difference between non linearity and THD

[preethi19]

Hi i am trying to study an analog multiplier. And the output of multiplier is Iout=2*ix*iy/Ib.. So ix and iy are input currents and Ib is the bias current. The gain can be adjusted by Ib. So i am little confused with these three terms
Offset
Non linearity and
THD

I understand say if we plot for the above equation in matlab we get ideal output value which cannot be the same from the simulated circuit. So how much the simulated deviates from the ideal is offset.

So what is non-linearity then??? I thought it was nothing but offset..

THD being total harmonic distortion occurs when unwanted harmonics mixes with our output degrading the output. So what is the difference between non-linearity and THD???

So can anyone pls tell if gain Ib mismatch (due to current mirror) causes non-linearity error more or THD more? Plssss help!!!!

 

[JesusML]

Suposse that you have two systems with A1(x)= G*(x+1) and A2(x) = G*(x+1)^2 representing its respectives behaviour. Now, introduce an input x= Cos(wt). Then, A1(x) = G*B*Cos(wt) + G and A2(x)= G*(B^2*Cos^2(wt)+2*B*Cos(wt)+1) =(after some algebra).. = G*(3/2+ 2*B*Cos(wt)+ B^2*Cos(2wt)/2). From above we see that if in the first system we introduce a signal with the form Cos(wt), we obtain at the output the same signal scaled with a constant G displaced by G, while in the second, we obtain components with multiples of the fundamental frecuency (2wt, 3wt, etc, depending of the system´s grade). So, we can conclude that the first system es linear while second is not. A linear (and time invariant, which is another topic) CANT generate harmonics while a non-linear can. The THD is the measure of how much a signal is distorted. So, non-linearity is a property while THD is a measure. I think that the offset in the multiplier is the level of DC generated at the output (0 volts ideal) but, due mistmach you will have a non-zero DC voltage at the output.

Hope will help
Best regards
Jesus

 

[barry]

Your ideal transfer function is a straight line. Non-linearity is the deviation from that straight line, not just offset. Offset is a fixed difference from the ideal line; if you drew a line parallel to the ideal line but shifted above or below it, the difference in the Y direction is offset. Harmonic distortion is defined as the ratio of the power of the harmonics to the power of the fundamental.

 

[LvW]

THD is one method to express the consequences of a non-linearity as a "figure of quality"

 

[crutschow]

The is no "difference" between the two. The are just different ways of expressing the effects of non-linear behavior.

THD is a measure of the effect the non-linearity has on a sine-wave.

The non-linearity spec is usually used to determine the deviation from the ideal transfer function for DC or slowly-varying signals

 

[Audioguru]

If a sinewave is squashed or expanded symmetrically at both top and bottom or if a load line is amplified or attenuated symmetrically at both ends then the distortion has odd-numbered harmonics.
If a sinewave is squashed or expanded only the top or only the bottom or if a load line is amplified or attenuated at one end then the distortion has even-numbered harmonics.