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# Dyanmic Range of Analog Circuit

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#### samiran_dam

##### Full Member level 2
Dear All,

I need to know how do one measure the dynamic range of an analog circuit?

DR = 20log(Vin_max/Vin_min) dB.

Is this definition correct?

For a practical design, should I consider the maximum input-referred noise voltage as Vin_min? For Vin_max, should I consider the limitation imposed by given THD (%)?

Regards
Sam

For practical purpose your formula is correct. But by definition it is only derivative of ratio Voltage (for your case).
If you are working on Amplifier then you should have to consider THD.

Good luck

Hi Saikat,

Thanks for the reply. I am working on active filter. So active filter being the class of circuit concerned, are my assumptions right, or does it need some more modifications?

Regards
Sam

No, need no modification for your filter.
Actually forget to mention that Amplifiers, Filters etc. or such ckt which mainly deals with a BW of signal, to design such network or ckt. one should consider THD. So don't worry. Go ahead my friend.
Good luck.

Hi Saikat,

I have some questions:

According to the definition of DR, the lowest possible amplitude of input voltage is defined by maximum of input-referred noise voltage. My question is - why is it not dictated by the linearity range of OTA? Because, trying to reduce the input amplitude after a certain value will violate the saturation criterion of the input transistor of the OTA.

May be I am wrong. Please clarify. Actually I am in real confusion about how to measure the DR. If you know, please suggest me some relevant literatures which explains it from the basic level.

regards
Sam

Because, trying to reduce the input amplitude after a certain value will violate the saturation criterion of the input transistor of the OTA.
Amplifiers (either OTA or different types) are generally operated with a bias current. At low signal levels, you can expect |Isignal| << |Ibias|, so linearity will be always achieved.

Yes you are right. After posting the previous reply, I realized vagueness of the question. Thanks again.

DR = 20log(Vin_max/Vin_min) dB.

Sam

yes you are right this is the right equation to find the dynamic range. I am designing LPF and I found the DR throught this equation. the max input voltage is meant by the value of the input voltage when you get a THD 0f -40dB (1%), the min. indicates the niose floor (mignitude).

I hope this may helps.

Good Luck

---------- Post added at 10:46 ---------- Previous post was at 10:46 ----------

DR = 20log(Vin_max/Vin_min) dB.

Sam

yes you are right this is the right equation to find the dynamic range. I am designing LPF and I found the DR throught this equation. the max input voltage is meant by the value of the input voltage when you get a THD 0f -40dB (1%), the min. indicates the niose floor (mignitude).

I hope this may helps.

Good Luck

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