Near ideal transistor or similar on/off switch

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hello,

 

i am measuring(whit 1s interval) a voltage point in a electro-chemical system, via an op amp configured as a buffer(voltage follower) since i need measuring point to have reaaaaaally high impedance, to prevent effecting very sensitive chemical process. I am measuring voltages in range of 20 to 200mV.

I got that part up and running, but the other thing is that while i am not measuring, that same point must be shorted to ground. So i tried that whit a mosfet (p16nf06l), but the problem is that while mosfet in closed state leaked very little current (uA), it is still a LOT comparing it to an op amp that i use(AD8542ARZ1), which has couple of pA and even if i close the mosfet while measuring, the value that i measure is around 40mV, instead of actual 180mV. 

 

So basically i need to find a component or a circuit that will be able to COMPLETELY(or near completely) disconnect the grond from emasuring point while i measure, and then after, when i open it, starts conducting. When in conducting phase it does not mather if it has couple of 10 Ohms, but when it is in non-conductive state, the leake current should be <100pA. And i cant use mechanical realys. 

 

Thanks in advance.

This topic has a solution.

Last Edited: Tue. Jun 5, 2018 - 10:18 AM
This reply has been marked as the solution. 
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Hi Klemko

 

An analog switch could do the trick.

 

Example: https://www.vishay.com/docs/6990...

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Thanks, this solves it :D

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Here is a search for low leakage switches:

https://www.digikey.com/products...

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Adding an external 40kΩ resistor to each input makes the MAX328/MAX329 an excellent fault-tolerant multiplexer. Low leakage (less than 1pA at +25°C) and 2.5kΩ on-resistance allow the circuit to sustain 110V AC faults indefinitely while maintaining an error of less than 40nV for normal signals (i.e., 1pA times 40kΩ).

 

 

When in the dark remember-the future looks brighter than ever.   I look forward to being able to predict the future!

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Thanks all :D

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The MOSfet you selected: p16nf06l is a TO220 Power mosfet. low leakage is not a very important design parameter in these power fets.

Surprized: 2n7000 has a whopping 10nA leakage (old design?)

Others (SOT23) mosfet's I've looked at also have a pretty high drain/source leakage.

 

In a service manual of an old HP bench top  / 17" multimeter they used J-Fets to switch sensitive signals ( 8 digits of accuracy or so).

Gate was pullled to -15 V to close the fet, but simultaneously a buffered version of the drain voltage was put on the source to minimize any leakage.

Leakage of the gate is presumalby several orders of magnitude lower than that of Source / Drain.

 

For the records:

There are also optocouplers which are made from a LED and 2 back to back Fet's.

These can switch DC or AC signals with no electrical connection between input and output.

"optomos" is a good magic word for these devices, but that name might be trademaked or fouled by other foul language.

https://duckduckgo.com/html?q=optomos

 

Edit:

The document mentioned is the service manual of the HP 3456A

It is a 385 page, 15,4MB pdf with an extensive description on the techniques used to get 6 digits of presicion out of 1980's technology.

It is a very good read for anybody interested in high accuracy measurement techniques.

They even go to the effort of using teflon standoffs, but then still include guard rings around the teflon standoffs just to be sure.

I believe this "Voltmeter" has been the reference for about 20 years and is still a highly appreciated instument.

https://duckduckgo.com/html?q=HP+3456A+service+manual+pdf

Doing magic with a USD 7 Logic Analyser: https://www.avrfreaks.net/comment/2421756#comment-2421756

Bunch of old projects with AVR's: http://www.hoevendesign.com

Last Edited: Wed. Jun 6, 2018 - 01:33 PM
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Pretty cool method

but simultaneously a buffered version of the drain voltage was put on the source to minimize any leakage.

That's also known (terminology) as guarding. 

When in the dark remember-the future looks brighter than ever.   I look forward to being able to predict the future!