sample and hold ADC rail to rail opamp

[maori]

sample and hold adc

i am a newbiee , i have done a sample ADC using rail to rail opamp.

what i know is that rail to rail have higher outputswing..

i want to ask,

1) what is the function of sample and hold in ADC.i know that it hold the interval and sampling it again...but what is it effect in ADC.the advantage of it

2)what is the advantage of using rail to rail opamp than other opamp.
is there any disadvantage or drawback by using rail to rail amplifier

i really hope for any help that i can get...such as oppinion.if there have any reference that anyone want to share i really appereciate it.thank Q

 

[acbalbason]

adc ti op amp

hi!

rail to rail op amps? hmmm, the output swing is the entire voltage range. for example your vdd is 3.3V, the op amp gain is relatively flat from 0 to 3.3V. it is like having a big pulse, if you plot gain versus input voltage. the advantage? a very stable gain. you can trust your amp to have the same gain even though your input does not reach VDD. you can achieve lower power consumption that way.

- al

 

[tekno1]

rail to rail op amp

1) what is the function of sample and hold in ADC.i know that it hold the interval and sampling it again...but what is it effect in ADC.the advantage of it

2)what is the advantage of using rail to rail op amp than other op amp.
is there any disadvantage or drawback by using rail to rail amplifier

A.1) ADC has limited input BW therefore for High frequency input signals one need a sample and hold stage to prevent errors and increase accuracy since we need to know time value relationship.

A.2) Input side it is necessary since it allows wide input range. For low power supplies it is important since with shrinking power voltages if input CMR shrinks dramatically.
At the output also output common mode range is important since for example to keep SNR or effect of offset errors to minimum etc. There is one important issue with output being rail-to-rail, in reality output do not go all the way to rails with sufficient gain (feedback loop does break) so amplifier is good only to couple hundred mV to rails if current drawn from output.

Talking this issues within narrow frame of this reply window is hard but the best thing I will suggest to you that to go to Analog Devices, MAXIM, TI, Linear technology etc. company web sites and read app notes. This is the best way since those app notes are simple and clear enough and for to educate the users.