There are Choppers based on LDRs, but due to the resistance of the LDRs the noise tends to be relatively high. So this is more like the 5-10 nV/sqrt(Hz) level and higher. There can still be some effective charge injection from cross conduction, as the switching is slow.
There are nV amplifier based on JFET switches and a transformer at the input - these can get really low in noise (well below 1 nV/sqrt(Hz), but charge injection needs a carefull trim to get it low. This could be a way for a DIY solution - it is old style, but relatively simple. AFAIK Keithley has such an amplifier for the K2001/2002. TiN can probably tell more on these.
There is some App. note from Maxim on a chopper amplifier build with CMOS switches at the input, they do bet pretty low noise, but not so shure about the charge injection.
An improved version could be found somewhere on this web page:
http://www.janascard.cz/aj_Zakazkova_vyroba.html#USBI have tried a similar (but using full wave chopper with 4 switches and thus lower noise - a bit like the input stage of the DA1281 DMM) circuit on the bread board, to get an idea if it works OK. The results were not that bad, though the bread board is obviously not the way to do nV stuff. So the main point was that is was stable and not oscillating and the charge injection spikes not excessive.
Compared to the ready made AZ OPs the switches are usually not as well matched and not that well fit for the purpose. On the other side a DIY chopper can use a relatively low chopping frequency and larger coupling / filtering capacitors that are not available on a chip. A lower chopping frequency gives less current noise and a JFET based amplifier can have an 1/f noise limit lower than the CMOS process used with the AZ OPs. Especially DIY an extra adjustment, e.g. to minimize charge injection, is also possible.
For my purpose a simple AZ OP was good enough - they are not that bad and could make a pretty simple system if the source is low impedance.
So the low noise chopper is at hold for the moment. I want to get the meter part working fírst.
Chopping works pretty well also to very low frequencies. The slight residual 1/f noise one sometimes sees with AZ OPs seems to come from a limited gain of the chopper part and than relatively high 1/f cross over of the main amplifier to start with. A seprate build amplifier can be better in this respect. The main limit are thermal effects.