Chandu,
The purpose of the VCNTL pins are for Time-Gain Compensation (TGC), where the gain of the VCAT block is varied over time. This can be used in the ultrasound system to offset natural attenuation at depth of field. The VCNTL is an analog gain control used to vary the gain in time. You can ramp the DC voltage on these pins to vary the attenuation.
Thanks,
Chuck Smyth