This paper describes the sample-hold and analog-digital converting technique in fast digitizer and digital storage oscilloscope.The quantization error

sampling error

dynamic performance and effecfive bit--frequencycharacteristics of adove digital instrument are also described.The paper points out the difference between real-time and equivalent-time sampleing technique in measureming of single shoot signal and reputitive signal. According to the recent IEC draft "Digital recorder for measurements in HV impulse tests"

we introduce that how to select the rated resolution and sampling rate of digitizers employed for testing of self-restoring and non-self-restoring insulation.