Measuring THD3 in an OTA

Dear Friends,

Hi. I'm going to simulate an ultra LV OTA for calculating THD3 for Vin 520mVp-p with Hspice 2006 I need commands. I've used .four command but it gives third-order harmonic only at a given frequency?? How about Total Harmonic Distotion at total frequency sweep??

Thanx

From that FFT, you can add all harmonic components (usually ~5th or 7th) to calculate THD.
THD = toal harmonic distortion.
What is THD3? Is this IP3?

THD is defined as "Ratio of the total power of 2nd and higher harmonic components to the power of the fundamental power".

What you have to remember is..

1) THD value is almost always a function of the amplitude of the input (output) signal level, and thus the corresponding signal amplitude must be also specified.

2) Typically ~5th harmonics are counted (sometimes ~7th..but...dependes)

3) Watch out for the filtering effect in a single tone test. Assume that your amp has lowpass characteristic and the applied signal is close to the cutoff frequency, all harmonics are attenuated. This does not mean this amplifier is good in THD.
What you can do is intermoduation test!!!


B

Dear Friend

While I do thank you I would like to tell you that by THD3 I meant Third-order harmonic for a specified input. Do you know any command in Hspice simulator??

Thanx