Weston Cells

[doktor pyta]

An analog switch is rather not an option due to ESD input diodes. That's why LT uses discrete transistor.

[guenthert]

@guenthert

the rationale for adding the buffer to 10 Weston cells is to:

1. provide low output impedance
2. provide protection against shorting the cells. It happens and it has bad effect.

I know that main advantage of my ovenized setup is the low noise.

Scaling 10.18 to 10.00000 should not introduce much of error /drift/ tempco.

According to the calibration reports my set of Weston cells drifts about 1ppm/ year.

      Well, while I can't really argue against the argument of protection, as I'm exactly that clumsy guy who'd cause such an accident, the use with a reference divider would limit the chance of such.

https://www.ebay.com/itm/325039540790?epid=1501497714&hash=item4badddca36:g:kcIAAOSwNHRh9xba&shqty=1&isGTR=1#shId
(not affiliated)
At that price I was thinking of buying it myself, but then, I don't have a standard cell.

[1audio]

Why not a good relay running from the same power as the buffer? Off would be limited by contamination. Only connects when the opamp is under power.

[doktor pyta]

I have concerns about using a typical non-latching relay and coil heating the contacts. So rather latching relay or small signal SSR like AQY221N2M.

[Kleinstein]

A latching relay would need extra measures to make sure it really turns off relialy when the power shuts down. A photo-mos SSR could indeed be a viable choice, though I would not consider the metioned type, as there is no need for a low R_on, more like get a type for low leakage and slightly higher voltage rating.

[ramon]

Which ones has better specs?

[Gyro]

On the face of it, the AQY221N2M as it appears to have the lowest typical leakage at the highest voltage, although they all seem to match by the time you get below 5V... However the CRITICAL spec is the 10nA Maximum. There is little information there, so this might apply at the upper end of the operating temperature range, but might not.

It looks as if you can get the manufacturer to select for <1nA maximum leakage, but your best bet would probably be to purchase a few standard parts and hand select yourself for the lowest leakage at ambient temperature and target voltage.

[Gyro]

Why not a good relay running from the same power as the buffer? Off would be limited by contamination. Only connects when the opamp is under power.

This application is pretty much the definition of dry switching. Very few contact materials could be relied upon to provide reliable contact closure. About the only type of relay that would be guaranteed to provide reliable switching would be a Mercury wetted type. Of course these aren't manufactured any more due to the toxic content, but are still available surplus on ebay.

Given the toxic content of the Standard cell itself, a Mercury wetted relay would be rather appropriate. The only really serious issue would be that the relay would have to be maintained vertical to avoid accidental closure (a few are position independent), however you would probably want to avoid inverting the standard cell anyway to avoid shifts in voltage.

[Kleinstein]

The actual leakage is usually way lower than the maximum specs. A weston cell amplifer is more like a one off part, so testing the part before use should not be an issue.
The temperature effect is documented for quite a few parts: expect the leakage current to go up about exponentially.

The specs for the usual small MOSFETs are somewhat similar: the limit on the leakage even tends to be worse due to being more price sensitive parts.

[TimFox]

Yes, the formerly-available Hg-wetted contact relays are great for dry switching;  I use them for low-level audio.
Be careful about the orientation however, which was mentioned above. 
If you tilt the unit too far from the indicated direction, the mercury will flow internally and short out all the contacts. 
This reverses quickly when you re-orient the relay to near the correct direction. 
Of course, you are not supposed to tilt a standard cell, either.

[Gyro]

I use them for low-level audio

Me too.