Berti wrote on Jul 16th, 2008, 11:33pm:Interesting approach. But I am not sure. Probably you just shift the problem, because
the last (usually strongest) buffer will couple into the analog (clean) supply.?
Regards
Hi Berti,.
As usual, there is a trade off involved. Certainly your clean supply will get slightly dirty when you connect the gates driving the switches to it. Then the question is what is worse?
1) getting the analog supply slightly dirtier but not enough in order to degrade the injected noise into the OTA (assuming a not too large PSRR) but keeping the (sensitive) signals being driven by the switches from the (much) dirtier digital supply OR
2) Living with the signal driven by the switches - now connected to the dirty supply - being corrupted by such noise, but keeping the OTA with a super clean supply.
In most cases I would be inclined for option 1) since it is usually the signal driven by switches (for example low noise signal coming into a SC filter) the one that suffers mostly the effect of noise. At the same time the OTA is supposed to be more immune to noise than the incoming signal. Despite the relative low PSRR the OTA might have
it will always be better than the unprotected incoming signal which is totally exposed to capacitive coupling.
Certainly there can be exceptions and eventually option #2 might be better.
Roland:
Using guard rings is another good technique for isolating noise from analog sensitive circuits. Please be aware of the role a guard ring might play: it could be used for
collecting noise - in which case you should connect it to an already dirty supply line or for
isolating the noise. In this case it should be connected to a clean supply.
I´m not sure about increasing the impedance of the AC power. I guess that increasing its resistance will help filter the spikes, but at some point it might hurt and I'm not totally clear in which way.
Hope this helps
Tosei