A digitizer converts the echo waveforms returned by the ultrasonic transducer into digital information using an ADC. For applications that require well-shaped waveforms in the time-domain, a sample rate 10 times higher than the resonant frequency of the transducers is required. In these applications, a transducer that has a resonant frequency of 5 MHz requires 50 MS/s to accurately represent the shape of the signal. However, for applications that require less amplitude and echo-timing accuracy, 4-5x the resonant frequency is acceptable.
Vertical resolution establishes the minimum voltage step size within a voltage range. 16-bits is equivalent to 65,536 (2^16) steps. When a 16-bit ADC is applied to a voltage range of 0-10 Volts, the minimum voltage step size is 0.153 mV (10 V / 65,536). However, when an 8-bit ADC is applied to the same range, the minimum voltage step size is 39 mV (10 V/256). In ultrasonics, the voltage amplitude is proportional to the amount of energy echoed by the discontinuity or flaw. The front face and back face of the part generally reflect the most energy, while flaws reflect much less. To see the energy reflected from small flaws, the signal from the transducer must be amplified or the digitizer must have high resolution. Amplifying the signal to detect a small flaw can cause the front surface and back surface reflection voltages to swing outside the voltage range of the digitizer. On the other hand, additional resolution permits the user to zoom in on small flaws and not distort main surface reflections at the same time. High resolution also relaxes the amplification levels required by the P/R component.