Analog Bandwidth
Analog bandwidth describes the frequency range (in Hz) in which a signal can be acquired accurately. This limitation is determined by the inherent frequency response of the input path, which causes loss of amplitude and phase information. Analog bandwidth is the frequency at which the measured amplitude is 3 dB below the actual amplitude of the signal. This amplitude loss occurs at very low frequencies if the signal is AC coupled and at very high frequencies regardless of coupling. When the signal is DC coupled, the bandwidth of the amplifier extends all the way to the DC voltage. The following figure illustrates the effect of analog bandwidth on a high-frequency signal.
As indicated in the specifications, the NI 4070/4071/4072 has a voltage measurement bandwidth of 300 kHz. This specification means that only signals ≤300 kHz can be accurately acquired. Note that this number is different than the maximum sampling rate of the NI 4070/4071/4072 (1.8 MS/s). The Nyquist Theorem implies that the maximum signal frequency that can be effectively acquired is 900 kHz. However, the bandwidth actually limits the maximum frequency to 300 kHz. Refer to sample rate for more information.
Bandwidth Considerations
The resistance and the capacitance present in the signal path can affect high frequency signals. The larger the capacitance and the resistance, the lower the bandwidth of the signal path. The NI 4070/4071/4072 input capacitance can be found in the specification. Note that additional capacitance can be introduced in the cabling that is used, and any other front-end accessories such as switches. This capacitance not only impacts the performance of the waveform acquisition, but can also impact the source of the signal by loading it. Having a low-impedance source and minimal added capacitance is best for optimal operation when acquiring high frequency signals. Use short cable lengths of a low-capacitance, low resistance, and low dielectric absorption for best results.