Somewhere in the training sessions prepared by PA I thought I had seen this, but cannot find it now - any leads on where that might be? The one I recall was kind of like a THD power equation.
Oh and Art, another piece I am putting into this article is relatively recent bench verification that the HD terms add in phase for cascaded stages. I took that data on the 2stage THS3217 - 1st seperately then cascaded, I would say this equation is working pretty well,
The math combining two cascaded stages for each distortion term is shown as Eq. 1.
Eq. 1
This is written as SFDR, but actually the HD2 and HD3 (etc) terms would be combined individually. Legacy literature would often report the combination as a THD power combination as in Eq. 2, which is totally incorrect, (page 43, ref. 3).
Eq. 2
Much more recent bench testing of a 2-stage amplifier (THS3217), where the input and output stages were measured separately, then combined in cascaded stages, showed these same results (Fig. 4). Here the input D2S stage was tested over frequency with a 2Vpp into 200ohm load while the OPS stage in that same unit was tested separately producing 5Vpp into 100ohm load. These separate measurements were combined using Eq. 1 to predict the cascaded distortion then the same part was measured in a cascaded configuration producing the same swings and loading at each stage. Clearly Eq. 1 appears to correctly combine the cascaded distortion performance.
Figure 4. Calculated and measured two stage amplifier distortion comparison.
