The OPS in the THS3215 provides one of the best HD solutions available through high power levels and frequencies. Figure 6-31 and Figure 6-32 show the swept-frequency HD2 and HD3, where the second harmonic is clearly the dominant term over the third harmonic. Typical wideband CFA distortion is reported only through 2 V_PP output, while Figure 6-31 and Figure 6-32 provide sweeps at 5 V_PP and 8 V_PP into a 100 Ω load. These curves show an approximate 20 dB per decade rise with frequency due to loop-gain roll-off.

The distortion performance is extremely robust as a function of load resistance (see Figure 6-33 and Figure 6-34). Normally, heavier loads degrade the distortion performance, as shown by the HD2 in Figure 6-33. However at frequencies greater than 30 MHz, the HD2 actually improves slightly as the output load is increased from 500 Ω to 100 Ω.

One of the key advantages offered by the CFA design in the OPS is that the distortion performance holds approximately constant over gain, as seen in the full-path distortion measurements of Figure 6-9 and Figure 6-10. Here, the D2S provides a fixed gain of 2 V/V driving a 200 Ω interstage load and using the internal path to drive the OPS at gains from 1.5 V/V to 10 V/V. Hold the loop-gain approximately constant by adjusting the feedback R_F value with gain to achieve vastly improved performance versus a voltage-feedback-based design.

Testing a 5 V_PP output from the OPS with the supplies swept from the minimum ±4 V to ±7.5 V in Figure 6-35 and Figure 6-36 show:

1. The 1.5 V headroom on ±4 V supplies and ±2.5 V output voltage results in degraded performance. At the lower supplies, target lower output swings for improved linearity performance.
2. The HD2 does not change significantly with supply voltages above ±6 V. The HD3 does improve slightly at higher supply-voltage settings.

From these plots at ±7.5 V supplies, a 5 V_PP output into 100 Ω load shows better than –60 dBc HD2 and HD3 performance through 30 MHz. This exceptional performance is available with the OPS configured as a standalone amplifier. Combining the standalone OPS performance with the D2S (see Figure 6-3 and Figure 6-4) does not degrade the full, signal-path distortion levels. With the D2S and OPS running together at a final 5 V_PP output and 30 MHz, the HD2 changes to –63 dBc and HD3 changes to –59 dBc on ±6 V supplies. Lower output swings for the combined stages provide much lower distortion. The 2 V_PP output curves on Figure 6-3 and Figure 6-4 show –61 dBc for HD2 and HD3 at 50 MHz.