Chapter 3
Using the NI PXI-4204
3-4
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Measurement Considerations
This section provides more information about the type of signal connection
made to the NI PXI-4204 and important factors that can affect your
measurement.
Differential Signals
All of the analog inputs of the NI PXI-4204 are differential. In general,
a differential measurement system is preferable because it rejects not only
ground loop-induced errors and common-mode voltages, but also the noise
picked up in the environment to a certain degree.
Lowpass Filter
The NI PXI-4204 includes a 2-pole Butterworth filter per
channel with two software-selectable cutoff frequencies to
reduce signal noise and improve accuracy. You can
programmatically configure the filter bandwidths on a per
channel basis for cutoff frequencies of 6 Hz or 10 kHz. The
6 Hz filter setting attenuates differentially connected 60 Hz
signals by 40 dB.
T/H Circuitry
You can enable the NI PXI-4204 track-and-hold circuitry to
enable SS/H. This allows you to acquire synchronized
measurements across multiple channels. You cannot enable
or disable SS/H on a per channel basis. It is disabled by
default. Enabling SS/H results in slower maximum sample
rates and slightly degraded accuracy.
With SS/H disabled, the NI PXI-4204 uses a multiplexed
architecture that enables the measurement of multiple
channels using a single analog-to-digital converter (ADC).
The multiplexing architecture of the NI PXI-4204 results in
measurements between channels that are separated in time.
The time delay between channels is determined by the sample
rate at which you acquire measurements. For most
low-frequency measurement applications, this time delay or
phase delay is not significant.
Table 3-1.
Signal Conditioning Functional Blocks (Continued)
Signal Conditioning Component
Description
