Help to analyze the Gain-by-2 switched cap based Amplifier

[eegchen]

hi,
I am designing a 1.5 bit Sub-ADC. The Gain-by-2 Amp is shown in the graph below.

The sampling speed is 200MHz, Supply is 1.2. The waveform is only Gain-by-2 Amp result. the input is 300mvP-P. And the ideal output waveform should be 300mv--900mV and DAC output is fixed at 600mV.

Gain of OTA is more than 100dB, unit gain bandwidth is 600MHz, output current is 1mA.( BTW, i use ideal switch,common mode is 600mv)

Could anyone analyze the waveform for me? what caused the settling behavior so slow and it seems not settle down at all? What do i need further improvement.?
To me, the waveform is like a pole-zero doublet. i only learn that Cascode folding OTA has pole-zero doublet problem. How about this circuit ?


Thanks

Best
Gang

 

[sutapanaki]

Re: Help to analyze the Gain-by-2 switched cap based Amplifi

I think you will have to understand first why your waveform is not symmetric and then think about pole-zero doublets.

 

[eegchen]

Re: Help to analyze the Gain-by-2 switched cap based Amplifi

thanks,

That is what i am thinking, why it's not symmetrical. It's obvious caused by OTA.

Is it because OTA has different discharge and charge cap?

Best

 

[sutapanaki]

Re: Help to analyze the Gain-by-2 switched cap based Amplifi

I don't think it is the charge-discharge. You see, the blue half doesn't even reach deltaV=300mV and is kind of going down. The red one is more than 300mV and increasing.

 

[eegchen]

Re: Help to analyze the Gain-by-2 switched cap based Amplifi

So is it the reason of not symmetrical output swing ?

I don't think it is the charge-discharge. You see, the blue half doesn't even reach deltaV=300mV and is kind of going down. The red one is more than 300mV and increasing.

 

[sutapanaki]

Re: Help to analyze the Gain-by-2 switched cap based Amplifi

may be not the asymmetrical swing per se, but more the reason that's causing it.