Input Coupling

You can configure each AI channel of your DSA device to be either AC or DC coupled, with the exception of the NI 9233 and NI 449X devices, which are AC coupled only. If you select DC coupling, any DC offset present in the source signal is passed to the ADC. The DC-coupled configuration is usually best if the signal source has only small amounts of offset voltage, less than ±100 mV, or if the DC content of the acquired signal is important.

If the source has a significant amount of unwanted DC offset (bias voltage), you should select AC coupling to take full advantage of the input dynamic range. Selecting AC coupling enables a single-pole, high-pass resistor-capacitor (RC) filter into the positive and negative signal path. Refer to your device documentation for additional information on the filter circuitry.

Use the NI-DAQmx Channel attribute/property AI.Coupling to set the input coupling mode on your DSA device.

If you create a virtual channel for acceleration or sound pressure measurements with your DSA device, the input coupling for the channel defaults to AC. For other types of virtual channels, the input coupling defaults to DC.

Note  NI-DAQmx does not compensate for the settling time or delay introduced by the RC filter.

Using AC coupling results in a drop in the low-frequency response of the AI circuitry. The AC coupling circuitry is usually characterized by a 3 dB cut-off frequency. However, the roll off from the high-pass filter can have a measurable effect even at frequencies several times greater than the 3 dB point.