The method for sizing the amplifier in pipeline ADC design

[sheen]

Pipeline ADC design

Hi

I'm designing 8-bit pipeline adc and using folded cascode design for amplifier. Can someone tell me correct method to size the amplifier or any website which tells this method. I somehow sized my amplifier but gain was very less. Also can someone tell me how I can increase the gain of the OTA ?

Thanks for help !!

Sheen

 

[Vamsi Mocherla]

Re: Pipeline ADC design

You should make sure that your gain error in the opamp is less than 1/2 LSB or a maximum of 1 LSB. Hence for an 8 bit device you should atleast have a gain of 60 dB for your amplifier. Well, sizing the transistors can be found in any basic book. Try Allen Holberg for this. It will help you

 

[opamp741]

Re: Pipeline ADC design

first try to get the Gain, UGB, SR kinda specs from ur basic topology..
for example if u r using 1-true-bit/stage..ur gain error and settling error shd be around 1/4th of 1/2^7..
ur gain error will generate Gain spec of opamp while ur settling error spec will generate ur UGB spec..
now fix ur load cap..now u can get input transistor Gm by CL*UGB..now if ur vdsat is 100mv..u can approximate ur current by 0.5(vdsat*Gm)..
isnt it very simple :spoko:

 

[Vamsi Mocherla]

Re: Pipeline ADC design

That is good. I forgot to mention about the settling spec

 

[andy2000a]

Pipeline ADC design

which type gain stage suit for pipeline ?? some textbook use cap array become D/A stage

 

[Vamsi Mocherla]

Re: Pipeline ADC design

Well, Cap array can be used for Piple D?A part. Actually, you can do all the things using a single S/H circuit if you do a 1 or 1.5 step resolution in every stage

 

[yeewong_su]

Pipeline ADC design

For 8bit Pipeline ADC
maybe the sample Freq is > 140MHz
And you should use the simple two stage opamp and gain >70db

 

[Vamsi Mocherla]

Re: Pipeline ADC design

I am not sure whether you want to use the 2 stage amplifier for a clock frequency of 140 MHz. It is way too high. You will need an amplifier which has a GBW of about 400 MHz for fast settling. For 8 bit applications, you can work with an amplifier of gain of around 60 dB. So go for differential telescopic cascode