GND problem in an Attenuator circuit

Dear all :
In Attenuator circuit ,
1. why we need connect ground pin to capactor to ground ?
2. why we apply capacitor across MOS gate-drain ?

Thanks.

Re: about GND problem...

1)Even if you removed C442, it wouldn't distrub you.It would still work. Such type of schemes are useful when there's a risk of slight fluctuation to the point where you have connected the other part of the capacitor.
Usually you'll find these schemes when capacitor is connected to some source like +9V. Such capacitors are connected as close to the pin as possible. So +9V can be called as a name of a NODE(or NET ALIAS if you talk in terms of CAD)
Remember capacitor takes T=RC times to changes it's voltage level. If voltage get change for a fraction of time, you won't see it on the PIN on which the IC is connected, because it would take finite time to see the level change on the other end.
2)As for C435,C453 case. The output coming from V5 might go across the C453 into the input. Yes, it can. If that's a DC signal then it surely is going to be blocked, don't know whether the output frequency is high enough to go across the capacitor. You need to know the output of your frequency.
Please give us a clear picture to the surrounding components of IC and the type of signal being fed into the IC and being taken out of the IC.

Re: about GND problem...

1. I think that you said is difference to my question.
put cap at VDD is for reduce ripple but my question is serial cap from gnd pin to GND. (see picture)

2. input is 45Mhz signal ,
v1,v2 is control voltage to contol attenuation.
0/vdd or vdd/0.

I had label in ckt,

thanks.

Re: about GND problem...

As this is an RF circuit, and as you said it's operate on 45Mhz.I believe you're getting digital data bits into it or analog data,whatever. With an update rate of thousands of times per second, so in other word you have entered into radio range and things aren't gonna remain as they appear, two straight parallel wires may start acting like an antenna.Almost like in a deep African jungle, in morning it appears very pleasant, you see birds,etc, in night, it becomes dangerous, you can hear horrible noises of animals and jungle becomes many times dangerous. So you need to pay attention on bypass,decoupling,etc; When you work at RF. As all are digital bits or even if you feed 45Mhz signal with analog value, then I would say all are voltages referred to some common ground.
At high frequency this ground pin starts offering resistance, but voltage sources must have Zero impedance in ideal case, so as to reduce the impedance offered that capacitor is used in here. This is what it appears to me. You must look at mathematical model of transmission line and how it works.It's RF.I don't know exactl mathematic formulation, but It appears from the circuit, it was done so as to minimize the impedance.Look at equation of transmission line, some RF handbook, some relevant material. So you can actually calculate as how much impedance would be offered with it and without it.Because in order for this IC to opearate, it takes Input and Output, but they must have return path also, so ground is taken as return path. if Input/Output is working on RF, then ground(return path) must also be taking signal in RF.