DC and AC Coupling
The NI 4070/4071/4072 offers two different modes for measuring AC voltage, and the NI 4065 offers one mode, as follows:
- AC Volts (NI 4070/4071/4072 and NI 4065)
- AC Volts DC Coupled (NI 4070/4071/4072 only)
AC Volts Mode
AC Volts mode on the NI 4065 and NI 4070/4071/4072 use a coupling capacitor to remove DC offsets before digitization. This functionality is standard in traditional DMMs. A coupling capacitor is available for situations where a large DC offset must be blocked before digitization. To measure AC voltage in the presence of large DC offsets, such as ripple on a DC power supply, use AC Volts mode, which uses a coupling capacitor to eliminate the offset.
AC Volts DC Coupled Mode
The rms algorithm employed by the NI 4070/4071/4072 digital signal processor (DSP) is completely insensitive to any DC component of the signal being measured. Therefore, for applications without large DC components, such as AC powerline and audio signals, the AC coupling capacitor used to block the DC signal component is not always necessary on the NI 4070/4071/4072. This capacitor can be bypassed by using AC Volts DC Coupled mode. This mode does not have the long time constant associated with the input coupling capacitor and thus offers a very short settling time.
DC Coupling on AC Current Measurements
AC current measurements are always DC coupled and therefore always offer quick settling time, but these measurements are subject to overload if the DC component of the signal exceeds the limits of the chosen range.