Simple pure sine-wave test source...

[Mark Hennessy]

Came across an LTC design note (DN564) from July 2017 with an simple/obvious and neat approach - uses a very low power single-rail op-amp and a monolithic squarewave generator - the op-amp implements a very narrow BW bandpass filer to pick out  the fundamental from the square wave, resulting in a -100dBc (0.001% THD+N) sine wave.

Hmm...

Have you built and tested this?

Firstly, it actually doesn't promise -100dB performance:

One does not expect to generate a sine wave with –100dBc distortion using a 5V low power op amp. All the same, a bandpass filter using the LTC6258 can combine with an easy-to-use low power oscillator to create a sine wave at low cost, low voltage and extremely low dissipation.

I read that as "we're not aiming for anything like -100dBc, but it'll be a plausible sine wave nonetheless".

With a 10kHz square wave, the 3rd harmonic is so far away (20kHz) we can afford to use a simple low Q band-pass filter to extract what, by definition, is a pure sine-wave at the fundamental.

Well, 20kHz is only an octave away from 10kHz, but that aside, it's 30kHz that we need to attack. And 50kHz, 70kHz, 90kHz, etc.

The 30kHz harmonic will be a third of the amplitude of the fundamental, so ~10dB down. So our bandpass filter needs to be around 90dB down at 30kHz to get -100dB performance. But according to the graph on page 1 of that PDF, 30kHz is only ~30dB down relative to 10kHz. Which suggests that the distortion performance will be ~1% at best.

Quite apart from that, the LTC6258 has pretty mediocre distortion - the data sheet says 0.025% at 500Hz into 4k, and it will be higher at 10kHz and higher again into lower load impedances (see R5). BTW, "single-rail op-amp" is marketing-speak - just about any op-amp can work from a single supply. Op-amps that are marketed as "single rail" usually allow their inputs and outputs to get closer to the rails than a typical general-purpose op-amp, but naturally the data sheet should always be consulted to see if that's the case. Pay particular attention to the output loading conditions, as some are only "rail-to-rail" when very lightly loaded. And to achieve this, compromises have to be made, which is why I've yet to see a "rail-to-tail" op-amp get anywhere near the audio performance of the venerable 5532 (or even the TL072, frankly).

This sort of technique is commonly used for non-critical, fixed-frequency applications. But I've never seen it used in high-quality audio measurements - in the analogue domain, you need a Wien bridge oscillator at the very least, and preferably a state-variable filter oscillator.

Sorry to pour cold water on this, but hopefully it helps to put some perspective on it. It's still a useful circuit in its own right - but high performance it's not. Impressively low power consumption though - the NE5534 (single 5532) takes 4mA!