Bruce Griffiths wrote:
Joseph M Gwinn wrote:
The integration function requires a low frequency cutoff (either a
servoloop or a resistor shunting the integration capacitor) to avoid
integrator saturation.
This inevitably distorts the triangle wave, however it should be
possible to reduce the triangular wave distortion by predistorting the
integrator input current.
Yes, there would need to be some kind of drift compensation (I favor a
opamp servoloop), but given that we are trying to measure ZCD jitter
(versus long-term wander), isn't this good enough? The distortion
will be
small and stable, and so will not cause jitter.
Joe Gwinn
Yes one shouldn't lose sight of the goal which isnt a perfect triangular
wave, but merely a low jitter one.
Indeed.
The major problem is the Johnson noise of the resistors used in the
integrator.
If for example one uses a simple RC filter using 25k plus 10uF and
drives it with a 10Hz square wave the output noise at dc is about
20nv/rtHz.
The output slew rate with say a 5V amplitude square wave is about 1V pp
and the zero crossing jitter due to Johnson noise is on the order of 3ns.
Hmm... what about an LR replacement of the RC chain over the op-amp?
Just a thought. It is a bit uncommon, but is essentially the same in
function. Could prove interesting since the LR balance could possibly
allow for lower noise selections.
Cheers,
Magnus
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