Keysight DAQ970A Guia Del Usuario página 222

4 Measurement Tutorials
Rejecting Power-Line Noise Voltages
A desirable characteristic of an integrating analog-to-digital (A/D) converter is its ability to reject spurious signals.
Integrating techniques reject power-line related noise present with DC signals on the input. This is called normal
mode rejection or NMR. Normal mode noise rejection is achieved when the internal DMM measures the average of
the input by "integrating" it over a fixed period. If you set the integration time to a whole number of power line cycles
(PLCs) of the spurious input, these errors (and their harmonics) will average out to approximately zero.
When you apply power to the internal DMM, it measures the power-line frequency (50 Hz or 60 Hz), and uses this
measurement to determine the integration time. The following graph shows the attenuation of AC signals measured
in the DC voltage function for various A/D integration time settings. Note that signal frequencies at multiples of 1/T
exhibit high attenuation.
Temperature measurements
A temperature transducer measurement is typically either a resistance or voltage measurement converted to an
equivalent temperature by software conversion routines inside the instrument. The mathematical conversion is
based on specific properties of the various transducers. The mathematical conversion accuracy (not including the
transducer accuracy) for each transducer type is shown below:
Transducer
Thermocouple
RTD
Thermistor
Errors associated with temperature measurements include all of those listed for DC voltage and resistance
measurements elsewhere in this chapter. The largest source of error in temperature measurements is generally the
transducer itself.
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Conversion Accuracy
0.05 °C
0.02 °C
0.05 °C
Keysight DAQ970A/DAQ973A User's Guide