Attila wrote:
I have a "small" side project, which involves keeping a voltage stable to better than 100nV over the period of several seconds. * * * I know the basic literature on noise and how to deal with that. What I am interested in are the real world problems, how big they actually are and how to deal with them.
Noise is just one of the concerns, and potentially not the hardest one to deal with. Settling time will be a real issue. If the full-scale output is 1v, you need to wait for the system to settle to within nearly 24 bits.
First, the LP filters will have a prolonged settling tail to 100nV (meaning that the basic regulation must show 100nV stability for longer -- probably much longer -- than you are contemplating), so drift may be much more of an issue than you anticipate.
Second, everything -- the reference, the DAC, the buffer amps, the resistors, and even the capacitors -- will exhibit temperature effects. You will need an extremely stable thermal environment. You will also need to deal with changes in the self-heating of the DAC, the amplifiers, and the resistors when you change the voltage, which will likely produce considerably longer settling tails than the LP filters. The wider the range of output voltages, the worse the self-heating effects will be.
Of course, having to wait for settling to within 24 bits will put a real limit on how fast you can change the voltage (how long you have to wait after commanding a change before you can rely on the output voltage). And what is the downstream circuitry going to be doing during this settling time? Are you going to need a S&H that can hold to within 100nV for seconds at a time?
This isn't quite so much of a problem if each step is small, but settling to within 24 bits is a pretty big deal even in that case. And probably much slower than you are expecting.
Best regards, Charles _______________________________________________ volt-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts and follow the instructions there.
