THD meter circuit needed

[Martin Czech]

Yes Darla and Layla. At least one of them has EXTERNAL ADCs.
Is it Layla?

Some ADC chips are said to have a mute circuit, if
no signal is applied. So an experimenter shorting the input may be
fooled. This seems also be true for some DAC.

I think a battery driven osacillateur a la vienese (Wien bridge)
with some OP37 or 27 or even 5534 can be a very good signal source.
Depends all on amplitude regulation circuitry and time constants.

m.c.