Shielding
AC voltages that are extremely large compared to the DC signal to be measured may produce an
erroneous output. Therefore, to minimize AC interference, the circuit should be shielded, with the
shield connected to the Model DMM7510 input low (particularly for low-level sources). Improper
shielding can cause the Model DMM7510 to behave in one or more of the following ways:
•
Unexpected offset voltages
•
Inconsistent readings between ranges
•
Sudden shifts in reading
•
Higher overall noise in the measurements
To minimize pick-up, keep the voltage source and the Model DMM7510 away from strong AC
magnetic sources. The voltage induced due to magnetic flux is proportional to the area of the loop
formed by the input leads. Therefore, minimize the loop area of the input leads and connect each
signal at only one point.
To minimize noise, you may need a closed metal shield that surrounds the source. This shield should
be connected to input LO in most cases. In some situations, you may get better noise performance
with the shield connected to chassis ground.