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4 Measurement Tutorials
Low-level AC measurement errors
When measuring AC voltages less than 100 mV, be aware that these measurements are especially susceptible to
errors introduced by extraneous noise sources. An exposed test lead will act as an antenna and the internal DMM
will measure the signals received. The entire measurement path, including the power line, act as a loop antenna.
Circulating currents in the loop will create error voltages across any impedances in series with the instrument's
input. For this reason, you should apply low-level AC voltages to the instrument through shielded cables. You should
also connect the shield to the input LO terminal.
Be sure to minimize the area of any ground loops that cannot be avoided. A high-impedance source is more
susceptible to noise pickup than a low-impedance source. You can reduce the high-frequency impedance of a
source by placing a capacitor in parallel with the instrument's input terminals. You may have to experiment to
determine the correct capacitance value for your application.
Most extraneous noise is not correlated with the input signal. You can determine the error as shown below:
Correlated noise, while rare, is especially detrimental. Correlated noise will always add directly to the input signal.
Measuring a low-level signal with the same frequency as the local power line is a common situation that is prone to
this error. You should use caution when switching high-level and low-level signals on the same module. It is possible
that high-level charged voltages may be discharged onto a low-level channel. It is recommended that you either
use two different modules or separate the high-level signals from the low-level signals with an unused channel
connected to ground.
Measurements below full scale
You can make the most accurate AC measurements when the internal DMM is at full scale of the selected range.
Autoranging occurs at 10 % and 120 % of full scale. This enables you to measure some inputs at full scale on one
range and 10% of full scale on the next higher range. Note that the measurement accuracy will be significantly
different for the two cases. For highest accuracy, you should use manual ranging to select the lowest range possible
for the measurement.
Temperature coefficient and overload errors
The internal DMM uses an AC measurement technique that periodically measures and removes internal offset
voltages when you select a different function or range. When manual ranging to a new range in an overload
condition, the internal offset measurement may be degraded for the selected range. Typically, an additional 0.01% of
range error may be introduced. This additional error will remain until the next periodic removal (typically 15
minutes).
Keysight DAQ970A/DAQ973A User's Guide
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