Harmonic Distortion And Board Layout

How Do You Know What Is Really Going On?

In the absence of an ideal quad amplifier to put onto one’s board, it is pretty hard to measure the effect of a single amplifier channel on itself. Clearly, a given amplifier channel doesn’t just perturb its own inputs, but also perturbs other channels’ inputs. Ground currents flow past all of the different channel inputs, with differing results, but all are affected by each output. This effect is measurable.

Table 2 shows what happens when just a single channel is driven with harmonics measured on the undriven channels. The undriven channels show a tiny signal at the fundamental frequency (board crosstalk), but they also show distortion products which, in the absence of any significant fundamental signal, are directly caused by the ground currents. The Low Distortion Layout of Figure 6 shows greatly improved second harmonic and Total Harmonic Distortion (THD) due to the near elimination of ground current effects.

Summary

In simplified terms, on a PCB, the ground return current flows back to the different bypass capacitors (for each supply voltage) and to the supplies themselves in proportion to their conductance. High-frequency signal currents return to the small bypass capacitors. Lower frequency currents, such as in the audio range, might flow mostly to the larger bypass capacitors. Even lower frequency currents might flow directly to the supply wiring and ignore the bypass capacitors altogether. The specific application determines which current path is most critical. Fortunately, it is easy to protect against all ground current paths by using common ground points and ground bypass capacitors on the output side (if possible). Readers familiar with high-frequency amplifiers will be concerned about adding another constraint to board layout. The cardinal rule in high-frequency board layout is to place the high-frequency bypass capacitors as close to the package supply pins as possible. Modifying this rule to improve distortion is not much of a change, as can be seen by comparing Figure 5 and Figure 6. The distortion improvements come at a cost of adding about 0.15 inches of trace to the high-frequency bypass connections. For example, this had only a small effect on the AC response of the FHP3450 with performance being improved: the 0.1 dB gain flatness improved from 47 MHz to 55 MHz. Also improved was differential gain and phase, with differential gain improving from 0.1% to 0.08% and differential phase from 0.03 to 0.01 degrees (all AC coupled). Board layout is crucial in wringing all of the performance from a quality amplifier. The issues discussed here are not by any means limited to high- frequency amplifiers. Lower frequency signals, such as audio, generally have much tighter distortion requirements. The ground current effects might be reduced at lower frequencies, but if the required distortion performance improves correspondingly, ground currents can still be a significant issue.