about the linearity of amplifier

[jiangwp]

In the design of the sigma-delta ADC/DAC , the nyquist ADC/DAC , the linearity is a very important parameter. So what is the way to represent the linearity. and How to do represent it. How do we select the linearity of amplifier of the ADC/DAC system. How do we improve the linearity of the amplifier?

 

[Moss]

Where you want to represent this amplifier? In simulation package?

How do we improve the linearity of the amplifier

With negative feedback! Fundamental rule is more "deep" negative feedback - more linear amplifier. If you seleck OP with high enough gain, it will be possible to use enough negative feedback, and the amplifier will be "linear"

 

[jiangwp]

To Moss:

Maybe you are right.

Generally, we use the THD to represent a opamp linearity.
But what is the relation between THD and some parameters of OPAMP?

The larger gain of OP, The better linearity of OP.

But How do we slelection the gain of between after we have known a linearity of a system.

And is there some equation to represent the relation?

 

[Amuro]

The negative feedback helps improving the linearity of the amplifier through the parameter "loopgain", which is the product of Amp's openloop gain and feedback network. If the loopgain is really high by designing the high openloop gain, which of course have to sacrifice with some bandwidth, then the distortion components will be reduced a lot esp. the high order components.

However, there are still the other parameters of amplifier you have to concern. The linearity of the system depend on how large of the signal you want to handle compare to how good of signal swing is allowed on your amp. This would be refered to the output stage because it is the part that have to deal with the largest signal in your amp.

First limitation is the output architecture. Your output swing is limited by the supply voltage minus some voltage headrooms. That's why the rail-to-rail output stage like common source is usually occupied as the output stage in low-voltage operation. You may can imagine, if the output signal is bigger than your output stage can handle, that signal will be cliped and then cause the distortion.

Second parameter is the Slew Rate. If you operate the system in low freq., this would not be a problem since your SR value usually more than enough. But if you want to use in higher freq. toward the GBP, please beware and check that the signal amplitude doesn't exceed SR/ωmax, where ωmax is the maximum freq. of that signal. Otherwise you will not get the same shape of the signal anymore.

Finally, as I can think of for now, is for fully differential amp., the CMFB also can limit the voltage swing of the output. In addition to check the stability of CMFB loop, you should check for that your CMFB circuit doesn't limit the output voltage swing too.

Hope this help you get some idea. I also wait to hear the other comments on this issue too.

Amuro.

 

[terryssw]

The linearity in ADC/DAC can be represented as INL/DNL in no. of LSBs in static way, and also the THD and dynamic range in dynamic consideration.

The non-linearity in ADC/DAC can be composed by many factors, such as opamp linearity, differential mode charge injection, gain-error in pipelined ADC, offset mismatches in flash ADC, etc. You must know which part of the non-linearity most affect your systems then you can determine how to do the treatment.