Making an auto ranging uCurrent (switching between 0.01, 10 and 10kOhm)

[markbng]

Hi,

I want to make an auto ranging uCurrent. The measured current goes through a 10 mOhm, 10 Ohm or 10k resistor. I want to be able to switch between these measurement resistor branches with a switching N-fet or something. so all these resistors are connected with each other in 1 point and the other end goes to the drain of a N-Fet. all source connections are connected to ground. The source of the selected measurement resistor value has a high voltage to enable this resistor. But my problem is the 10k range. This measures nA (10e-9). if other branches leak nA, then I have a problem. What FET can I use that has a low capacitance and very high impedance (GOhm)? Or do you have another solution. How can I switch between the measurement resistors? I will probably change the FET in the design. First the FET and than the measurement resistor to ground. Can someone help me in this part of the design. I want to measure nA, so only a leakage of a couple of pA is exeptable. Any help is appreciated.

Kind regards,
Mark

 

[KlausST]

Hi,

I want to make an auto ranging uCurrent.

What is this?
* A current source in the microAmperes range?
* Or a measurement device (meter) for currents in the microamperes range?
* Or someting else?
...
DC? AC? What frequency range, what waveform?
****

autoranging?
What range?

The measured current goes through a 10 mOhm, 10 Ohm or 10k resistor. I want to be able to switch between these measurement resistor branches with a switching N-fet or something.

If you have a circuit (solution) in mind, then show a schematic or at least a draft.

But if you are looking for a solution, then give clear requirements.
Like: Values with units, ranges, tolerance, accuracy, precision, resolution...

What FET can I use that has a low capacitance and very high impedance (GOhm)?

Why "low capacitance"? A MOSFET model (datasheets) usually comes with 3 different capacitances. Which one?
What does "low" mean? as value..

Nothing about voltages, power supply, how to be measured, how to be controlled?

Klaus

 

[Easy peasy]

3 way switch would seem to fit the bill

 

[D.A.(Tony)Stewart]

if your resolution is 1 mV and need low input offset V and low input bias current for a Howland Current Source, there are many to choose from.

Here's my list from one source Iin < =1pA Vio<=10uV in stock.

https://www.digikey.ca/en/products/filter/linear/amplifiers/instrumentation-op-amps-buffer-amps/687?s=N4IgjCBcoJwOwBYqgMZQGYEMA2BnApgDQgD2UA2iAAwB0VVATAAQAOAgiMbfQMysdc6VAKz9O1OmDGDG0iVPbjucUYpmqBEqgjnc%2Ba8LpowjADiMqLRgGwalNa0buCdB2vs20GziQw-3mNxowU0DPYLsAXWIWABcoEABVADsAS1iAeXQAWXxMXABXACd8EABfYnhhZBA0SCw8IlIKcComAFaANXFzLvFHPuJRQZA%2BEeYRsBo4Du7iKeG5wxHaRYDZ%2BzAJpdopPuiQOISU9Kzc-OLSipAAWgYautiigqaySEpqyLLr6xrUgBMEjcwFQIDF4pAQOJYgBPFilSH5NDfIA

But you need high current for mohm resolution so use an emitter follower and/or a one of the above 170 items with 85mA drive capability and 0.01 pA input current. You won't need 50 MHz GBW but this certainly has come a long way from the uA741

 

[danadakk]

Small signal relay, like the ones used in IC testers ?

What is a Signal Relays? | OMRON Device ＆ Module Solutions - Americas

Signal relays are used for lower than 2A load switching, and for telecom & network devices, test & measurement devices, and security devices.

components.omron.com

You would have to do a cal cycle to manage its contact R, but leakage would be 1,000 Meg Ohms
min on many.... Many low current drive.


Regards, Dana.

 

[D.A.(Tony)Stewart]

What do you expect for accuracy, resolution and sensitivity ?​