01-03-2018, 01:32 PM
Sorry again 
TBH, the filter is almost certainly the limiting factor here - I'm guessing that you'd need a fair bit more stop-band attenuation before the op-amp THD starts to dominate. Though it's impossible to quantify that without seeing a graph of THD vs frequency for the op-amp. It would be tempting to add a second filter, which should get to around 0.05% in theory (some tweaking or component selection might be needed).
I remembered that this technique is used in the Radiometer SMG40 that I repaired a while back. The 19kHz output on the rear panel is a bit "spiky triangle", but good enough for the purpose. The main output has a complex LC filter than removes harmonics from the complete composite signal (along with other "magic" - they had some clever folk working on that design).
TBH, the filter is almost certainly the limiting factor here - I'm guessing that you'd need a fair bit more stop-band attenuation before the op-amp THD starts to dominate. Though it's impossible to quantify that without seeing a graph of THD vs frequency for the op-amp. It would be tempting to add a second filter, which should get to around 0.05% in theory (some tweaking or component selection might be needed).
I remembered that this technique is used in the Radiometer SMG40 that I repaired a while back. The 19kHz output on the rear panel is a bit "spiky triangle", but good enough for the purpose. The main output has a complex LC filter than removes harmonics from the complete composite signal (along with other "magic" - they had some clever folk working on that design).
