Read the various datasheets and application notes for chopper stabilized amplifiers, the highest precision bipolar operational amplifiers, and low input bias current operational amplifiers. Linear Technology and Analog Devices are great sources for this information. The major issues in rough order of importance are:
1. Thermocouples and temperature gradients - this is a huge problem and special attention will need to be directed toward the layout and maintaining an isothermal environment. Careful design is required to get the specified drift performance out of chopper stabilized (10nV/C) and low drift operational amplifiers (100nV/C before trimming or grading). 2. Leakage - even at low impedance levels, minor amounts of leakage will cause significant errors. 100nV at 10k is only 10pA so this is a real problem for RC filters where long time constants require high impedance levels and capacitor leakage also needs to be considered; avoid Mylar/polyester/PET and high dielectric constant ceramic capacitors. In the past, through-hole construction allowed "air wiring" sensitive nodes but that is not practical with surface mount parts. Guards will need to be used in the layout which is impossible with the smaller surface mount packages. 3. Resistor self heating - even precision resistors become a problem at this level of stability because of self heating which also complicates the thermocouple problem. High resistance values lower self heating but make errors from leakage current and current noise worse. 4. Dielectric absorption - this will be a problem if fast settling is required. Avoid Mylar/polyester/PET and high dielectric constant ceramic capacitors. 5. Pink Noise - 1/f noise increases as frequency decreases. Chopper amplifiers have flat 1/f noise so are invaluable below about 1 Hz. On Tue, 1 Nov 2016 13:14:16 +0100, you wrote: >Hi, > >I have a "small" side project, which involves keeping a voltage stable >to better than 100nV over the period of several seconds. Ie. a DAC >produces an output and a chain of opamps and low pass filters feeds >it to the consuming device. The absolute value and drift over more >than 10-100s is not that important. > >As I lack a lot of knowledge in this field, I would like to ask >whether someone can point me at literature or give me some terms >to search for that help me to figure out whether this is actually >feasible and how I could achieve that. 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. > > Attila Kinali _______________________________________________ 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.
