Author Topic: Analog Switches / Transistors  (Read 408 times)

Lasse

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Analog Switches / Transistors
« on: March 07, 2019, 02:12:16 PM »
Hi,

I'm trying to understand how the analog switching of the Current Ranger is working. Unfortunately I see only the image of the schematic, which does not show any designators for the transistors.

I'm mostly interested in the FETs on the lower left (switching ranges), and also on the NPN for switching on the low pass filter (though I guess that that one is less critical). Could you provide me with the type (best-case) or the relevant specifications (is low Rds_on enough to look for?)

Kind regards,
Lasse

Felix

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Re: Analog Switches / Transistors
« Reply #1 on: March 07, 2019, 04:00:25 PM »
It almost sounds like you just want some part numbers. But ... What exactly is your end goal?

If you just want to understand, here's the explanation in a nutshell:
We need ideal switches to toggle the shunts for the different measurement ranges. The same is true for the LPF BJT.
The closer to this ideal we can get, the less artifacts, leakage, noise, voltage drop (burden voltage) they exhibit into the amplifier circuit and into the DUT.
That's all.

Lasse

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Re: Analog Switches / Transistors
« Reply #2 on: March 15, 2019, 10:29:47 AM »
Hi Felix,

I'm trying to understand the circuitry to to refresh my analog knowledge, as I've been doing digital only the last few years. Finally I'm trying to build my own current meter, but this is only a private project (non commercial, most likely for me only). So, part numbers would help for the build, but giving me hints on what to take care for during the selection process of the FET would help me more to improve my knowledge. On a first shot, I would look for low Rds_on, but what are other main characteristics?

Rdson lies in the microohm range, which is probably the reason why it can be ignored for the micro ampere scale. If I understand the schematics correctly, in the milliampere range, Rdson adds to the burden voltage, but not to the measured value.


Based on your reply, I think following specs are important:
- low Rdson
- low DS/GS/GD Leakage

How does the FET introduce noise (which parameter should be low/high)?


I understand if you don't want to help me forking your project, please let me simply know in this case. I somehow assumed your project was open hardware, but maybe I got that confused with the uCurrent.

Kind regards,
Lasse

Felix

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Re: Analog Switches / Transistors
« Reply #3 on: March 15, 2019, 12:03:45 PM »
It's really just a simple matter of understanding ohm's law, mosfets and how they behave based on how they are biased, and their different regions of operation etc.
All these resources are widely available on the web. Along with the schematic and source code, that is more than enough to figure out what is going on.
There are various ways of routing the measured current through the shunts to avoid creating massive current burdens. I have chosen this topology as opposed to a stacked topology as found in the uCurrent, because it's lower burden voltage.

I think this is a pretty "open source" project, I have given away almost everything. Much more so than what you will find with other products, and you know that open source does not usually come with instructional videos and a 300 page manual, or otherwise you'd have no code and no schematic.