If a circuit has a charge pump IC to generate a negative rail, and a ADC nearby running at an appropriate frequency for an audio Nyquist...say
44khz, has anyone tried using the ADC clock as input for the charge pump to reduce noise injection from the latter into the former?Or is this a non-issue with proper layout?
This aliases the ripple to a DC component. Assuming the sample event occurs at the same phase against the ripple waveform, and that the ripple doesn't change in waveform profile, then it will remain constant and DC, and can be subtracted after a one-point (or better) calibration (i.e., zeroing).
Or the phase can be adjusted so the sample event coincides with a zero crossing of the ripple, nulling it.
Or an averaging sampler can be used which integrates over the whole sample period, rather than a short sampling aperture, and the phase will be unimportant (this is characteristic of sigma-delta ADCs, of course).
If the desired signal doesn't even include DC (i.e., it's AC coupled, typical of an audio application), the DC can be filtered digitally without any need for matching phase; the frequency difference (Fripple - Fs) only need be within the stop band by a suitable margin (-xx dB?). Of course, if phase is locked, frequency difference is guaranteed at zero, which gives infinite attenuation on most high pass filter designs.
Tim
-- Seven Transistor Labs Electrical Engineering Consultation Website: http://seventransistorlabs.com "bitrex" wrote in message news:5592080f$0$5936$4c5ecfc7@frugalusenet.com... > If a circuit has a charge pump IC to generate a negative rail, and a ADC > nearby running at an appropriate frequency for an audio Nyquist...say > 44khz, has anyone tried using the ADC clock as input for the charge pump > to reduce noise injection from the latter into the former? > > Or is this a non-issue with proper layout?
That wouldn't work on an ICL7660 (they run at 10kHz), but some of the 'compatibles' can run faster - IIRC one from Microchip and the LM7660. You should be able to override the oscillator by feeding a cmos clock into the external cap pin.
In English, man! Give me the idiot's version, please!
The Cliff's notes version is, "yes, it'll help a lot, because it'll put all the spurs at DC, where they're easy to get rid of."
Cheers
Phil Hobbs
Thanks!
How about : It'll probably work great, unless your requirements are tighter than you think, or the charge pump's phase wanders due to cheapness or the demands of voltage regulation.
-- Tim Wescott Wescott Design Services http://www.wescottdesign.com
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