AnsweredAssumed Answered

It's all about settling time

Question asked by Jimmyjie on Nov 22, 2013
Latest reply on Nov 26, 2013 by Jimmyjie

Fellows,

 

My idea is having forward settling time less than ADC conversion time.

 

AD7982 conversion time is 710ns;

 

To get 18 bit accuracy, ADG1204 transition time plus settling time < 400ns, AD8475 specified the 16 bit settling time 50ns, I guess the 18bit settling time should be less than 100ns. RC filter settling time should be less than 200ns if i chose the value right.

 

So the forward settling time is sqrt(400^2+100^2+200^2)=458 ns.

 

 

Catch(11-22-18-58-35).jpg

 

Here comes the problem, AD8475 requires very low impedance input source to achieve gain accuracy and high CMRR. So I need find a fast settling buffer to drive AD8475, and the supply voltage for this buffer should be no less than 24V to cater to the input signal.

 

I went through and found AD8021 might be the only one suitable.  But it only specified 1V step, 0.01% settling time, which is 23ns. How can I know the 18 bit settling time with large input, like +8V step with ±12V supply ?

 

Please correct tme if i'm wrong and it's appreciated if you had suitable OpAmp to recommend.

 

Cheers,

Jason

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