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variable gain current amp

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Simon:
No, you are totally missing the key challenges. I have come up with a solution. One does not simply got a MOSFET that is significantly lower resistance than a current shunt. So I am prepared to loose 100mV. at 5A that is 20mOhms. A 10mOhm MOSFET will introduce a massive error. I have found MOSFETS for over 500A that are 300µOhm. I will use 2 in parallel.

But I need 100mV to look like 5.00 on my voltmeter and I will get 10.00 with 20mohm. If I use a 10mOhm shunt I start to loose accuracy. So I am using my low range shunt as a voltage divider with 2 1Ohm resistors in series and my measurement comes off the centre tap. The combined series and parallel resistances when the higher current shunt is switched in is near bang on 20mOhm still. While the 2Ohms will also drop 100mV at 50mA which being tapped off the middle will be 50mV @ 50mA which I can represent on a 200mV panel meter. The decimal points can be turned on at will so I can make it all work without even needing a current amp.

Jay_Diddy_B:
Simon,

Take a look at this:

https://www.analog.com/en/products/ltc6102.html#product-overview

The LTC6102 is zero drift, so it is optimised for wide dynamic range.



Results







Linearity at low currents





The non-linearity is in the input stage, so post amplification will not fix it.

Regards,

Jay_Diddy_B

 6102 range switch.asc (1.71 kB - downloaded 26 times.)

Jay_Diddy_B:
Hi,

HP did this in their the 66332A Power supply:



They use two shunts, each one is four terminals. A MOSFET switch shorts the sensitive range shunt. The MOSFET is not in the measurement circuit for the high current range.

You can implement like this:



In this configuration the RDSon doesn't impact the reading. The low range shunt is optimised for low currents.

Regards,
Jay_Diddy_B

Simon:
I am looking at that chip. Yes it has a pretty high gain. Yes if you use two circuits and just bypass the low current one sure it works, but then I'd need to duplicate everything and analogue multiplex to the output to read it.

Jay_Diddy_B:
Hi,

I think that double measuring and shorting shunts is the way to get really high dynamic range.

Dave did a review of device designed to this in EEVBlog 1304. Look at 31:00 mins plus. There are multiple shunts and a pair of shorting MOSFETs.

Quitech Otti USB power measurement and battery analyser: https://www.qoitech.com/

The question is how good do you need to be?

Regards,
Jay_Diddy_B


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