HugoFranca wrote on Jul 29th, 2008, 6:29am:Thank you for the answers.
I think that my specifications was too tight and that I have now a clearer picture of the problem.
So I think that I will ignore this small phase that is introduced and use switches with Ron of around 200Ω which are easy to do.
The circuit will NOT settle within, say 1/4LSB, at the end of the sampling phase, but I think that there is no big deal with this, right?
Cheers,
Hugo
I am afraid that your understanding of the specs is still wrong. While the value of Ron you have chosen seems more reasonable, please bear in mind that an ADC is a sampling system, and switches are nonlinear blocks.
Unless your sampled signal settles to a sufficient accuracy within the sampling phase, you will get horrible distortion, and there is no point building a 10-bit ADC when the sampling switch is only 5-bit accurate.
If you don't believe me, just run a simple transient simulation of the switches and the cap, and look at the spectrum of the sampled signal.
There are 2 design challenges to resolve here, which depend on your application.
1. Settling accuracy for static signals. If there is already a sample-and-hold before your ADC (say), then your ADC will be digitizing static signals, more or less. So, you just need 2*Ron.C such that you have more than 7.7 time constants (for 66 dB settling accuracy) in the sample phase.
2. If you have a dynamic signal on the other hand, and this may be quite fast. You quote 20 MHz input, then the requirements are quite different. There is probably no hope to get good linearity, unless you bootstrap your switch. Because now, your switch timeconstants have to be so small that the switch tracks the changing input all the time. That is probably the reason why you got this 14.5 Ohm number. But with a bootstrapped switch, you will reduce the nonlinearity in the switch Ron, which will allow operation with a tolerable size switch (and a not so tolerable size for the bootstrap capacitor).
3. The nightmarish scenario if you are making a subsampled system, that is, your input signal still has a small bandwidth but the absolute frequency is very high. Then bootstrapped switches are the only way as well, but even they may have a hard time.
So, check if you have scenario 1 or 2 before you begin switch design, and do a transient analysis for distortion.
Regards
Vivek