The Nyquist theorem is the fundamental basis for all sampled data systems. It states (as you a ppear to be aware) that to accurately reproduce a continuous signal from the sampled representation the sampling rate must be greater than twice the bandwidth of the signal.
To process a signal with a 40 kHz bandwidth requires sampling at greater than 80 kHz. The USB-6008 has a sampling rate of 10 kHz for one channel. So, it cannot acquire the data you need.
You need to obtain a DAQ device which has a much faster sampling rate. If your ultrasonic signal is pulsed or swept in frequency, the bandiwdth of the modulation must also be considered. I would look for a DAQ device which could sample at least 100 kHz and possibly even faster.
Lynn
