ytass wrote on Aug 31st, 2006, 4:51pm:2. Can you please explain further what you mean by "present a switched capacitor load at the input of your amplifier"? Why would this cause problems, (and then make CHS a requirement)?
4. I am a bit confused as to what you mean by "your net noise is proportional to the various capacitances that you use". Can you please explain further? I was also of the understanding that SC/DT circuit has lower power dissipation that CT circuit.
Cheers
Hi,
For more details, see pg. 500-513 of the text by Gregorian and Temes (Analog MOS ICs for Signal Processing), and/or Section 2.2 in
http://www.designers-guide.org/Analysis/sc-filters.pdfThese deal with the issue of noise in sampled systems.
More specifically:
2. If you have a switched-capacitor circuit, then the input branch is a capacitor which is connected to the input (vin) each cycle. This charging and discharging is not always tolerable in every system. Firstly, if the biological signal source in your system does not have sufficient drive capability, then you will be loading it severely. A switched-capacitor circuit is essentially a time-varying low-impedance load. Mostly, you would place a high-impedance network at the input, like the gate of a CMOS transistor, so that the signal source sees no load and needs to provide no current.
With standard CDS (offset sampling at input), this is not possible. However, with chopper stabilization, you can chop at another point in the system such as the output. You can of course use a modified form of CDS where you sample the offset at the output of your preamp and then cancel it (see the excellent textbook by Razavi where he discusses this in Section 13.2). The requirement is that the gain of the stage be small enough that you do not saturate it while measuring the offset, but this should be possible if you use a multi-stage preamp and perform offset cancellation on each stage.
4. Regarding noise, please read the above references. Essentially, the thermal noise power taken in a sample (sampled system) is constant = kT. When you normalize this to a voltage stored on your cap, the voltage noise level is kT/C, where C is the sampling cap. The noise contributed to an output sample by an amplifier in a sampled system is also proportional to kT/Cc, where Cc is the compensation cap, and the proportionality constant is dependent on the amplifier configuration and design. Thus, in a sampled system, reducing noise => increasing C => increasing power.
Thus, wideband noise from all frequencies is aliased down in each sample.
If you have a purely continuous-time system on the other hand, only the noise contained within the bandwidth of your system comes into play.
However, since you are anyway taking samples of the input and averaging them, you have a sampled-data system at some point. The key is to design the system in such a way that the net noise required is achieved without too much power consumption.
I hope this helps. But the topic you are interested in is quite a comprehensive one.
Regards
Vivek