Analog switches for volt meter range switching

[leonerd]

Hi all,

Dredging up an old topic, but I found this post on precision resistor networks used as range switches in volt meters yesterday and it exactly answers about 4 different questions I had all in one. But it leaves me with some more.

Lets consider current flow for a moment. Looking again at the 1V input, with the range switch set to the lowest divide ratio of 10:1, that 1V is flowing through a total resistance of 11.111...Mohm. I make that a current flow of 90nA. What sort of analog switch chip are you going to use for that?

I see "leakage current" of channels specified sometimes in units of nA already (e.g. the randomly-picked MAX351 guarantees to source or sink no more than 0.4nA, which is 0.4% of our signal here). Does that matter? I admit I don't quite understand what the leakage current is measuring, and whether it's important in this application.

Given our switch is shorting a resistor to the low-impedance ground supply, I don't think charge-injection is an important parameter here, as ground will conduct that all away.

Channel resistance is fairly important, at least at the highest end of the divider chain - a 10ohm channel on that 1K resistor is a massive 1% of error. Maybe on higher ratio / lower resistance end of the chain we'd be better off using discrete FETs instead of an analog switch part?

Can someone suggest a suitable analog switch we could use for precision voltage measurements based on this sort of divider?

[Kalvin]

You can use discrete MOSFETs and handpick/screen those which perform best. You should be able to find some parts achieving very low leakage current of 10pA and better quite easily. For example this JFET https://www.digchip.com/datasheets/parts/datasheet/513/SST4118-pdf.php has leakage below 1pA at 25 C. Typically leakage current is specified at 25 C, and it will increase considerably at higher temperatures. There are matched MOSFETs, for example http://www.aldinc.com/pdf/ALD110808.pdf and others with datasheet specifying typical leakage current of 10 pA, but handpicking suitable devices your should be able to find parts with sub-pA currents.