About sampling instant of S/H

[urian]

Hi,there!
As we know and can be seen from the pic below, the bottom plate sample technique is widely used in S/H to reduce charge injection from sampling switch. The sampling instant is said to be determined by the CLK Φ1p which is opened in advance with respect to Φ1 in many literatures. Recently, however, an engineer told me that in fact, the sampling instant is determined by the Φ1 instead of Φ1p. I doubt whether this is the turth or not.

 

[monsoon]

Sampling instant is determined by Φ1p only. The reason being after Φ1p goes down, voltage across the sampling capacitor Cs cannot change because one of its terminal is floating. This is also the reason why there is no charge injection from the sampling switch.

 

[urian]

Hi, monsoon
My option is the same as you. But he said that the voltage can change because of parasitic cap or whatever, then I dont know how about the practical situation.

 

[erikl]

Below pls. find a comprehensive consideration of S/H circuits. See page 9 of the PDF for a Bottom Plate Sampling circuit.

 

[urian]

Thx, erikl
It seems that theory and practice coincide with each other very well with regards to bottom plate sample.